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Depolarization and hyperpolarization in stereocilia of the inner ear

Depolarization and hyperpolarization in stereocilia of the inner ear


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It's a well mentioned fact that when the stereocilia of the cochlear hair cells bend in one direction, the hair cell depolarizes, and when the stereocilia bend in the other direction, the cell hyperpolarizes. When the basilar membrane vibrates, the stereocilia are bent back and forth, creating depolarizations in the hair cells followed by hyperpolarizations. What I'm having trouble understanding is why this is significant. This does not determine the frequency of the sound wave, as that is determined by the location along the basilar membrane that the wave impinges on. I don't see how this would determine amplitude either, seeing as a greater amplitude would only create more drastic bending of a greater number of hair cells. Can anyone shed some light on this?


There are roughly two modes of pitch coding in the cochlea: place-coding and temporal coding. The place-theory is the most prevalent accepted model of how the cochlea realizes pitch coding (e.g., Zwislocki, 1991). Basically, it is based on a frequency-to-place Fourier transformation on the incoming sound, where each frequency is coded on a different place on the basilar membrane, as described accurately in the question.

However, there is another, much overlooked way of coding pitch, namely temporal coding. Up until about 1 kHz, spiral ganglion cells in the auditory nerve and acoustical brain stem regions (such as the inferior colliculus) have been found to respond in a phase-locked pattern (Du et al., 2011). Electrophysiology in auditory nerve fibers illustrates the phase-locked activity in response to low-frequency sounds (Fig. 1). This phase-locking behavior of neurons in the auditory system is called the frequency-following response (FFR).


fig. 1. FFR in auditory nerve fibers. Upper trace shows the stimulus, middle trace a single fiber, and the lower trace the compound activity of many fibers. The group-response faithfully codes the stimulus waveform. Source: New York University.

However, as you rightfully state in your question - why would inhibition of auditory nerve fibers (ANFs) be helpful? Figure 1 illustrates nicely that the auditory nerve FFR is rectified, i.e. only the upper half is coded, while negative action potential counts obviously don't exist; ANFs fire, or they don't, they don't fire negative action potentials. However, one has to realize that a relatively large proportion of ANFs in healthy ears spontaneously fire, i.e., in the absence of sound the auditory nerve is still remarkably active. Spontaneous firing rates vary from 0 to more than 100 spikes/s (Jackson & Carney, 2005). Hence, suppression of spontaneous activity codes pitch too.

Moreover, as a theoretical and un-referenced side note - the auditory system processes acoustic information on the sub-millisecond level. Left/right sound localization is performed by the auditory system by resolving inter-aural time differences (ITDs). By analyzing the delay at which sound arrives in one ear with respect to the other, the localization in the horizontal plane can be estimated. ITDs do not exceed 0.8 ms, given the speed of sound and the size of the human head. This is well below the time it takes for any action potential to develop(!). Hence, by deploying both phases of the sound, one phase being excitatory, the other inhibitory at the ANF level, no time is lost when a sound wave happens to enter the cochlea with its inhibitory flanking phase first.

References
- Du et al., Neurosci Biobehav Rev (2011)
- Jackson & Carney, JARO (2005); 6: 148-59
- Zwislocki, Acta Otolaryngol (1991); 111(2): 256-62


The depolarization and hyperpolarization of stereocilia in the ear is significant for its role in transduction. Hair cells are involved in changing mechanical energy into changes in membrane potential, a process called transduction.

When the cells are displaced and move in the direction of the tallest stereocilium, K+ enters the cell and causes depolarization which allows more transduction channels to open. This depolarization opens voltage gated calcium Ca2+ ion channels. The influx of Ca2+ causes neurotransmitter release from the basal end of the hair cell to the auditory nerve endings which sends signals to the brain.

Movement of the hair cells in the opposite direction causes hyperpolarization which prevents influx of K+ and closes the Ca2+ channels at the base resulting in smaller release, or no release of the transmitter. This allows hair cells to generate a sinusoidal receptor potential in response to a sinusoidal stimulus which preserves the temporal information present in the original signal up to frequencies of 3 kHz.

So the depolarization and hyperpolarization of hair cells is significant because the constant release of transmitter drives the spontaneous activity in the auditory and vestibular nerve fibers. The interaction of Ca2+ influx and Ca2+ dependent K+ efflux lead to electrical resonances that enhance the tuning response properties within the inner ear.

For a more detailed response see: http://www.ncbi.nlm.nih.gov/books/NBK10867/ and http://homepage.psy.utexas.edu/homepage/class/psy394U/hayhoe/perception/Chapt.%201%20Hair%20cells%20of%20the%20cochlea%20copy%203.pdf


Mechanism of Sound Transduction

Even though the structure of cochlea seems to be complicated, its basic operation is relatively simple. Inward motion at the oval window caused by ossicles pushes the perilymph in the scala vestibule towards the apex of the cochlea. The fluid pressure, through the helicotrema, then travels back down the scala tympani to the round window. This causes the membrane at the round window to bulge out. In this process, the flexible basilar membrane that sits in between scala tympani and scala vestibule bends in response to sound.

How does the basilar membrane respond to sound?

Two structural properties of the basilar membrane determine the way it responds to a sound wave:

         1.   The apex of the membrane is 5 times wider than the base.

         2.   The base of the membrane is about 100 times stiffer than the apex.

Figure 1: Schematic representation of the basilar membrane within the cochlea (uncoiled for illustration of greater clarity) showing its length variation from the base toward the apex.

The wave travelling along the membrane is analogous to that runs along a rope when you give it a snap while holding it in your hand. The distance the wave travels up from the base to the apex of the basilar membrane depends on its frequency: high-frequency waves cause great deal of vibration at the stiffer base and dissipates most of the energy before propagating further in contrast, low-frequency waves travel all the way up to the floppy apex before complete dissipation of energy. The unique response of the basilar membrane sets up a place code whereby different locations of membrane are maximally bent or deformed at different sound frequencies.

Figure 2: Displacement of the basilar membrane in reponse to sound waves of low frequency (top) and high frequency (bottom).

Image produced by the author of the website.

How is the place code converted into neural coding of pitch?

Since the organ of Corti is rigidly connected to the basilar membrane, any movement of the membrane in response to sound waves will cause the structures within the organ of Corti, including the hair cells, to move as a unit. When sound causes the basilar membrane to move upward, the reticular lamina moves up and in towards the tectorial membrane, causing the stereocilia of outer hair cells to bend due to their attachment at the tips to the membrane. Likewise, downward movement of basilar membrane causes bending of stereocilia in the opposite direction. In fact, stereocilia from inner hair cells are similarly bent, presumably as a result of endolymph movement. The stereocilia on a hair cell are made to stick together by cross-link actin filaments and hence move as a unit.

Figure 3: The bending directions of the hair cells depend on whether the basilar membrane is moving upward (a), resting (b), or moving downward (c). 

Bending of the stereocilia leads to the generation of neural signals

Recording from the bony structure in vivo has technically been a difficult issue. Therefore the transduction mechanism has been revealed mostly by in vitro studies of isolated hair cells from the cochlea. Recordings from these cells have shown that bending of stereocilia in a direction causes depolarization of the hair cell.    As they bend in the other direction, the cell hyperpolarizes instead. Consequently, when the sound wave causes the stereocilia to bent backward and forward, hyperpolarization and depolarization of the hair cells occur in alternative manner from the resting potential.

Figure 4: Hyperpolarization and depolarization of the hair cell depending on the bending direction of the stereocilia. The hair cell receptor potential follows the changes in air pressure closely during a low-frequency sound.


2015 Hearing & Balance

  1. The mechanoreceptors that transduce fluid movements induced by sound and head movement are found in the inner ear, deep within the petrous portion of the temporal bone. The complex accessory structures that make up the labyrinths are necessary for both hearing and balance, while the external and middle earsmodulate sounds before they are transduced by the cochlea.
  2. Large modified apocrine glands deep in the dermis of the external auditory meatus produce cerumen (ear wax), which normally protects the tympanic membrane . If the wax becomes impacted, and/or is pushed deep into the ear canal, it can produce a conductive hearing loss.
  3. Vibration of the tympanic membrane causes the malleus and incus to pivot, resulting in the stapes footplate vibration at the oval window. The gain in pressure due to the actions of the ossicles prevents some of the energy loss inherent in an air/fluid transition and results in air conduction being better than bone conduction (the basis of the Rinne test).
  4. A conductive hearing loss is an increase in hearing thresholds due to a decrease in sound transmission through the external and/or middle ears. Air conduction is reduced but bone transmission is unaffected. Common causes of CHL include a ruptured eardrum, intra-tympanic fluid (usually caused by otitis media), and otosclerosis.
  5. The inner ear consists of 2 structural regions that contain different fluids: the bony (osseous) labyrinth, which contains Na + -rich perilymph, and the membranous labyrinth, which contains K + -rich endolymph. The histology in each of the 6 receptive areas of the inner ear is similar.
  6. Inner ear receptors are divided into two types both types convert mechanical energy into receptor potentials . TYPE I (INNER HAIR CELLS) are the true sensory receptors that convey information to the brainstem. TYPE II (OUTER HAIR CELLS) function as biological amplifiers, essentially acting as motor units.

Inner ear transduction is DIRECTIONAL: displacement toward the tallest stereocilia (positive deflection) results in DEPOLARIZATION . In the cochlea, this occurs when the basilar membrane moves toward scala vestibuli. Negative deflection (toward scala tympani) results in HYPERPOLARIZATION .

The SEMICIRCULAR CANALS detect head rotation (angular acceleration). The OTOLITH ORGANS (UTRICLE and SACCULE) detect gravity (linear acceleration). The vestibular system is involved in balance and posture, co-ordination of head and body movements and in fixating the visual image on the fovea.

SEMICIRCULAR CANALS WORK IN PAIRS , with depolarizaation occuring in the SAME direction as the head rotation (HORIZONTAL CANALS: head left &rarrdepolarization left, hyperpolarization right). The natural pairing is of LEFT ANTERIOR with RIGHT POSTERIOR CANAL (and vice versa).

The VESTIBULO-OCULAR REFLEX is a 3 neuron arc (hair cell/vestibular nerve, vestibular nuclei, cranial nerve motor nuclei) that is used to adjust eye position to compensate for changes in head position (i.e., it keeps the visual image centred on the fovea). Remembering the pairings listed in fact #4, there is depolarization/excitation/contraction in one of the pathways of the pair, and hyperpolarization/inhibition/relaxation in the other . Rotation of the head in one direction results in rotation of the eyes in the opposite direction.

NYSTAGMUS consists of a slow drift of the eyes in one direction (PURSUIT) followed by a rapid recovery movement in the opposite direction (SACCADE) . The direction is named for the fast component i.e., a RIGHTWARD NYSTAGMUS consists of slow movement of eyes to the left, followed by fast recovery to the right . The PURSUIT is controlled by vestibulo-ocular reflex the SACCADE by higher centers (e.g., cortex). Nystagmus can be observed in normal people following stimulation of the vestibular system in the absence of stimulation, it is a sign of underlying pathology.

The caloric test is used to assess brain function. In a person with a normally functioning cortex, injection of cool water into the right ear, will produce a LEFTWARD NYSTAGMUS (COLD=OPPOSITE, WARM=SAME &rarr COWS). If the patient is COMATOSE , the SACCADE WILL BE ABSENT (the VOR, which operates in the brainstem is still functional and the pursuit will be intact). If the patient is BRAIN DEAD, both the PURSUIT and SACCADE WILL BE ABSENT.

The middle ear transfer function determines the absolute threshold of hearing at each frequency in normal individuals &ndash the cochlea is so sensitive, it can transduce any signal that reaches it. This implies that anything that alters middle ear function (like an infection) will significantly impact hearing thresholds.

Sound waves pass through the cochlea INSTANTANEOUSLY . The traveling wave pattern on the basilar membrane is established more gradually and is INDEPENDENT of how the motion is initiated i.e., don't need to deliver sound via the oval window --- can use bone! The traveling wave establishes a frequency vs. place relationship along the length of the cochlea, with high frequencies being transduced in the base , and low frequencies in the apex.

Outer hair cells use their receptor potential to exert force on the basilar membrane ---thereby generating a POSITIVE FEEDBACK MECHANISM which amplifies the vibration of the membrane in a nonlinear, highly frequency specific manner. This force produces its own fluid wave, which is conducted back through the perilymph, vibrating the middle ear apparatus and generating sounds that are emitted from the ear (OTOACOUSTIC EMISSIONS).

The STRIA VASCULARIS produces the endolymph (high K+) and the endocochlear potential (+80 mV). Many of the ion transporters of the stria are the same as those in the kidney, so drugs that affect renal function are often ototoxic &ndash esp. loop diuretics (which affect the Na+/K+/2Cl- transporter).

Sounds are localized by the differences in timing and intensity between the two ears. Lateral superior olive (LSO) neurons localize high frequency stimuli by comparing interaural intensity differences (IIDs) medial superior olive (MSO) neurons use interaural timing differences (ITDs) to localize low frequency stimuli.


Human Biology (1st Edition) Edit edition

1. Cochlear hair cells—convert sound waves to electrical signals.

The cochlear hair cells are located in the cochlear duct in the inner ear. They are called the auditory receptors. These hair cells have many stereocilia and one kinocilium. The stereocilia are linked together by fine fibers called tip-links. They protrude into the endolymph. The longer stereocilia meshed in the overlying tectorial membrane. This endolymph is rich in K + . Transduction of the sound stimuli occurs, when the hair cells are deformed by the movements of the basilar membrane. When the stereocilia bend towards the kinocilium, the tension is caused in the tip-links, which open the cation channels in the adjacent stereocilia. This causes inward K + and Ca 2+ current and graded by depolarization. Bending the cilia away from the kinocilium closes the mechanically gated ion channels and allows repolarization and graded hyperpolarization. The intracellular Ca 2+ increased during depolarization and the neurotransmitter release by the hair cells also increase. This stimulates the afferent cochlear fibers to transmit faster stream of impulses to brain, for auditory interpretation. During hyper polarization this effect is reversed. Activation of hair cells occurs at points, where there is vigorous basilar vibration.

The depolarization and the hyperpolarization of the outer hair cells cause stiffness of basilar membrane, which amplifies the responsiveness of inner hair cells. The outer cells are more in number they send little information to the brain.

Hence, the correct option is (c) convert sound waves to electrical signals .


Depolarization and hyperpolarization in stereocilia of the inner ear - Biology

The vestibular system is a complex set of structures and neural pathways that serves a wide variety of functions that contribute to our sense of proprioception and equilibrium. These functions include the sensation of orientation and acceleration of the head in any direction with associated compensation in eye movement and posture. These reflexes are referred to as the vestibulo-ocular and vestibulospinal reflexes, respectively. The centrally located vestibular system involves neural pathways in the brain that respond to afferent input from the peripheral vestibular system in the inner ear and provide efferent signals that make these reflexes possible. Current data suggest that the vestibular system also plays a role in consciousness, and dysfunctions of the system can cause cognitive deficits related to spatial memory, learning, and navigation.[1][2][3]

Cellular

There are vast amounts of both afferent and efferent cellular connections involved in the vestibular system. Most of the afferent nerve signals come from the peripheral vestibular system found in the inner ear within the petrous temporal bone. The inner ear contains a bony labyrinth and a membranous labyrinth. The bony labyrinth is filled with a fluid known as "perilymph" which is comparable to cerebrospinal fluid and drains into the subarachnoid space. Suspended within the bony labyrinth is the membranous labyrinth that contains a fluid known as endolymph unique in composition due to its high potassium ion concentration. Endolymph in the vestibular system is produced by the Vestibular Dark Cells which are similar to the Stria vascularis of the cochlea. Endolymph within the membranous labyrinth surrounds the sensory epithelium and interacts with hair cells within the vestibular apparatus providing the high potassium gradient to facilitate depolarization of the hair cell and afferent nerve transmission. [4] The vestibular apparatus comprises the utricle, saccule, and superior, posterior, and lateral semicircular ducts. The sensory neuroepithelium in the utricle and saccule is the macula, and the sensory neuroepithelium in the semicircular ducts is the crista ampullaris. Both neuroepithelial structures contain specialized mechanoreceptor cells called "hair cells." Hair cells contain a vast number of cross-linked actin filaments called stereocilia that are connected at the tips by &ldquotip links.&rdquo The stereocilia contain cation channels at their apex and are organized in rows by length, with the tallest stereocilium connected to an immobile kinocilium. The kinocilium, the only true cilium, is made of the characteristic 9 + 2 microtubule arrangement. [5][6] Hair cells are divided into Type 1 hair cells and Type 2 hair cells. Type 1 hair cells have a high variability of resting discharge while Type 2 hair cells have a low variability of resting discharge. Acceleration of endolymph results in the movement of stereocilia, leading to either depolarization or hyperpolarization depending on the direction of the inertial drag. Movement towards the kinocilium causes the interconnected tip links to pull open cation channels resulting in an influx of potassium ions and depolarization. The depolarized hair cell releases glutamate to afferent nerve receptors and neurotransmission to the vestibular ganglion. Movement in the opposite direction to the kinocilium causes stereocilia to converge resulting in tip links closing the cation channels. Lack of potassium influx causes hyperpolarization of the hair cell and inhibition of glutamate release to the afferent nerve. [5][6][7] The vestibular ganglion, also known as Scarpa ganglion, contains thousands of bipolar neurons that receive sensory input from hair cells within the macula and crista ampullaris. Afferent axons from the vestibular ganglion join to become the vestibular nerve. The vestibular nerve then joins the cochlear nerve to become cranial nerve VIII, the vestibulocochlear nerve. Afferent nerve signals carried by the vestibulocochlear nerve are then interpreted by the central vestibular system within the brain. The central vestibular system unites the peripheral signals from both ascending pathways to elicit eye, head, and body motor responses for control of balance and orientation.[6]

Development

The development of the peripheral vestibular system begins with the formation of the otic placodes from surface ectoderm in the third week. During the fourth week, the otic placodes become the otic pits when they become surrounded by embryonic mesoderm. The otic pits then develop into the otic vesicles. The upper portion of the otic vesicle becomes the vestibular apparatus. As the otic vesicle lengthens, a division occurs between the ventral saccular portion and the dorsal utricular portion. The ventral saccular portion becomes the adult saccule and cochlear duct while the dorsal utricular portion forms the utricle and semicircular canals. Ossification of the system begins at 19 weeks gestation and reaches adult size by 25 weeks except for the internal aperture of the vestibular aqueduct that continues to develop until birth. Hair cells and otoconia develop at seven weeks with differentiation of Type 1 and Type 2 hair cells occurring between 11 and 13 weeks.[8][9]

Function

The vestibular system functions to detect the position and movement of our head in space. This allows for the coordination of eye movements, posture, and equilibrium. The vestibular apparatus found in the inner ear helps to accomplish this task by sending afferent nerve signals from its individual components. The utricle and the saccule are responsible for sensing linear acceleration, gravitational forces, and tilting of the head. The neuroepithelium found in the utricle and saccule is the macula which provides neural feedback about horizontal motion from the utricle and vertical motion from the saccule. Embedded within the otolithic membrane of the macula are small calcium carbonate crystals known as otoliths that assist in hair cell response to the inertial drag of endolymph. Angular acceleration and rotation of the head in various planes are sensed by the three semicircular ducts that are oriented at right angles to one another. Each of the semicircular ducts contains a dilation near the opening to the utricle. This dilation is called the ampulla which contains a neuroepithelial structure called the "crista ampullaris." The crista ampullaris is coated by a gelatinous protein-polysaccharide substance known as the cupula which holds the hair cells in place. Unlike the macula, the crista ampullaris does not contain otoliths. In addition to the functions associated with the peripheral vestibular system, the central vestibular system allows for processing and interpretation of afferent signals and the output of efferent signals. Efferent signals include the vestibulo-ocular reflex, which allows the eyes to remain fixed on an object while the head is moving. This is accomplished by coordinating movement between both eyes involving the parapontine reticular formation and output to various extraocular eye muscles involving the oculomotor and abducens nerves. The vestibulospinal reflex maintains balance and posture through the coordination of spinal musculature with head movement. Cognitive functions that involve the central vestibular system are based on established neural pathways, although many pathways are still unknown. The known central vestibular connections include the vestibulo-thalamo-cortical tract, dorsal tegmental nucleus to entorhinal cortex tract, and nucleus reticularis pontis oralis to hippocampus tract. These tracts form a series of complex connections that play a functional role in self-motion perception, spatial navigation, spatial memory, and object recognition memory.[3][2][6][1]

Mechanism

The mechanism involved with the function of the peripheral vestibular system involves the acceleration of endolymph within the various structures of the vestibular apparatus. Head movement in various directions is responsible for this acceleration that results in the stimulation of the stereocilia of hair cells. When the head stops accelerating, hair cells return to their baseline position which allows them to respond to further changes in endolymph acceleration. Depending on the direction of acceleration, the inertial drag of the endolymph will push the stereocilia either towards or away from the fixed kinocilium. Movement towards the kinocilium causes tip links to pull open cation channels resulting in depolarization of the hair cell via potassium ion influx. Movement away from the kinocilium results in closure of the cation channels and hyperpolarization and reduction in afferent firing rates. Depolarization results in the opening of calcium channels. Calcium channel opening results in neurotransmitter release across the synaptic cleft, leading to nerve transmission to the vestibular ganglion. Nerve signals pass through the 20,000 bipolar neurons in the vestibular ganglion and leave along the vestibular nerve. The vestibular nerve joins the cochlear nerve and enters the brainstem at the pontomedullary junction. The primary processor of vestibular signals is the vestibular nucleus complex that extends from the rostral medulla to the caudal pons. Many signals are sent from the vestibular nucleus to either the thalamus, cortex, or cerebellum that help to process and adjust efferent signals to postural or ocular muscles. Of note, the hippocampus plays an important role in spatial memory, including the functions of navigation and orientation.[6][1][7]

Related Testing

Many tests can help determine if the vestibular system is functioning properly. The oculocephalic reflex is a simple test used to determine if the brainstem of a comatose patient is intact using the reflexes of the vestibular system. The test involves rotating a patient&rsquos head horizontally, which should activate the vestibular system on the ipsilateral side of rotation. This results in the patient&rsquos eyes slowly deviating to the side opposite of head movement if the brainstem is intact. If the brainstem is not intact, the eyes will follow the movement of the head to the ipsilateral side.

Caloric testing is a test that uses differences in temperature to diagnose damage to the acoustic nerve. A small amount of cold water or air is gently delivered into one of your ears. Your eyes should show an involuntary movement called nystagmus. Then they should turn away from that ear and slowly back. If water is used, it is allowed to drain out of the ear canal. Next, a small amount of warm water or air is gently delivered into the same ear. Again, your eyes should show nystagmus. Then they should turn toward that ear and slowly back. [10]

More specific tests for components of vestibular function includes videonystagometry, the most common vestibular function test. The test is divided into three parts including ocular motor, positional testing, and caloric testing. Other diagnostic modalities include rotation tests and video head impulse testing (VHIT). Both of these tests use devices to monitor eye movements when the head is rotated in various directions to test the integrity of the vestibulo-ocular reflex.[3][11]

Clinical Significance

Dysfunction of the vestibular system can manifest symptomatically as vertigo, nausea, vomiting, visual disturbance, hearing changes, and various cognitive deficits. The relationship of the vestibular system to cognition is not well understood, but many patients with vestibular dysfunction exhibit impairment of spatial navigation, learning, memory, and object recognition. The pathophysiology of vertigo can be defined as peripheral or central. Peripheral vertigo is more common than central vertigo, and three of the most common etiologies include benign paroxysmal positional vertigo, Meniere disease, and viral labyrinthitis.

Benign paroxysmal positional vertigo (BPPV) is the most common cause of peripheral vertigo and lasts for seconds to minutes. Most cases are idiopathic but the pathophysiology is believed to be due to displaced otoconia in the posterior semicircular canal that causes an inappropriate sensation of movement. BPPV is diagnosed based on a thorough history and use of the Dix-Hallpike test with associated reproduction of vertigo symptoms and nystagmus. There are many movement techniques used to treat BPPV however, the Epley maneuver is often cited as being one of the most effective. The technique involves rotating and tilting the head and body in various ways to reposition displaced otoconia in the inner ear. For acute, severe exacerbations of BPPV, anti-vertigo medications are indicated to help with symptom control.

Meniere disease is another cause of peripheral vertigo that can last for hours and also manifest with symptoms of hearing loss and tinnitus. The pathophysiology of Meniere disease is the expansion of endolymph volume within the membranous labyrinth. This volume expansion impacts both the vestibular apparatus and cochlea that are both filled with endolymph. Expansion of endolymph within the cochlear duct results in defects in hearing that differentiate it from BPPV. Meniere disease is diagnosed based on clinical criteria, and currently, no curative treatments exist. Symptoms are managed with anti-vertigo medications, low-salt diet, and surgical decompression of the endolymphatic sac as a last option.

Viral labyrinthitis, also known as "vestibular neuritis," is another cause of peripheral vertigo that can be attributed to inflammation of the vestibular nerve, secondary to a viral infection. Symptoms include a simultaneous loss of hearing and balance function in the affected ear that can last from days to weeks. Treatment involves symptom control with anti-vertigo medications, and symptoms will typically resolve in one to three weeks. In addition to the pathologies listed, vestibular function in the elderly has been well studied as a contributing factor to dizziness and imbalance leading to falls. This is due to the significant loss of both Type 1 and Type 2 hair cells that occurs most prominently between the ages of 65 and 70.[3][12][7][13]

Salicylate (aspirin) can reversibly eliminate outer hair cell electromotility to induce hearing loss. Salicylate competitively binds to the motor protein reversibly inhibiting electromotility. Other symptoms of aspirin toxicity include nausea, vomiting, tinnitus, hyperpnea, and disorientation. Other common ototoxic agents include aminoglycosides, furosemide, cisplatin, and quinines.[14]


Tuning in to the inner ear

Did you hear that sound? If so, you can thank your stereocilia.

These tiny fibers form bundles sitting atop sensory hair cells deep inside your inner ear, and give them their name. Stereocilia are as fragile and scarce as they are vital to your ability to hear.

Basile Tarchini, Ph.D.Investigating inner ear development, focusing on the role of cytoskeleton polarization in sensory function and hearing loss, with a goal to inform therapies for sensory cell regeneration. Basile Tarchini studies stereocilia, which convert sound into hearing through complex signaling operations with the brain. The Jackson Laboratory (JAX) assistant professor&rsquos work has revealed unexpected aspects of how stereocilia develop.

Normal inner-ear stereocilia grow in a &ldquostaircase&rdquo formation, with a short-to-tall graduation of hairs in the bundle, arranged like kids in a class photo. &ldquoThis staircase-like architecture of the hair bundle is essential for hearing and considered instrumental for direction-sensitivity to sound stimuli,&rdquo Tarchini says.

Here, in broad strokes, is how you hear. Sound waves enter the external ear and swirl down the ear canal until they reach the eardrum and set it vibrating. Tiny middle-ear bones connected to the eardrum amplify the sound waves and deliver them to the auditory portion of the inner ear, or cochlea.

Shaped like a snail shell and filled with fluid, the cochlea is divided into an upper and lower part by an elastic partition called the basilar membrane. In this liquid environment, the sound waves become fluid waves that travel along the basilar membrane. Inner-ear hair cells on the basilar membrane literally ride these waves.

The stereocilia at the top of the hair cells sway and bend in the flow. &ldquoThere are tiny links between stereocilia,&rdquo Tarchini says, &ldquolinking the tallest to the next tallest and so on. Tension on those links cause pore-like channels at the tips of the stereocilia to open up, and ions rush into the cells, creating an electrical signal.

&ldquoThis whole structure acts as a motion sensor.&rdquo

The auditory nerve carries the electrical signal to the brain, which recognizes and interprets the sound. Amazingly, the hair cells are arranged along the basilar membrane like the keys of a piano, high to low: Those near the entrance of the cochlea are responsible for detecting high-pitched sounds like birdsong and those close to the center of the &ldquosnail&rdquo sense lower-pitched sounds like far-off thunder.

In a healthy human cochlea, just about 16,000 hair cells handle this elaborate choreography of sound signaling, and only 4,000 of them are true sound receptors. By comparison, the retina of the human eye has about 127 million photoreceptors &mdash rods and cones &mdash to process visual signals.

Not only are hair cells rare, but they&rsquore also vulnerable to environmental damage. Sustained loud noise from working in construction or the military, or attending a 1980s hair band tribute concert, can kill hair cells, and some antibiotics and cancer drugs also cause hair cell destruction.

Humans develop their hair cells very early in life &mdash starting about 10 weeks post-conception. And humans, like mice and other mammals, are born with all the hair cells they will ever get, so once they&rsquore lost they&rsquore gone for good. On the other hand, birds, fish, and other non-mammals have the capacity to recover lost hearing through various regenerative processes.

The auditory epithelium of a young postnatal mouse. Tarchini's protein of interest, shown in blue, is polarized at the flat surface of the cells toward the top of the image and is also found in a higher amount at the tip of the short stereocilia that shoot off the surface. Sound deflects these stereocilia to open gated channels, and the depolarization of the green hair cells results in the release of neurotransmitters at the base of the cell, which is captured by the red nerve terminals, and then relayed to the brain.

Working with mice, Understanding the fine architecture of hearingWork by a team including JAX Assistant Professor Basile Tarchini, Ph.D., is shedding light on the mechanism that directs the assembly of the staircase pattern of the hair bundle. Tarchini discovered a signaling pathway that regulates the distinctive short-to-tall organization of stereocilia during development. If this signaling pathway is disrupted, he showed, stereocilia are shorter and of more even height, and the animal is deaf. Understanding the basic mechanisms underlying hair cell development holds the promise to unlock regeneration potential in adults and restore hearing following injury.

The staircase organization of the stereocilia bundle also means that each hair cell shows directionality, like the magnetized needle of a compass. In addition, neighboring hair cells orient their bundles in concert, in the same way a collection of compasses would all point to the north magnetic pole. Working with colleagues at The Rockefeller University, Tarchini showed that Protein Daple coordinates inner-ear single-cell and organ-wide directionalityJAX, Rockefeller research team shows mice lacking Daple show developmental defects in hair cells and bundles. a single protein, Daple, is required to shape the architecture of the stereocilia bundle in individual hair cells and establish their concerted orientation in the surrounding organ. In mice lacking Daple, hair bundles are misshapen and misoriented in a pattern indicating both single cell and organ-wide defects.

Tarchini was born in Switzerland, and French is his first language. He obtained his B.Sc. and Ph.D. in biology at the Université de Genève. There, as a graduate student, he worked in the laboratory of Denis Duboule, an eminent professor in the department of genetics and evolution. Tarchini then obtained a fellowship from the Human Frontier Science Program, a prestigious international program of research support, and completed his postdoctoral fellowship at the Institut de Recherches Cliniques de Montréal in Canada. There he worked with Prof. Michel Cayouette whose laboratory studies cell fate determination in the retina.

New interest in the inner ear, together with a lifelong tendency to take the less-traveled path, led Tarchini to change his research path.

&ldquoI was concerned that my retina project wasn&rsquot moving fast enough and wasn&rsquot sufficiently promising&rdquo he recalls. &ldquoI had this idea to look in a different organ, and I had heard the inner ear was an amazing system when it came to cell polarity. But I knew nothing about the inner ear.&rdquo

This began, Tarchini says, as a &ldquovery risky and ineffective foray into the inner ear,&rdquo and involved teaching himself techniques because no one else in the lab had the knowledge to train him. &ldquoSo I lost a lot of time, but it turned out to be an investment in my future research. I was very lucky Michel is a particularly open-minded and hands-off person, and he gave me the freedom and time to explore a different system.&rdquo

The move to studying the inner ear allowed Tarchini to successfully navigate the tricky situation of any postdoc starting his or her own lab after working in the lab of an established scientist. &ldquoIn the end, this inner ear work proved to be really interesting, and I was able to leave the Cayouette lab and seamlessly continue the same research independently, without having to worry in any way whether I was stepping on the toes of my previous advisor. And that was fantastic.&rdquo

Tarchini&rsquos former mentors continue to watch Tarchini&rsquos progress with interest and pride. &ldquoBasile is a fantastic scientist,&rdquo says Cayouette, &ldquoand I would say that his greatest assets are that he is thorough, meticulous and rigorous in both experimental planning and execution. Basile is also obviously very intelligent and dedicated. He taught himself everything and ended up publishing beautiful papers on the cochlea, in a retina lab! That was very impressive. I have no doubt that Basile will continue to make important contributions and will become a leader in his field.&rdquo

&ldquoOn page 60 of his Ph.D. thesis,&rdquo Duboule relates, &ldquoBasile quoted a citation from H.L. Mencken: 'For every complex problem there is an answer that is clear, simple, and wrong.&rsquo This says a lot about him and his very high scientific standards &mdash and about his slight touch of nihilism too.&rsquo'

Tarchini joined the JAX faculty in 2015. A year later he secured his first federal research funding, a five-year, $1.9 million grant from the National Institute on Deafness and Other Communication Disorders.

In person, everything about Tarchini is precise and measured, from his orderly office overlooking Bar Harbor&rsquos spectacular Frenchman Bay to his dapper appearance (in contrast to most scientists his age, who favor the sporty-scruffy-perennial grad student look). He may be wearing an exquisite sweater knitted by his wife Dayana Krawchuk, JAX&rsquos exuberant scientist-social media manager. When they&rsquore together she tells the anecdotes and he provides the pithy punch lines.

Tarchini is also an accomplished jazz musician who once considered taking the path of the professional performer instead of the scientist. He recently performed on bass at a concert at the Bar Harbor town library with JAX President and CEO Edison Liu on piano.

A musician-scientist who studies hearing? Actually, Tarchini says with a laugh, &ldquothe older I get, the more I like quiet. I can&rsquot stand background music, for example!&rdquo

And in fact, his research interests, while staying in the inner ear, are moving to include the vestibular system, which is located right next to the cochlea.

&ldquoThe inner ear is basically two systems in one, auditory and vestibular,&rdquo he says. &ldquoIt&rsquos something we take for granted, the ability to perceive where our body is in space, to walk upright, to sense gravity. But it&rsquos incredibly important that it functions correctly: otherwise, you couldn&rsquot get out of bed in the morning.&rdquo

Tarchini has already shown he can boldly and successfully shift his research focus. Stay tuned for interesting discoveries.


Physiology of external, middle and inner ear

This organ extends from the apex to the base of the cochlea and consequently has a spiral shape.

20,000 outer hair cells and 3500 inner hair cells in each human cochlea

Ninety to 95% of these sensory neurons innervate the inner hair cells only 5–10% innervate the more numerous outer hair cells, and each sensory neuron innervates several outer hair cells.
most of the efferent fibers in the auditory nerve terminate on the outer rather than inner hair cells. The axons of the afferent neurons that innervate the hair cells form the auditory (cochlear) division of the eighth cranial nerve.

basilar membrane is relatively permeable to perilymph in the scala tympani, and consequently, the tunnel of the organ of Corti and the bases of the hair cells are bathed in perilymph.

increase progressively in height

along the perpendicular axis, all the stereocilia are the same height

tight junctions between the hair cells and the adjacent phalangeal cells prevent endolymph from reaching the bases of the cells

is embedded in an epithelium made up of supporting cells, with the basal end in close contact with afferent neurons. Projecting from the apical end are 30 to 150 rod-shaped processes, or hairs.

have cores composed of parallel filaments of actin. The actin is coated with various isoforms of myosin

outer hair cells respond to sound, like the inner hair cells, but depolarization makes them shorten and hyperpolarization makes them lengthen.

Very fine processes called tip links tie the tip of each stereocilium to the side of its higher neighbor, and at the junction are cation channels in the higher process that appear to be mechanically sensitive.

When the shorter stereocilia are pushed toward the higher, the open time of these channels increases. K+—the most abundant cation in endolymph—and Ca2+ enter via the channel and produce depolarization.

one hypothesis is that a molecular motor in the higher neighbor next moves the channel toward the base, releasing tension in the tip link

This causes the channel to close and permits restoration of the resting state. The motor apparently is myosin-based. Depolarization of hair cells causes them to release a neurotransmitter, probably glutamate, which initiates depolarization of neighboring afferent neurons.

When transduction channels open large potential difference of 150mV exist between the endolymph (80 mV= endolymphatic potential) and the hair cell interior is -70 mV that drives the cation into the steriocilia

This high pd increases the sensitivity of the system and helps in the formation of receptor potential by increasing conductance of cations thru the apical membrane of hair cells

perilymph is formed mainly from plasma.

On the other hand, endolymph is formed in the scala media by the stria vascularis and has a high concentration of K+ and a low concentration of Na+

Cells in the stria vascularis have a high concentration of Na+–K+ pump.

The acoustic impedence of water is higher than that of the air an dwithout impedence matching most of the sound reaching the cochlea is reflected back instead being transmitted into the cochlear fluid

It
The pressure gain ensures that more than half the sound energy striking the tympanic membrane is transmitted to the cochlea


To overcome the inertia of the ossicles some energy is lost, but the sound pressure is magnified in the middle ear in 2 ways:
d/t lever action within the ossicular chain
d/t smaller area of the oval window in comparison to the tympanic membranethis magnification of sound in the middle ear is called impedence matching
It is also imp to overcome the impedence in the innerear so as to transmit the sound from the air to the liquid medium


The middle ear is said to match the impedence of the external ear to the inner ear

Middle ear magnifies the pressure of the sound by 3 mechanisms

Major : area of tm is 20 times area of foot plate of stapes
F=P*A
When a sound impinging to the TM with a given force is transmitted to the stapes pressure is much higher at the stapes
Ability to move the fluid is depends on the pressure than the force
Thus the area diff amplifies the pressure

Since the area of the footplate is const increase in force increases the pressure

Leveraction of the osicles—malleus longer than incus, incus displaced less than malleus but with greater force
This lever action provides amplification by 1.3 times

Curved membrane mechanism: tm is rigidly fixed near its rim
Not tightly stretched, hangs loose like a tent, thus sound produces less displacement at the center than at the periphery so the force exerted on the malleus is greater


Depolarization and hyperpolarization in stereocilia of the inner ear - Biology

The vestibulocochlear nerve, also referred to as the eighth cranial nerve (CN XIII), is a sensory afferent nerve that transmits electrochemical impulses from the inner ear to the brainstem. It is composed of three separate nerves that run parallel to one another, the two vestibular nerves and the cochlear nerve. The superior and inferior vestibular nerves receive sensory input from the vestibular labyrinth, which is made up of the otolith organs and the semicircular canals and is responsible for balance and coordination of movements. The cochlear nerve receives sensory input from the cochlea, which is involved in hearing.[1]

Structure and Function

The ear is organized into three different anatomical structures: the outer, middle, and inner ear. The outer ear consists of the pinna, external auditory canal, and tympanic membrane and is responsible for the transmission of sound waves from the external environment.[1] The middle ear is an air-filled space that contains the three ossicles (malleus, incus, and stapes), which are bones responsible for transmitting vibrations from the tympanic membrane to the inner ear. Vibrations are transmitted from the malleus through the incus to the stapes, which is in contact with the cochlear oval window. The inner ear is located within the bony labyrinth of the temporal bone and contains the cochlea, semicircular canals, utricle, and saccule. These organs make up the membranous labyrinth that is within the bony labyrinth, separated only by perilymph. The membranous labyrinth contains a fluid known as endolymph, which plays a vital role in the excitation of hair cells responsible for sound and vestibular transmission.

The cochlea is a spiral-shaped fluid-filled organ located within the cochlear duct of the inner ear. The cochlea contains three distinct anatomic compartments: the scala vestibuli, scala media (also referred to as the cochlear duct), and scala tympani. The scala vestibuli and scala tympani both contain perilymph and surround the scala media, which contains endolymph. The endolymph within the scala media originates from cerebrospinal fluid (CSF) and is secreted by the stria vascularis, which is a network of capillaries located in the spiral ligament. The perilymph in the scala vestibuli originates from blood plasma, whereas the perilymph in the scala tympani comes from CSF. Endolymph and perilymph vary significantly in their concentration of ions, which is essential to the overall function of the cochlea. Endolymph is rich in potassium and low in sodium and calcium, whereas perilymph is rich in sodium and low in potassium and calcium. This difference in concentration allows for a positive endocochlear potential. The difference in concentration of potassium ions among the three fluid compartments within the cochlea enables proper transduction of current along with the hair cells.

Vibration from the stapes gets transmitted through the oval window, which is an opening into the inner ear through which the middle and inner ear communicate. Vibrations across the oval window initiate a perilymph wave that propagates along the scala vestibuli, with high frequency sounds dissipating earlier at the base of the cochlea and low-frequency sounds dissipating later towards the apex of the cochlea. The perilymphatic wave terminates at the round window, another point at which the middle ear communicates with the inner ear. In contrast to the oval window, the round window does not articulate with the stapes. Rather, the round window membrane is located inferomedial to the oval window and functions to counteract the fluid shift created in the cochlea. The presence of the round window allows for fluid to move more freely through the cochlea, thereby improving sound transmission.

As vibration transmits across the oval window, perilymph gets pushed towards the cochlear apex, which causes the scala media to become compressed. Within the scala media, there is a tectorial membrane that sits atop the organ of Corti. The compression of the scala media causes the tectorial membrane to change the position of cells within the organ of Corti.

The organ of Corti is located within the scala media and is responsible for converting mechanical forces into electrical impulses. It contains 15000 inner and outer hair cells that are arranged tonotopically throughout the cochlea to help distinguish between sounds of varying frequencies. The hair cells have projections known as stereocilia and kinocilia that are in contact with the tectorial membrane. Vibrations transmitted to the tectorial membrane cause displacement of stereocilia, leading to the displacement of the adjacent kinocilia. Movement of the kinocilia triggers depolarization of the hair cell, leading to an influx of calcium and the release of specific neurotransmitters that act at the cochlear ganglion. This activity produces an action potential that is propagated along the cochlear nerve and along auditory pathways, where it eventually reaches the cochlear nuclei located in the brainstem.[2]

The inner ear also contains the vestibular organs that are responsible for balance and position. The vestibular organs include the semicircular canals, utricle, and saccule. To understand the anatomy of the vestibular organs, it is helpful to separate the vestibular organs based on their specific functions. The semicircular canals, including their ampullas, are responsible for angular acceleration (rotational movement of the head), whereas the utricle and saccule are involved in linear acceleration.[1]

There are three semicircular canals anterior, posterior, and lateral. Each semicircular canal is located in a different plane (x,y, and z) and connects to the utricle via an ampulla, which is a widening of the canal. Within the ampulla, there are sensory epithelia known as cristae that contain projections of hair cells. Above the hair cells and cristae, there is a gelatinous cupula. As the head rotates in various directions, endolymph flowing through the semicircular canals displaces the gelatinous cupula that rests above the cristae leading to excitation of the hair cells embedded within the cristae. The hair cells become depolarized or hyperpolarized depending on the direction in which endolymph flows.[7]

The utricle and saccule each contain a macula, which is the fundamental end-organ (the equivalent of the crista within the ampulla described in the previous section) involved in detecting linear acceleration. The utricle is involved in longitudinal acceleration, whereas the saccule is involved in acceleration along the vertical axis. Each macule contains hair cells and supporting cells that are surrounded by a gelatinous layer, which is covered by an otolithic membrane. Resting atop the otolithic membrane are otoconia, which are heavy calcium carbonate crystals. Linear acceleration of the head causes a shear force between the otolithic membrane and macula, causing displacement of the hair bundles. Similarly to the hair bundles within the ampulla of the semicircular canals, displacement of the hair cells in the macula leads to the generation of a potential depending on the direction of movement. Movement towards the kinocilium causes the opening of channels and a subsequent depolarization of the cell. Movement away from the kinocilium causes the closure of channels leading to hyperpolarization of nerve fibers.

Embryology

During week 4 of embryologic development, the pre-placodal region of the ectoderm, which is at the anterior border of the neural plate, begins to thicken.[3] The ectoderm then forms into the otic (also referred to as auditory) placode, which is a derivative for the structures that eventually form the inner ear.[4] The ectoderm invaginates toward the mesoderm, forming the otic vesicles and neuroepithelial cells.[5] The utricle and saccule derive from the otic vesicles. By week 5, the cochlear duct forms from the otic vesicle and endolymph begin to accumulate within the membranous labyrinth. Next, a wall forms within the cochlea leading to the formation of two separate cavities, the cochlear duct and scala vestibuli.[4] The cochlear duct is further separated by the basilar membrane, forming the scala tympani. The cochlear duct begins to form hair cells that reside within the tectorial membrane.

Blood Supply and Lymphatics

The blood supply to the inner ear is via the labyrinthine artery (LA), also known as the internal auditory artery.[6] The LA usually arises from the anterior inferior cerebellar artery (83.6%), but can also arise from the basilar artery (12.3%).[7] It enters the internal acoustic meatus alongside the vestibulocochlear nerve and supplies both the facial and the vestibulocochlear nerves.[8] The LA then divides into three arteries while coursing through the internal acoustic canal:  (1) anterior vestibular artery (AVA), (2) vestibulocochlear artery (VCA), and (3) cochlear artery (CA).[7] The VCA separates into the cochlear and vestibular branches. The cochlear branch eventually forms an anastomosis with the CA, which makes up the main vascular supply to the cochlea. The vestibular branch and the AVA are responsible for vascular supply to the vestibular system.

Vestibular and cochlear aqueducts are responsible for venous drainage of the inner ear. The anterior and posterior spiral modiolar veins drain blood from the cochlea. The anterior and posterior vestibular veins drain blood from the vestibule, connects with the vein of the round window (RW), and eventually empties into the inferior cochlear vein (ICV). The ICV then drains into the inferior petrosal sinus.

Lymphatic fluid in the inner ear plays a critical role, circulating inside the cells and transferring metabolites from the CSF.[9] The membranous labyrinth is filled with and surrounded by lymphatic fluid. Perilymph, also derived from the lymphatic system, is present between the membranous and bony labyrinth. Lymphatic fluid drains through lymphatic chains from the middle ear to the cervical lymph nodes. Studies in guinea pigs showed that the inner ear drains to the parotid nodes and the superficial ventral cervical lymph nodes.[10]

Nerves

The vestibulocochlear nerve transmits an electrochemical signal from the cochlea, semicircular canals, and vestibule through the internal acoustic meatus and into the posterior cranial fossa.[11]

Impulses begin in the hair cells located within the spiral ganglion of the cochlea. Depolarization of hair cells propagates to the cochlear nerve.

Impulses begin in hair cells located within the ampulla of the semicircular canals and the utricle and saccule. There is a vestibular ganglion, known as Scarpa&rsquos ganglion, that exists within the internal acoustic meatus at the junction at which the vestibular and cochlear nerve meet. The bipolar cells that comprise Scarpa&rsquos ganglion have dendritic processes that retrieve electrochemical impulses directly from the hair cells. Specifically, the superior vestibular nerve innervates the utricle and superior and lateral semicircular canals. The inferior vestibular nerve innervates the saccule and inferior/posterior semicircular canal. The bipolar cells then transfer the electrochemical impulse via axonal fibers to the vestibular nerve.[11]

The vestibulocochlear nerve refers to the point at which the vestibular and cochlear nerve course together through the internal auditory meatus. After entering the posterior cranial fossa, CN VIII enters the brainstem between the pons and medulla and synapses on nuclei within the pons. The cochlear nerve synapses on the dorsal and ventral cochlear nuclei. The vestibular nerve synapses on the superior, inferior, medial, and lateral vestibular nuclei.[11]

Muscles

Two important muscles within the middle ear are responsible for modulating the auditory signal:

The stapedius muscle is only one millimeter in length, making it the smallest skeletal muscle in the entire body. The stapedius is attached to the stapes and helps modulate the transfer of sound waves from the external environment to the inner ear. In particular, it serves to decrease the vibration of the stapes, thereby dampening the sound energy that reaches the cochlea. The stapedius muscle receives innervation by a branch of the facial nerve (CN VII). Dysfunction of the stapedius muscle can lead to hyperacusis, a disorder characterized by impaired tolerance to certain noises due to an inability to dampen sounds entering the middle ear.[12]

The tensor tympani also plays a role in sound modulation by tensing the tympanic membrane to prevent loud sounds from damaging the inner ear. The tensor tympani originates in the cartilaginous portion of the Eustachian tube that connects the pharynx to the middle ear. The muscle inserts on the medial portion of the malleus. It gets innervated by the mandibular division of the trigeminal nerve (CN V). The tensor tympani is activated during talking, chewing, coughing, and laughing.[13]

Middle ear myoclonus (MEM), one of many causes of pulsatile tinnitus, is due to dysfunction of either the tensor tympani or stapedius muscle. It is often characterized as a clicking sound with the involvement of the tensor tympani and as a buzzing sound when due to the dysfunctional movement of the stapedius. It has also been described as a tapping, throbbing, fluttering, or whooshing sound. The tinnitus is usually objective and, therefore, can be heard by the examiner. MEM has been treated successfully in the past with surgical removal of the involved tendon. However, there is still some controversy over what the best approach to treatment is, warranting more prospective controlled trials.[14]

Surgical Considerations

Cochlear implantation is a surgery whereby an electrode is inserted into the cochlea to bypass the cochlear function and provide a direct electrical stimulus to the cochlear nerve, thereby allowing for patients with complete or partial hearing loss to regain the ability to hear. Cochlear implantation is reserved for patients with sensorineural hearing loss that will not improve with the use of hearing aids. The surgery involves placing 12 to 22 electrodes within the cochlea that connect to an internal receiver/stimulator that is implanted just below the skin posteriorly to the ear. Two weeks after successful implantation of the internal device, the patient is ready to have the external device, which the patient wears over the ear, activated. A microphone in the external device captures sound and encodes it into an electric impulse using a speech processor. Next, this electrical stimulation code travels to the external transmitter that is attached by a magnet through the skin to the internal receiver/stimulator that was recently surgically implanted. The information transmits via a radio-frequency link to the internal receiver and then travels to the cochlea, where it electrically stimulates the auditory nerve.[15]

Patients with severe vertigo unresponsive to medical therapy can elect to undergo a labyrinthectomy whereby the organs that make up the labyrinth are surgically removed. The most common indication to perform a labyrinthectomy is in Meniere&rsquos disease that has already compromised a patient&rsquos hearing.[16] The success rate of this surgery in treating vertigo is greater than 90.5%, although a significant downside is that it leads to permanent hearing loss.[17]

Clinical Significance

Benign Paroxysmal Positional Vertigo

Benign Paroxysmal Positional Vertigo (BPPV) is the commonest cause of vertigo, accounting for 17% of all cases.[18] Patients with BPPV present with episodic dizziness upon changes in head position that usually lasts for less than 30 seconds. The pathophysiology of BPPV involves the displacement of otoconia that rests atop the otolithic membrane of the macule located in the utricle. Otoconia become displaced from the utricle to the semicircular canals, which leads to the unwanted perception of angular rotation of the head. BPPV is diagnosable with the Dix-Hallpike maneuver, which involves quickly moving a patient from an upright position to a horizontal position, with the head hanging over the side of the table and turned at a 45-degree angle. Reproduction of symptoms and the presence of nystagmus indicates a positive exam.  Treatment for BPPV involves particle repositioning maneuvers, such as the Epley, Semont, and Lempert maneuvers.

Vestibular neuritis also referred to as vestibular neuronitis, is a self-limiting form of vertigo that occurs due to a viral infection or due to residual inflammation following viral infection of the vestibular nerve.[19] Symptoms of vertigo begin over the course of several hours and can last anywhere from days to weeks. Patients may also experience disequilibrium, sweating, nausea, vomiting, and imbalance. Recovery can take many weeks and may require balance physical therapy.

Meniere&rsquos disease is a vestibular disorder caused by endolymphatic hydrops, which is an accumulation of endolymphatic fluid that distorts the endolymph within the labyrinthine system. Meniere disease characteristically demonstrates episodic vertigo, hearing loss, tinnitus, and aural fullness. Episodes of vertigo last from minutes to hours.[20] Current treatment options for Meniere disease include acetazolamide, thiazide diuretics, intratympanic steroids, antibiotics, and surgery. Although the exact pathogenesis is unknown, it most likely has multiple causes that result in the Meniere&rsquos symptom constellation.

Presbycusis refers to age-related hearing loss due to the loss of hair cells. Presbycusis categorizes into either sensory and neural. Sensory refers to hair cell dysfunction that begins at the basal end of the cochlea, leading to loss of high-frequency sounds. Neural refers to cochlear nerve cell atrophy, leading to a defect in speech discrimination.[11]

Sudden Sensorineural Hearing Loss

Sudden sensorineural hearing loss (SSHL) is the sudden loss of hearing due to pathology within the cochlea or vestibulocochlear nerve that cannot be explained by some other disease related to the outer or middle ear. The criteria for diagnosing SSHL include the sudden hearing loss (within 72 hours) of 30 dB in at least three sequential frequencies.[21] SSHL improves spontaneously in 45-65% of patients. The current treatments for SSHL are oral or intratympanic steroids, with the possible addition of hyperbaric oxygen if diagnosed early.

Tinnitus is the unwanted perception of sound, often in the absence of external stimuli. The sound is described as ringing, buzzing, humming, or swooshing and can have a detrimental effect on a patient&rsquos quality of life. Tinnitus can be either objective or subjective. Objective tinnitus is audible by both the patient and clinician and is due to sound created by the flow of blood within vessels or of the contraction of a muscle in nearby structures. Causes include dural arteriovenous malformation, carotid bruits, venous bruits, carotid sinus fistulas, carotid dissections, and stapedial myoclonus.[22] Subjective tinnitus, which is much more common, can only be heard by the patient and often correlates with hearing loss. Presbycusis and otosclerosis seen in elderly patients can lead to the perception of subjective tinnitus. Another cause of subjective tinnitus is damage to the hair cells within the cochlea secondary to noise trauma or ototoxic medications.[23] Furthermore, tinnitus has been shown to have high comorbidity with psychiatric illnesses such as depression, anxiety, and insomnia. Treatment for tinnitus varies based upon its underlying cause. When there is an identifiable cause of tinnitus, treatment focuses on the specific underlying etiology. However, with idiopathic tinnitus, treatment is aimed at mitigating triggers known to be associated with tinnitus. For example, patients with comorbid tinnitus and depression have benefited from cognitive behavior therapy and antidepressant therapy, but evidence for its efficacy is limited. Other treatments include addressing stress with cognitive behavioral therapy, implementing healthy sleep habits, and teaching patients strategies for coping with their symptoms.[24] Patients with associated hearing loss can benefit from hearing aids or cochlear implants.[25] Sound therapy has shown some efficacy in masking the perception of tinnitus and reducing the disturbance caused by tinnitus. Sound therapy involves playing a wideband sound through a generator placed around a patient&rsquos ears at a frequency similar to the patient&rsquos perceived tinnitus. Although many cases of tinnitus have no definitive treatment available, patients can benefit significantly from implementing coping strategies to help mitigate symptoms.

Vestibular schwannoma (VS), previously referred to as acoustic neuroma, is a benign tumor of the Schwann cells that myelinate the vestibular branches of the vestibulocochlear nerve.[26] VS most often present with symptoms of hearing loss, unilateral tinnitus, and vertigo. Although the majority of VS cases are unilateral, a small percentage of patients (4%) present with bilateral VS, most often associated with the autosomal dominant genetic disorder neurofibromatosis 2 (NF-2). VS is diagnosed by imaging, with contrast-enhanced magnetic resonance imaging being the preferred study due to its high sensitivity. Treatment for VS involves a multifaceted decision process that takes into account the size of the tumor, the rate of tumor growth, the severity of symptoms, and the patient&rsquos age and comorbidities.[27] There are many risks involved in surgical removal of a VS due to the many important structures that are in close proximity to the vestibular nerve. The risks and benefits of surgical removal of the tumor must be weighed against other treatment options such as watchful waiting and stereotactic radiation. 


Vertebrates (Mammals, Fish, Birds, Reptiles)

Modify Conductivity

Living organisms are about two‑thirds water. Modifying the concentration of dissolved ions in water inside or outside cells, or embedding large non‑polar molecules with cellular lipid membranes, alters electrical conductivity (i.e., the movement of electrons). Given that chemical reactions depend on the movement of electrons within and between molecules, modifying electrical conductivity can modify the level of chemical activity. For example, Geobacter sulfurreducens bacteria produce electron‑conducing protein nanowires in order to successfully complete metabolically important oxidation/reduction reactions.

Vertebrates (Mammals, Fish, Birds, Reptiles)

Subphylum Vertebrata (“jointed”): Mammals, fish, birds, reptiles

Vertebrates are a subgroup of chordates, which all have a flexible, rod-shaped structure that supports the body called a notochord. Non-vertebrate chordates include tunicates, hagfish, and lancelets. In vertebrates the notochord ultimately becomes part of the spine, usually encased in bony joints. All chordates also have a dorsal hollow nerve cord that forms the nervous system and pharyngeal slits that open outside the body during development (and persist to form gills in aquatic animals). Lastly, chordates have a tail at the back of the body––it’s just that sometimes you need an x-ray to see it.

Ion channels in inner‑ear receptor cells switch electrical conductivity depending on lateral deflection of the sensors where they are located.

Used with permission from the Wellcome Trust, http://images.wellcome.ac.uk. License: UK CC‑NC‑ND 2.0. No cropping is allowed. Colour‑enhanced, close‑up image of stereocilia on the outer hair cells of the cochlea. When the stereocilia are deflected by sound, the fine tip links that connect them are stretched. This causes ion channels to open allowing potassium and calcium ions to flow into the cell. This in turn sets off nerve signals that carry the sound information to the brain. If the tip links fail to function properly, sound signals will not be transferred to the brain and deafness will result. Conversely, if the ion channels remain open, signals will be constantly flowing to the brain. This may help to explain tinnitus. Scanning electron microscope.

Scheme of a mammalian cochlear inner hair cell. Experimental results indicate that there are only one or two transduction channels close to the tip of each stereocilium. K+ influx through open transduction channels causes depolarization inside the hair cell, repolarization occurs then because of K+ outflux through the lateral cell body membrane. Reprinted with permission from Rattay, F, Gebeshuber, IC Gitter AH. The mammalian auditory hair cell: a simple electric circuit model. Journal of the Acoustical Society of America. 103(3): 1558‑1565, 1998.. Copyright [1998], Acoustical Society of America.

Used with permission from the Wellcome Trust, http://images.wellcome.ac.uk. License: UK CC‑NC‑ND 2.0. No cropping is allowed. A scanning electron microscope image of the sensory hair bundle of an inner hair cell from a guinea pigs hearing organ in the inner ear. Vibrations made by sound cause the hairs to be moved back and forth, alternately stimulating and inhibiting the cell. When the cell is stimulated it causes nerve impulses to form in the auditory nerve, sending messages to the brain.

“In vertebrates, hair cells are found in all peripheral structures used in hearing and balance. They play the key role in the mechano-electrical transduction mechanism. Inner hair cells (IHC) and outer hair cells (OHC) are found in the mammalian cochlea. Figure 1 [available in Gallery] schematically illustrates a typical inner hair cell. The apical part of the cell including the hairs (stereocilia) enters the endolymphatic fluid, which is characterized by its high electrical potential and its high K+-ion concentration. The stereocilia of one hair cell are connected through tip links and lateral links. The transmembrane voltage of 270 mV for OHC and 240 mV for IHC is mainly caused by the K+-ion concentration gradient between cell body and cortilymph. Current influx that changes the receptor potential occurs mainly through the transduction channels of the stereocilia: stereociliary displacement to the lateral side of the cochlea causes an increase of transduction channel open probability and hence depolarization of the receptor potential, whereas stereociliary displacement to the medial side results in a decrease of the transduction channel open probability and hence hyperpolarization.” (Rattay et al. 1998:1558)


ANATOMY OF THE MEMBRANOUS LABYRINTH

Early physician-anatomist Andreas Vesalius in his work entitled “De humani coporis fabrica” (Vesalius, 1543 ) and Bartolome Eustachi in his work entitled “Epistola de auditus organis” (Eustachi, 1564 ) provided early but incomplete descriptions of human inner ear anatomy and both of these physician-scientists' supported the theory postulated by Aristotle (Ross, 1906 ) and later by Galen (Galen, 1542 ) that the inner ear was filled with a type of purified air, that is, “aer ingenitus.” In 1740, Antonio Valsalva published his anatomical observations (Valsalva, 1740 ) on the human auditory system in which he pointed out the importance of the ossicular chain and the oval window for hearing and also observed that the innervation target for the auditory nerve was not the osseous spiral lamina as previously suggested by Professor Claude Perrault (Hawkins, 1988 ), but was instead the membranous portions of the cochlea and that these areas of sensory epithelium represented, in the opinion of Professor Valsalva, the true receptors of sound. It was the discovery of Professor Domenico Cotugno who dissected cochleae from fresh temporal bones and published in his report entitled “De aquaeductus auris humanae internae anatomica dissertatio” not only the anatomical structure of both the cochlear and vestibular aqueducts but also his important observation that the cochlea contained a liquid and not air as maintained by both Aristotle and Galen (Cotugno, 1775 ) thereby breaking with the centuries old concept of “aer ingenitus.” This liquid within the bony labyrinth that Professor Cotugno observed was termed “liquor Cotunni” to honor his discovery of this watery fluid and later became known as perilymph. Because Cotugno did not observe the inner membranous component of the cochlea, his observation of liquid within the inner ear only addressed perilymph located within the outer chambers of the cochlea, that is, scala tympani and scala vestibuli. It was Professor Cotugno's contention that there was a neural tissue partition suspended in the labyrinth's perilymph and that the acoustic nerves were like strings that oscillated within this perilymph and transmitted the sensation of hearing to the auditory centers of the brain. The observation of a liquid present within the cochlea's inner membranous compartment, that is, scala media, would have to wait for the sharp observational skills of one of his anatomical colleagues, that is, Professor Antonio Scarpa. The name of Antonio Scarpa is most closely associated with Scarpa's Ganglion which is the peripheral ganglion of the vestibular sensory epithelial receptors and so named to honor the anatomical contributions of Professor Scarpa to inner ear anatomy. The name of this famous 18th century physician-anatomist has also been closely associated with the early descriptive anatomy of the bony and membranous labyrinths with the first identification and detailed description of the human bony and membranous labyrinths published by Professor Scarpa in 1789. His work entitled “Anatomicae disquisitions de auditu et olfactu” (Scarpa, 1789 ) was published while he was Professor and Chair of Anatomy at the University of Pavia. In this publication, Scarpa described in detail the anatomical features of dissected human membranous labyrinths aided by his dissection of the inner ears of animals and birds. Antonio Scarpa's descriptive anatomical work on the vestibular portion of the human inner ear with three curvilinear canals located in the bony portion of the vestibular labyrinth encasing three membranous semicircular canals with associated ampullae was presented in his original 1789 publication. He noted the attachment of these semicircular ducts to the mucosal lining cells that invest the walls of the bony canals and the association of these semicircular canals with a utricle (termed by Scarpa as a common cavity in relationship to the semicircular ducts) and the presence of a saccule (referred to by Scarpa as a small spherical vestibular pouch). Innervation of the three ampullae and the maculae of both the utricle and the saccule were described as occurring via fibers emanating from the acoustic nerve. He described the vestibular (Scarpa's) ganglion as a small, plump, reddish chamber enclosed within the middle of the acoustic nerve. Scarpa based on his anatomical observations of the innervation of the inner ear by what he understood to be various branches of the auditory nerve mistakenly attributed the sense of hearing to all the sensory receptor structures that he had observed to form the membranous labyrinth which included all the vestibular sensory receptors. In Scarpa's anatomical studies of the cochlea, he describes in detail the osseous spiral lamina, the series of fine nerve fibers that emanate from the cochlear nerve, and the presence of the three scala, that is, media, vestibuli, and tympani, with a connection between the scalae tympani and vestibuli via a small apical turn area of communication termed the helicotrema. It was Antonio Scarpa who also noted the presence of a clear fluid within the semicircular ducts of the canals and also present within the cochlea's scala media which he called “Scarpa's fluid,” now known as endolymph. This represented a major advance that would at a later date aid in our understanding of cochlear function. Antonio Scarpa was a gifted anatomist and also a gifted artist with all his descriptive narrative of inner ear anatomy accompanied by his excellent drawings that depicted dissected specimens of cadaver temporal bones revealing the structure of both the bony and membranous labyrinths. His very important contributions to the early understanding of the anatomical structure of the human inner ear and a description of his professional life can be found in a more recent publication by Canalis et al. ( 2001 ). Another important contribution to inner ear anatomy by Antonio Scarpa occurred when he was still at the University of Moderna, Italy and was performed prior to his descriptive work on the membranous labyrinth (Scarpa, 1772 ). This work involved the anatomical aspects of the human round window membrane and addressed the structure–function of this membrane with a translation of this work found in a paper by Sellers and Anson ( 1962 ). Professor Scarpa suggests in his book on “Anatomical Observations on the Round Window” that it was Professor Fallopia at the University of Padua who first described both the oval and the round windows and was responsible for the naming of both of these inner ear structures (Fallopia, 1562 Sellers and Anson, 1962). Antonio Scarpa referred to the oval window as the secondary tympanum and in addition to a detailed description of its anatomy he suggested that this membrane covering the round window opening acted along with the oval window as a transmitter of sound energy into the cochlea hence his reference to this structure as the secondary tympanum. Antonio Scarpa provided a detailed anatomical description of both the round window membrane and its attachments as well as a similar detailed description of the niche in which it is located. According to Scarpa, Valsalva was a strong proponent of only the oval window in cooperation with the tympanic membrane and the ossicular chain for the transmission of sound energy into the cochlea (Valsalva, 1740 ) while Scarpa developed a strong argument for an additional contribution from the round window via the sound waves created within the tympanic cavity (Scarpa, 1772 ). It has now been shown that indeed the path of sound transmission proposed by Valsalva was correct with ossicular coupling via the oval window providing the major conversion of sound wave energy into fluid wave energy within the cochlea. Acoustic coupling that transmits sound energy to both the round and oval windows is now known to provide only a very small input from the sound energy within the middle ear cavity, therefore in a normal functioning middle and inner ear, the dominant transfer of sound energy occurs through the actions of the tympanic membrane/ossicular chain and oval window-stapedial footplate route (Rosowski and Merchant, 2005 ). It is important to note that all of Antonio Scarpa's descriptive anatomies of the bony and membranous labyrinths were accomplished without the aid of either advanced histological techniques or a compound microscope and that all the illustrations which accompanied his descriptive text were his own hand drawn illustrations.

With the development of advanced histological techniques and compound microscopes came advances in the understanding of the anatomical structures of the sensory receptor epithelium located within the membranous labyrinth. The Marquis Alphonse Corti while working in the laboratory of Professor Albert von Kölliker in Würzburg, Germany performed his anatomical-histological investigations of the organ of hearing (Corti, 1851 ) which later became known as the organ of Corti when Professor von Kölliker referred to the inner and outer pillar cells as the rods of Corti and the intervening tunnel as the tunnel of Corti (von Kölliker, 1852 ) with the entire structure of the organ of hearing eventually being referred to as Corti's organ (Fig. 1). Professor Corti was an Italian nobleman-physician and scientist working in the anatomical-histology laboratory of a German Professor, that is, von Kölliker, and who published his original report of the fine structure of the organ of hearing in French. This pivotal paper by Corti (who retired from scientific investigation the year after his reporting the description of the organ of Corti to assume his new role as Baron Corti following the death of his father) was the first histological description of the fine structure of cochlear receptor epithelium. His descriptive anatomy of the human organ of Corti was soon followed by papers on the descriptive anatomy of the cochlear receptor by Professors Deiters ( 1860 ), Claudius ( 1856 ), Hensen ( 1863 ), Boettscher ( 1869 ), and Nuel ( 1872 ). An earlier paper examining the avian ear structure described cells in the bird auditory receptor as auditory teeth which became known in the spiral limbus of the mammalian cochlea as the teeth of Huschke ( 1835 ). Each one of these early anatomist had some unique cell type or cell free space within the organ of Corti named after them and most of these cell types are seen and labeled in an excellent anatomical drawing of a radial section through Corti's organ from the second turn of a 6-day-old rabbit cochlea (Fig. 2) that was sketched by the artistically gifted Professor Hans Held ( 1909 ). In that same publication, Professor Held depicted the macula of the utricle in a drawing of this structure that roughly depicted the relationship between the vestibular sensory hair cells, the supporting cells and the otolithic membrane which he termed a cupula (depicted without otoliths) without reference to the presence of two different types of hair cells that are present within the vestibular sensory epithelium, that is, Types I and II vestibular hair cells (Fig. 3). An accurate description of the two different types of vestibular hair cells would have to wait for the development of the transmission electron microscope by Max Knoll and Ernst Ruska in 1931 and its application to electron microscopic ultrastructural analysis of cells by Porter ( 1945 ). In 1956, one of the most thorough ultrastructural analyses of the anatomical differences in the two different types of vestibular sensory hair cells and their pattern of afferent and efferent innervation was provided by the elegant and thorough ultrastructural study of the cristae ampulares from the vestibular labyrinths' of guinea pigs (Wersäll, 1956 ). This was the Docent thesis of Jan Wersäll as he studied in the famous Histology Department of the Karolinska Institute and this was accomplished in the same laboratory where Magnus Gustav Retzius performed his exceptional anatomical studies on the comparative anatomy of inner ear sensory receptors and reported on the anatomical structure of the human auditory receptor (Retzius, 1881 , 1882 , 1884 ). Docent Wersäll summarized his characterization of the two different type of vestibular hair cell that he observed in the guinea pig crista in a simple but elegant schematic drawing found in Fig. 9 of his thesis that was published as Supplement #126 in Acta Oto-Laryngologica (Wersäll, 1956 ). This schematic drawing depicts the major differences in both the shape differences of Type I and Type II vestibular hair cells as well as the differences in the characteristics of their afferent innervation (differences in the pattern of efferent innervation of Types I and II vestibular hair cells were not included) see Fig. 4. Later high resolution, low magnification electron micrographs (Harada, 1988 ) clearly show what Docent Wersäll had depicted in his informative schematic showing the different characteristics of innervation that characterize these two distinct types of vestibular hair cells (Fig. 5). The depiction of anatomical structure with drawings was well documented in the superb illustrations done by Professor Magnus Gustaf Retzius in his studies of the comparative anatomy of the inner ear of many different species of animals and birds, “Das Gehörorgan der Wirbelthiere” in two volumes (Retzius, 1981, 1984 ) while at the Histology Department of the Karolinska Institute. He is also known for his anatomy drawings depicting the structural features of the human membranous labyrinth, “Die Gestalt des membranosen Gehörorgans des Menschen” performed also at the Histology Department (Retzius, 1882 ). One of Retzius' most reproduced drawings is his elegant rendition of the human membranous labyrinth as it appeared after 6 months of gestational development (Fig. 6). Of particular note in this inner ear anatomy drawing, that is, Fig. 6, is Retzius' anatomical representation of the pattern of innervation for both the vestibular and auditory sensory receptors. A pioneer in the use of medical art drawings to depict complex anatomy was Max Brödel (Fig. 7) as he founded and was the first director the first academic Department of Medical Illustration, that is, Art as Applied to Medicine, Johns Hopkins University. During his academic career at Johns Hopkins Medical School working first in the Department of Anatomy at the invitation and with the encouragement of Professor Franklin Mall (Crosby and Cody, 1991 ), Professor Brödel used knowledge that he gained through many hours of careful anatomical dissections to artistically depict the complex anatomy of the human body including the relationship between the outer, middle and inner ears (Brödel, 1940 , 1946 ). Some of his medical drawings of ear anatomy are presented in Figs. 8–10. For otolaryngologist perhaps one of his best known medical illustrations is found in Figure 10 which was completed in 1941 and depicts the relationship between the cochlea, middle ear cavity with its ossicular chain and the tympanic membrane (an anatomic area of great importance to Neurotologist for the insertion of an electrode array during the process of cochlear implantation, see Bas et al., 2012 Eshraghi et al., 2012 Rask-Andersen et al., 2012). The last of the Brödel inner ear drawings was actually completed after the death of Professor Brödel by one of his former students, that is, P. D. Malone, 1945, based in large part on the preliminary sketches and anatomic studies performed at an earlier date by Max Brödel (Fig. 11A, B Brödel, 1946). These early medical illustrations of ear anatomy by Brödel have provided important insights into cochlea structure for both Otologists and Neurotologists.

A reproduction of three original drawings of the hearing receptor taken from Corti ( 1851 ), Z. Wiss. Biol. (explanation of Figs. 2–4 on pgs. 166–167).Translated from the original French article of Corti ( 1851 ), Z. Wiss. Zool. (Figs. 2–4 of the original Corti, 1851 paper correspond to the upper, middle, and lower panels seen in Fig. 1). Drawings depicting vertical slices (i.e., radial cross-sections) of the spiral lamina membrane enlarged approximately 450× magnification. (The epithelial layer that lines the vestibular surface of the spiral membrane lamina and the one that lines the tympanic surface have been removed Cats, dogs.). Fig. 2. Corti paper, 1851 upper panel—vertical slice of the spiral lamina membrane from its beginning closed to the vestibule. a.a. Periostetum that lines the bony spiral lamina (Blue color). b.b. Bony spiral lamina close to its free edge. c. Bundles of cochlear nerve contained between the bony laminae (b.b.) which form the free edge of the bony spiral lamina. d–w. Spiral membrane lamina (yellow color). d–w′. Indented zone. (Zona denticulata). d–d′–f. Habenula sulcata. d. Location where the periostetum of the vestibular surface of the bony spiral lamina changes its structure and thickens to form the habenula sulcata. e. Corpuscules that pad the fissures of the habenula sulcata. f–g. Teeth cells of the first row. g–f–h. Spiral fissure. (sulcus s. semicanalis spiralis). h. Inferior wall of the spiral fissure. k. Epithelial cells localized over the internal part of the habenula denticulata, and some of which block the spiral fissure at its opening. h–w′. Habenula denticulate. h–m. Apparent tooth. n–t. Teeth cells of the second row. n–p. Posterior branch of the teeth cells of the second row. o. Thickening of the posterior extremity of the posterior branch of the teeth cells of the second row. p–q. and q–r. Articular corner. r–t. Anterior part of the teeth cells of the second row. s.s.s. Cells of the cylindrical epithelium placed over the anterior branch of the teeth cells of the second row. l–v. Membrane working as a roof for the habenula denticulate. u. One of the epithelial cells localized between the pectinate zone and the membrane that works as a roof for the habenula denticulata. w′–w. Pectinate zone. (zona pectinata). x. Periostetum which lines the lamina spiralis ossea accesso-ria, and in which the spiral membrane lamina has its insertion (Blue color). y. Spiral path (internum). z. Its internal cover (referring to y.). Fig. 3. Corti paper, 1851 middle panel—vertical slice of the spiral lamina membrane, representing after it has completed from around 6 m enlargement since its origin in the vestibule. m′–m′. Apparent tooth cell. c′–c′. Expansion of the cochlear nerve spread over the tympanic surface of the habenula denticulata after having exited from the bony spiral lamina. Fig. 4. Corti paper,1851 lower panel—vertical slice of the spiral lamina membrane representing at around 0.5 μm just before its last ending in the top of the cochlea (the same letters indicate the same objects in Fig. 3—middle panel). z′. Internal spiral path with simple walls.

A modification of the drawing of a radial section through the organ of Corti (second cochlear turn) of a 6-day-old rabbit showing the different cell types that were named after a series of early anatomist who identified these cell types within Corti's organ. Original drawing by Hans Held (Held H, Untersuchungen über den feineren bau des ohrlabyrinthes der wirbeltiere II. Zur entwicklungsgeschichte des cortischen organs und der macula acustica bei säugetieren und vögeln, 1909, Leipzig, Bei BGTeubner, reproduced by permission).

A drawing of a radial section of the macula utriculus of a 6-day-old rabbit with labels to indicate the hair cells and the support cells. Original drawing by Hans Held (Held H, Untersuchungen über den feineren bau des ohrlabyrinthes der wirbeltiere II. Zur entwicklungsgeschichte des cortischen organs und der macula acustica bei säugetieren und vögeln, 1909, Leipzig, Bei BGTeubner, reproduced by permission).

A schematic drawing of Type I and Type II vestibular hair cells based upon the ultrastructural study of the adult guinea pig cristae ampulares from the Docent Thesis of Jan Wersall (Wersall J, Acta Oto-Laryngol, 1956, Suppl. 126, reproduced by permission).

A low-power electron micrograph from the cristae ampulares of an adult guinea pig showing both Type I (I) and Type II (II) vestibular hair cells and supporting cells (SC) as well as afferent button (NEa) and calyx (NC) nerve endings as well as efferent nerve endings (NEe) seen in Fig. 56 of Harada's book (Harada Y, The vestibular organs: S.E.M. atlas of the inner ear, 1988, Niigata, Nishimura Co. Ltd., reproduced by permission).

A drawing of the membranous labyrinth of a 6-month-old human fetus showing both the vestibular and the auditory sensory receptors with innervating nerves by Gustaf Retzius (Retzius G, Biol. Untersuch., 1882, 2, 1–32, reproduced by permission).

An image of Max Brödel in his later years at the Johns Hopkins University (Brödel M, Three unpublished drawings of the anatomy of the human ear, 1946, Philadelphia: Saunders, reproduced by permission).

A drawing that depicts the anatomical relationships between the outer, middle, and inner ears in humans drawn by Max Brödel in 1939 (Brödel M, Three unpublished drawings of the anatomy of the human ear, 1946, Philadelphia: Saunders, reproduced by permission).

A drawing of right membranous labyrinth of an adult human showing the major sensory receptors and their pattern of nerve fiber ingrowths from the vestibular and cochlear nerves drawn by Max Brödel in 1934 (Brödel M, The anatomy of the organ of hearing. 1940 year book of the eye, ear, nose and throat, 1940, Chicago: Year Book Publishers, reproduced by permission).

A drawing of the temporal bone showing the relationships between the external ear canal with tympanic membrane, middle ear with ossicular chain, the cochlea and the internal auditory canal with vestibular, cochlear and facial nerves drawn by Max Brödel near the end of his life in 1945 (Brödel M, Three unpublished drawings of the anatomy of the human ear, 1946, Philadelphia: Saunders, reproduced by permission).

A. Drawings of both the left and right inner ear showing the relationship between the vestibular membranous labyrinth and the cochlea which was based upon the preliminary sketches and anatomy studies of Max Brödel and completed after his death by his former student and colleague P.D. Malone in 1945 (Brödel M, Three unpublished drawings of the anatomy of the human ear, 1946, Philadelphia: Saunders, reproduced by permission). B. Labeled sketches of the drawings seen in Fig. 11A done by P.D. Malone in 1945 (Brödel M, Three unpublished drawings of the anatomy of the human ear, 1946, Philadelphia: Saunders, reproduced by permission).

Professor Sir Charles Oakley while at the Engineering Department of Cambridge University in the late 1940s along with his students is considered one of the perfectors of the Scanning Electron Microscope, that is, SEM. SEM depends upon critical point drying of biologic specimens then coating these processed specimen with a heavy metal in a partial vacuum (e.g., sputter coating with gold–palladium) and then bombarding the coated specimen with electrons in a partial vacuum so that secondary electrons emitted from the heavy metal coating of the specimen can be collected and provide a representation of the surface topography. One of the early investigators to take advantage of this technique of ultrastructural imaging for the documentation of inner ear surface anatomy was Professor David Lim (Lim, 1969 , 1986 Lim and Lane, 1969a , b Lim and Anniko, 1985 ). Most of David Lim's SEM ultrastructural observations of inner ear sensory receptor surface anatomy were performed in rodents with many of his studies focused on the morphology of the inner ear structures of adult guinea pigs (Lim, 2005 ). Professor David Lim was kind enough to provide me with some SEM images of both vestibular sensory receptor epithelia (Figs. 12–14) and the auditory sensory receptor epithelium (Figs. 15–17). In this special issue of the anatomical record Professor Helga Rask-Andersen and colleagues have provided ultrastructural images of the human auditory receptor and related their observations to the process of cochlear implantation (Rask-Andersen et al., 2012). It was the pioneering work of Professors Wersall, Lim, their students and other colleagues that encouraged the ultrastructural exploration of the auditory and vestibular receptors and the analyses of their structure–function using these ultrastructural techniques. Another pioneering investigator of ultrastructural studies was Professor Heinrich Spoendlin at the Ear Clinic, University of Zurich with his characterization of spiral ganglion neurons and the afferent and efferent innervation of the cochlear sensory receptor, that is, organ of Corti. Prior to the ultrastructural observations of auditory hair cell innervation it had been thought that outer hair cells (Figs. 15, 17) were the primary type of sensory hair cell responsible for audition. Professor Spoendlin demonstrated the two types of neurons present within the spiral ganglion, that is, Type I and Type II, and that the predominate neuronal type were the Type I neurons with only a small number of the Type II neurons present within this ganglion in several different species of laboratory animals (Fig. 18). Spoendlin's observations (Spoendlin, 1967 , 1969 , 1972 , 1979a , b , 1981 ) revealed that the Type I neurons had large, myelinated cell bodies and in adult animals only innervated the inner hair cells (Figs. 15, 16) while the Type II neurons possessed small, unmyelinated cell bodies and only innervated the outer hair cells (Figs. 15, 17) in adult animals. It was further noted as a result of these ultrastructural observations that each individual inner hair was innervated by many, that is, >10, Type I afferent neurons and that a single Type II afferent neuron would innervate en passant several, that is, >5, outer hair cells which Spoendlin summarized in his schematic drawing presented in Fig. 19. These observations of Spoendlin caused quite a reaction and ended up changing the perception of the auditory research community to now consider that the primary sensory receptor cell within the auditory receptor was the inner hair cell and not the outer hair cell. The Type I spiral ganglion neurons and their peripheral neuronal projections are the primary target of the electric pulses produced in a tonotopic pattern by the cochlear implant and its electrode array (Eshraghi et al., 2012 Rask-Andersen et al., 2012 Green et al., 2012 Budenz et al., 2012). Several years later the outer hair cells were found to posses evoked mechanical responses to intracellular currents (Brownell, et al., 1985 ) and then in several more years outer hair cell motility was found to depend upon a cytoskeletal meshwork attached to a motor protein (Kalinec et al., 1992 Zheng et al., 2000 ) that modulated their mechanical responses, that is, elongation or shortening of outer hair cell length) in auditory frequency range to electrical stimulation. These observations showed that although the outer hair cells were not the primary sensory cells of audition they were still very important in the modulation of hearing sensitivity and in providing discrimination (Liberman, 2005 ).

A low power scanning electron micrograph (SEM) of a guinea pig cristae ampulares showing this saddle-like structure with its hair cells, provided through the generosity of Professor David Lim. Bar = 100 μm.


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