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Potential between endolymph/cytoplasm and endolymph/perilymph

Potential between endolymph/cytoplasm and endolymph/perilymph


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I am studying for an exam and there is something I can't seem to understand. My textbook says that the endolymph contains 150mM potassium, 2mM Na+ and 130mM Cl-. The perilymph contains 5mM potassium, 140mM Na+ and 110mM Cl-. "Thus" the electric potential between perilymph and endolymph is +80mV (endolymph being positive).

I can see that there are chemical gradients but why is the endolymph so much more positively charged than the perilymph?


The basilar membrane is a pseudo-resonant structure [1] that, like the strings on an instrument, varies in width and stiffness. But unlike the parallel strings of a guitar, the basilar membrane is a single structure with different width, stiffness, mass, damping, and duct dimensions at different points along its length. The motion of the basilar membrane is generally described as a traveling wave. [2] The properties of the membrane at a given point along its length determine its characteristic frequency (CF), the frequency at which it is most sensitive to sound vibrations. The basilar membrane is widest (0.42–0.65 mm) and least stiff at the apex of the cochlea, and narrowest (0.08–0.16 mm) and stiffest at the base (near the round and oval windows). [3] High-frequency sounds localize near the base of the cochlea, while low-frequency sounds localize near the apex.

Endolymph/perilymph separation Edit

Along with the vestibular membrane, several tissues held by the basilar membrane segregate the fluids of the endolymph and perilymph, such as the inner and outer sulcus cells (shown in yellow) and the reticular lamina of the organ of Corti (shown in magenta). For the organ of Corti, the basilar membrane is permeable to perilymph. Here the border between endolymph and perilymph occurs at the reticular lamina, the endolymph side of the organ of Corti. [6]

A base for the sensory cells Edit

The basilar membrane is also the base for the hair cells. This function is present in all land vertebrates. Due to its location, the basilar membrane places the hair cells adjacent to both the endolymph and the perilymph, which is a precondition of hair cell function.

Frequency dispersion Edit

A third, evolutionarily younger, function of the basilar membrane is strongly developed in the cochlea of most mammalian species and weakly developed in some bird species: [7] the dispersion of incoming sound waves to separate frequencies spatially. In brief, the membrane is tapered and it is stiffer at one end than at the other. Furthermore, sound waves travelling to the "floppier" end of the basilar membrane have to travel through a longer fluid column than sound waves travelling to the nearer, stiffer end. Each part of the basilar membrane, together with the surrounding fluid, can therefore be thought of as a "mass-spring" system with different resonant properties: high stiffness and low mass, hence high resonant frequencies at the near (base) end, and low stiffness and high mass, hence low resonant frequencies, at the far (apex) end. [8] This causes sound input of a certain frequency to vibrate some locations of the membrane more than other locations. The distribution of frequencies to places is called the tonotopic organization of cochlea.

Sound-driven vibrations travel as waves along this membrane, along which, in humans, lie about 3,500 inner hair cells spaced in a single row. Each cell is attached to a tiny triangular frame. The 'hairs' are minute processes on the end of the cell, which are very sensitive to movement. When the vibration of the membrane rocks the triangular frames, the hairs on the cells are repeatedly displaced, and that produces streams of corresponding pulses in the nerve fibers, which are transmitted to the auditory pathway. [9] The outer hair cells feed back energy to amplify the traveling wave, by up to 65 dB at some locations. [10] [11] In the membrane of the outer hair cells there are motor proteins associated with the membrane. Those proteins are activated by sound-induced receptor potentials as the basilar membrane moves up and down. These motor proteins can amplify the movement, causing the basilar membrane to move a little bit more, amplifying the traveling wave. Consequently, the inner hair cells get more displacement of their cilia and move a little bit more and get more information than they would in a passive cochlea.

Generating receptor potential Edit

The movement of the basilar membrane causes hair cell stereocilia movement. The hair cells are attached to the basilar membrane, and with the moving of the basilar membrane, the tectorial membrane and the hair cells are also moving, with the stereocilia bending with the relative motion of the tectorial membrane. This can cause opening and closing of the mechanically gated potassium channels on the cilia of the hair cell. The cilia of the hair cell are in the endolymph. Unlike the normal cellular solution, low concentration of potassium and high of sodium, the endolymph is high concentration of potassium and low of sodium. And it is isolated, which means it does not have a resting potential of −70mV comparing with other normal cells, but rather maintains a potential about +80mV. However, the base of the hair cell is in the perilymph, with a 0 mV potential. This leads to the hair cell have a resting potential of -45 mV. As the basilar membrane moves upward, the cilia move in the direction causing opening of the mechanically gated potassium channel. The influx of potassium ions leads to depolarization. On the contrary, the cilia move the other way as the basilar membrane moves down, closing more mechanically gated potassium channels and leading to hyperpolarization. Depolarization will open the voltage gated calcium channel, releasing neurotransmitter (glutamate) at the nerve ending, acting on the spiral ganglion cell, the primary auditory neurons, making them more likely to spike. Hyperpolarization causes less calcium influx, thus less neurotransmitter release, and a reduced probability of spiral ganglion cell spiking.


Observations on the electrochemistry of the cochlear endolymph of the rat: a quantitative study of its electrical potential and ionic composition as determined by means of flame spectrophotometry

The relationship between the high positive potential and the high potassium and low sodium concentrations within the endolymph has been investigated in the adult rat. Very small, (2 nl.) uncontaminated samples of cochlear endolymph and perilymph have been collected and the endolymph potential measured at the times of collection. The sodium and potassium contents of the samples were estimated by means of total emission, integrative flame spectrophotometry. In the course of the procedure a number of serious problems were encountered, in particular that arising from the extremely small sodium content of the endolymph. For their solution a number of technical improvements were required, including the development of a new type of burner. A measure of the sensitivity thus attained is provided by the finding that, using test samples containing 4.8 x 10 -12 mequiv. (1.1 x 10 -13 g) of sodium, the standard deviation of the analytical results was ± 8*4 x 10 -13 mequiv. (±17.6%). With a solution comparable in composition to endolymph the standard deviation was ± 6% for sodium and ±1.3% for potassium. The analytical results showed the values of the sodium and potassium concentrations in the endolymph to be 0.91 and 154 mequiv./I. respectively. In perilymph these values were 138 and 6.9 mequiv./l. The average endolymphatic potential was +92 mV. During anoxia the positive endolymphatic potential was replaced by a negative potential reaching, on average, a maximum of - 42 mV after 4 1/2 min and thereafter slowly returning to zero. The principal ionic changes were a progressive increase in the endolymphatic sodium concentration from 3.6 mequiv./l. after 2 min anoxia to 32 mequiv./l. after 30 min anoxia and an associated decrease in the endolymphatic potassium concentration to 116 mequiv./l. after 30 min anoxia. These results establish that the low sodium content of the endolymph is maintained by means of an active transport mechanism which is probably situated in the stria vascularis. It thus appears that the characteristic composition of the endolymph is due to the active transfer of sodium and chloride from and potassium into the cochlear duct and that the mechanisms concerned are highly dependent on oxidative metabolism. The possible interrelationship of these mechanisms and the origin of the endolymphatic potential are briefly discussed but are considered to be still obscure.


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Materials and Methods

Information regarding the preparation of double-barreled K + -selective electrodes, vascular perfusion, and measurement of potential, aK + , and input resistance in the cochlear lateral wall is provided in SI Text .

Animal Preparation and Solutions.

The experimental protocol was approved by the Animal Research Committee of Osaka University Medical School. The experiments were carried out under the supervision of the Committee and in accordance with the Guidelines for Animal Experiments of Osaka University and the Japanese Animal Protection and Management Law. The guinea pigs were fed and allowed free access to water.


Discussion

For homeostasis in multicellular organisms, various compositionally distinct fluid compartments must be established through the barrier function of TJs in epithelial and endothelial cells. Recent identification of claudins, cell adhesion molecules responsible for the TJ barrier, opened a new way to perturb individual compartments and to evaluate the physiological relevance of each compartment at a whole-body level. To date, 24 members of the claudin family have been identified in humans and mice, and these are reported to be expressed in individual cell layers in various combinations and mixing ratios (for reviews, see Tsukita et al., 2001 Gonzalez-Mariscal et al., 2003). It has been shown that a particular compartment can be destroyed when the gene for major species of claudins constituting TJs of its delineating epithelial cell layer is homozygously knocked out in mice. For example, claudin-1 was expressed in large amounts in the epidermis and claudin-1-deficient mice showed severe dysfunction of the epidermal barrier, being dehydrated quickly after birth (Furuse et al., 2002). Endothelial cells of brain blood vessels primarily expressed claudin-5 and, in claudin-5-deficient mice, the blood-brain barrier was severely affected (Nitta et al., 2003). The Cld11 gene, which this study focused on, has already been knocked out by Gow et al. (Gow et al., 1999) and, in these mice, the compartments established by oligodendrocyte myelin sheaths and Sertoli cells were affected, resulting in neurological and reproductive deficits.

From the viewpoint of compartmentalization, the cochlea is most intriguing. In particular, endolymph is unique in its high K + concentration and positive electrical potential, the EP (for a review, see Ferrary and Sterkers, 1998). Accumulated evidence indicates that the stria vascularis is the actual site where K + is secreted into endolymph and the EP is generated (for reviews, see Wangemann et al., 1995 Wangemann, 2002). Interestingly, the stria vascularis itself constitutes a tube-like isolated fluid compartment delineated by two distinct, marginal and basal, cell layers. This tubular compartment runs spirally in tight association with the endolymph compartment, the scala media. The question thus naturally arose about stria vascularis of what, in terms of its K + secretion and the EP generation, is the physiological relevance of the establishment of such a peculiar compartment. One of the most conclusive ways to answer this was selectively to destroy the stria vascularis compartment and to examine the effects on hearing ability itself and the K + concentration and EP of the endolymph. We previously examined the expression patterns of claudins in the cochlea in detail, and found that claudin-11 was the major constituent of TJs of basal cell layers of stria vascularis (Kitajiri et al., 2004). In good agreement with this, TJs of these cells were reported to be characterized by parallel, densely packed strands (Janke, 1975a Janke, 1975b Gulley and Reese, 1976), which were also observed in claudin-11-based TJs of Sertoli cells (Gilula et al., 1976 Russell et al., 1985). Considering that the claudin-11 expression was restricted to the basal cell layers of stria vascularis, these findings led us to speculate that, when the Cld11 gene is knocked out, the stria vascularis compartment is selectively destroyed without affecting other cochlear compartments. We thought that this speculation could be evaluated experimentally, because claudin-11-deficient mice were reported to be born alive and grow without severe defects (Gow et al., 1999).

Then, we generated claudin-11-deficient (Cld11 -/- ) mice using a conventional homologous recombination method and examined the structure and functions of their cochlea, especially of stria vascularis, in detail. ABR measurements showed that Cld11 -/- mice suffered from deafness. As expected, in Cld11 -/- cochlea no obvious gross morphological malformations were observed and the tracer experiments clearly revealed that the barrier function of basal cell layers, but not marginal cell layers, of stria vascularis was severely affected. Very interestingly, when the K + concentration and EP were measured directly using K + -sensitive microelectrodes from the scala media, in Cld11 -/- cochlea, the K + concentration was maintained around a normal level (∼150 mM) but the latter was significantly suppressed down to ∼30 mV. These findings led to two conclusions. The first, that the barrier function of the marginal cell layer is sufficient for generation and maintenance of the high K + concentration of endolymph. This is consistent with previous electrophysiological data (Konishi et al., 1978 Wangemann et al., 1995). The second conclusion is that the basal cell barrier [i.e. the compartmentalization in stria vascularis (intrastrial space)] is indispensable for the generation and maintenance of the EP.

As regards the mechanisms for generation of EP, two distinct models have been proposed, a `single-cell model' and a `two-cell model' (Fig. 8) (for a review, see Wangemann, 1995). The single-cell model hypothesizes that Na + conductance of the basolateral membrane of marginal cells generates a large positive membrane voltage that is the source for the positive EP (Offner et al., 1987). In this model, the contribution of basal cell layers to the EP generation is not considered. In the two-cell model, K + conductance localized to the inner membrane of basal cells and to intermediate cells that are connected to basal cells through gap junctions, which were assumed to generate the source of EP (Salt et al., 1987 Kikuchi et al., 1995). In this model, the involvement of marginal cells in the generation of EP was limited to the maintenance of the low K + concentration in the intrastrial space. Recent detailed electrophysiological data appear to favor the two-cell model (Wangemann et al., 1995 Takeuchi et al., 1995 Takeuchi et al., 2000 Marcus et al., 2002), but it was technically difficult to evaluate conclusively the importance of the compartmentalization in the stria vascularis for the EP generation.

Two models for the mechanism behind generation of EP. The `single-cell model' hypothesizes that the Na + conductance of the basolateral membranes of marginal cells (blue lines) generates a large positive membrane voltage, which is the source for the positive EP (∼90 mV red zone). In this model, TJs in marginal cells (green) are thought to be essential for EP generation. In the `two-cell model', the K + conductance in the inner membranes (blue lines) of basal cells and of intermediate cells, which are connected to basal cells through gap junctions (GJ), are assumed to generate the source of EP (∼90 mV red zone). In this model, the involvement of marginal cells in the generation of EP was limited to the maintenance of the low K + concentration in the intrastrial space and TJs in basal cells (green) play a crucial role.

Two models for the mechanism behind generation of EP. The `single-cell model' hypothesizes that the Na + conductance of the basolateral membranes of marginal cells (blue lines) generates a large positive membrane voltage, which is the source for the positive EP (∼90 mV red zone). In this model, TJs in marginal cells (green) are thought to be essential for EP generation. In the `two-cell model', the K + conductance in the inner membranes (blue lines) of basal cells and of intermediate cells, which are connected to basal cells through gap junctions (GJ), are assumed to generate the source of EP (∼90 mV red zone). In this model, the involvement of marginal cells in the generation of EP was limited to the maintenance of the low K + concentration in the intrastrial space and TJs in basal cells (green) play a crucial role.

Set against this situation, the data obtained in this study clearly supported the two-cell model (Fig. 8). It is difficult to explain the downregulation of EP in Cld11 -/- cochlea by the one-cell model. Interestingly, the EP in Cld11 -/- cochlea was not suppressed completely down to 0 mV, but still showed ∼30 mV. It is likely that this voltage simply represents a residual electrical resistance between the intrastrial space and the spiral ligament, because that space is extremely small and tortuous in shape. Cld11 -/- mice would be useful in future experiments aiming to evaluate this speculation.

Claudin-11 was shown to constitute TJ strands between lamellae of myelin sheaths of oligodendrocytes in the brain, and between adjacent Sertoli cells in the testis (Morita et al., 1999b). In claudin-11-deficient mice, as established by Gow et al. (Gow et al., 1999), TJ strands were absent in myelin sheaths of oligodendrocytes and Sertoli cells, conclusively demonstrating that, in these types of cell, TJ strands are mainly composed of a single specific claudin, claudin-11. In this study, we established another line of Cld11 -/- mice and demonstrated that TJ strands in basal cells of stria vascularis in the cochlea were also singly composed of claudin-11. This situation is peculiar, because in most epithelial cellular sheets, TJ strands are composed of more than two distinct species of claudins as heteropolymers (for a review, see Tsukita et al., 2001). Interestingly, in addition to oligodendrocytes, Sertoli cells and cochlear basal cells, claudin-11 is reported to be expressed in renal epithelial cells of the thick ascending limb of Henle, in which, by contrast, claudin-11 appeared to form heteropolymers together with claudins 3, 10 and 16 (Kiuchi-Saishin et al., 2002). Through detailed analyses of claudin-11-deficient mice, we can now state that three important physiological processes, saltatory conduction along axons, spermatogenesis and hearing are fully dependent on the compartmentalization established by TJ strands consisting of a single species of claudin, claudin-11. A question then naturally arises: why is claudin-11 used singly for such important physiological processes, even though there are many other claudin species? It is still too soon to answer this question, but these findings indicate that the physiological relevance of the existence of many claudin species is not a simple safety measure based on functional redundancy. TJs are not a simple barrier: they show ion and size selectivity, and the tightness of their barrier function varies significantly depending on cell type. That cell-type-specific properties of TJ strands are determined by the combination and mixing ratios of claudins within individual TJ strands is widely accepted, so it is fascinating to speculate that TJs in oligodendrocytes, Sertoli cells and cochlear basal cells are highly specialized in terms of their barrier function, which could be why claudin-11 is used singly in these TJs. Cld11 -/- mice will provide a valuable resource in the future, not only for further study of the molecular mechanisms of hearing but also for gaining a better understanding of the physiological relevance of the existence of so many claudin species.


NCERT Exemplar Solutions for Class 11 Biology Chapter 21 Neural control and co-ordination

These Solutions are part of NCERT Exemplar Solutions for Class 11 Biology. Here we have given NCERT Exemplar Solutions for Class 11 Biology Chapter 21 Neural control and co-ordination.

VERY SHORT ANSWER QUESTIONS

Question 1.
Rearrange the following in the correct order of involvement in electrical impulse movement.
Solution:
The correct order of involvement in electrical impulse movement is as follows:
(i) Dendrites
(ii) Cell body
(iii) Axon
(iv) Axon terminal (vi) Synaptic knob

Question 2.
Which cells of the retina enable us to see coloured objects around us?
Solution:
Cone cells present in unable us to see the colours. There are three types of cones which possess their own characteristic photopigments that respond to red, green and blue light.

Question 3.
Arrange the following in the order of reception and transmission of sound wave from the ear drum. Cochlear nerve, external auditory canal, ear drum, stapes, incus, malleus, cochlea.
Solution:
The reception and transmission of sound waves occurs in following order – External Auditory canal —» Eardrum —» Malleus —> Incus —> Stapes —>• Cochlea —> Cochlear nerve

Question 4.
During resting potential, the axonal membrane is polarized, indicate the movement of H-ve and -ve ions leading to polarisation diagrammatically.
Solution:

Question 5.
Our reaction like aggressive behaviour, use of abusive words, restlessness etc. are regulated by brain, name the parts involved.
Solution:
Functions as aggressive behaviour, use or abusive words, restlessness, etc. The inner part of cerebral hemispheres and a group of associated deep structures called limbic lobe or limbic system along with hypothalamus are involved.

Question 6.
What do grey and white matter in the brain represent?
Solution:
A major component of CNS is Grey matter consisting of neutronal cell bodies, dendrite, unmyelinated axons, glial cells and capillaries. White matter is also a component of CNS and consists mostly of gilal cell and myelinated axons.

Question 7.
Where is the hunger centre located in human brain?
Solution:
Hypothalamus in human brain contains many centres which control urge for eating and drinking.

Question 8.
Complete the statement by choosing appropriate match among the following.

Solution:
A. -> (3), B. -> (4), C. -> (2), D. -> (1)

SHORT ANSWER QUESTIONS

Question 1.
The major parts of the human neural system is depicted below. Fill in the empty boxes with appropriate
words.

Solution:
The major parts of the human neural system is filled in the boxes with appropriate words

Question 2.
Neuron system and computers share certain common features. Comment in five lines.
Solution:
In various organs the sensory neurons is present to sense the environment and extend the message to the brain. So, it is equivalent to input device of computers.
Brain acts as the CPU, or Central Processing Unit. The information gathered by sensory neurons is processed by brain and it gives command to the concerned organ to act accordingly. This message is taken or conveyed by motor neurons which act as output devices.

Question 3.
What is the function described to Eustachian tube?
Solution:
The eustachian tube forms connection between the middle ear cavity with the pharynx. It helps in equalising the pressure on either sides of the ear drum. At the pharyngeal opening of the eustachian tube there is a valve which normally remains closed.
The valve opens during yawning, swallowing and during an abrupt change in altitude, when air enters or leaves the tympanic cavity to v equalise the pressure of air on the two sides of the tympanic membrane.

LONG ANSWER QUESTIONS

Question 1.
Explain the process of the transport and release of neurotransmitter with the help of a labelled diagram showing a complete neuron, axon terminal and synapse.
Solution:
The three main parts of a neuron include the
(i) Cell body
(ii) Axon
(iii) Dendrites
Stimulus or nerve impulse of any kind passes from one neuron to another via axon. This nerve impulse is wave of bioelectric/electrochemical disturbance that passes along the neuron during conduction of an excitation.

  • Within a synapse transport and release of a neuro transmiter occurs.
  • At a chemical synapse, the membranes of the pre- and post-synaptic neurons are separated by a fluid-filled space called synaptic cleft. Chemicals called neurotransmitters are involved in the transmission of impulses at these synapses.
  • The axon terminals contain vesicles filled with these neurotransmitters.
  • Upon arrival of an impulse (action potential) at the axon terminal, it stimulates the movement of the synaptic vesciles towards the membrane, where they fuse with the plasma membrane and release their neurotransmitters in the synaptic cleft.
  • The released neurotransmitters bind to their specific receptors, present on the post-synaptic membrane. This binding opens ion channels allowing the entry of ions, that can generate a new action potential in the

Question 2.
Explain the structure of middle and internal ear with the help of diagram.
Solution:
Ears are a part of statoacoustic organ meant for balancing and hearing the external ear in most mammals is a heap of tissue also called pinna. It is a part of auditory system.

The human ear consists of three main parts external ear, middle, ear and internal ear.

Structure of Middle Ear

  • The middle ear consists of three bones or ossicles-the malleus (hammer), incus (anvil and stapes (stir-up).
  • These bones are attached to one another in a chain-like manner.
  • The malleus is attached to the tympanic membrane and the stapes is attached to the oval window (a membrane beneath the stapes) of cochlea.
  • These three ossicles increase the efficiency of transmission of sound waves to the inner ear.
  • The middle ear also opens into the eustachian tube, which connects with the pharynx and maintains the pressure between the middle ear and the outside atmosphere.

Structure of Internal Ear

  • Thd inner ear consists of a labyrinth of chambers filled with fluid within temporal bone of the skull. The labyrinth consists of two parts the bony and membranous labyrinth. The bony labyrinth is a series of channels.
  • Membranous labyrinth lies inside these channels which is surrounded by a fluid called perilymph. The membranous labyrinth is filled with a fluid called endolymph. The coiled portion of the labyrinth is called cochlea.
  • The cochlea has two large canal separated by a small cochlear duct (scala media). An upper vestibular canal (scala vestibuli) and a lower tympanic canal (scala tympani). The vestibular and tympanic canals contain perilymph and the cochlear duct is filled with endolymph.
  • The wall of membranous labyrinth comes in contact with the fenestra ovalis at the base of scale vestibuli while the fenestra rotunda.

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Essay on Human Ear: Structure and Function

In this article we will discuss about the structure and function of human ear with its suitable diagram.

Ear has two important functional components:

1. Cochlea the hearing part containing receptor for hearing is located here.

2. The vestibular part having semicircular canals, the utricle, and the saccule are present here.

The receptor in these is responsible for the maintenance of equilibrium and posture.

Function of ear in general for hearing and also act as a direction detector:

a. Important protective role.

b. It modulates once own voice.

Ear has three parts the outer, the middle and the inner ear. The outer ear has the pinna, in lower animals this can move which helps in detecting the direction of sound waves. Sound waves which are captured by the pinna pass through the external auditory canal and vibrate the tympanic membrane.

The auditory tube is directed medially, downwards and forwards. The skin around the tube has lots of ceruminous glands which on exposure forms the ear wax. The direction of the external auditory tube as well as the ear wax protects the tympanic membrane from injuries.

The tympanic membrane is a fibrous structure. Its main function is to act as a resonator. Sound waves make the membrane vibrate. Tympanic membrane has a surface area of 68 sq mm. When the frequency of the sound wave is less than 2000 cps the entire membrane vibrates. If the frequency is more than 2000 cps, the membrane vibrates in segments approxi­mately 75% of the membrane vibrates.

Figure 10.23 (graphical representation) showing the relationship between the frequency of sound waves and the intensity of the sound. It shows that the sound frequencies between 2000 and 4000 cps are heard with the lowest intensities.

Middle Ear:

Contents of the Middle Ear:

The middle ear contains three bony ossicles namely the malleus, the incus and the stapes. These ossicles articulate with one another. The long process of the malleus articulates with the short process of the incus and forms a lever system. The handle of malleus is attached to the tympanic membrane and the foot plate of the stapes is attached to the oval window.

Through this mechanism, the vibrations of the tympanic membrane are conducted to the inner ear. The middle ear is also connected to the pharynx through the pharyngotympanic tube (Eustachian tube/auditory tube).

There are two small muscles in the middle ear. They are the tensor tympani and the stapedius. The tensor tympani when contracts make the tympanic membrane tense. The contraction of the stapedius pulls the foot plate of the stapes outwards. Both of these actions decrease the conduction of sound waves into the inner ear.

Functions of the middle ear:

2. Static pressure equilibration

3. Protective function—acoustic reflex (attenuation reflex)

4. Acts as a physiological filter.

5. Because of the impedance matching, it forms the preferential route of conduction.

As the sound waves are passing through the air medium, through the ear ossicles into the fluid medium of the internal ear, because it has to vibrate the fluid, a certain amount of sound energy is lost. This will give rise to a decrease in the sound intensity and the significance of the sound may be lost. The mechanism involved in minimizing the loss of sound energy is known as impedance matching.

The mechanisms involved are:

a. When the frequency of the sound wave is more than 2000 cps, only 75% of the tympanic membrane is thrown into vibration which is about 58 mm 2 . The foot plate of the stapes is about 3.2 mm 2 . The pressure applied over a larger surface area of the tympanic membrane is getting converged on to a much smaller area in the oval window. This magnifies the pressure acting on the oval window by about 14 to 17 times.

b. The handle of the malleus is longer than the short process of the incus and they articulate with each other forming a lever system. Because of this lever mechanism, there is an additional magnification by about 1.3 times. Therefore, the total magnification increased is by about 17 to 21 folds. Thus the loss of sound energy is minimized. If this mechanism fails, the person will have a hearing deficit of approximately 10 to 20 dB.

2. Static pressure equilibration:

For the proper functioning of the tympanic membrane as a vibrator, the pressure on either side of the membrane must be kept equal. Atmospheric pressure is the one which acts on the tympanic membrane from outside. Since the middle ear is connected to the pharynx, the pressure in the middle ear is also made equal to the atmospheric pressure.

Normally, the pharyngotympanic tube is kept closed. Whenever the pressure in the middle ear falls, the tube opens up connecting the middle ear to the pharynx and the pressure is equalized.

If the fall in the pressure in the middle ear is too much as it can happen when an unconscious person is brought to the sea level, there is a possibility that the tympanic membrane may rupture. This results in a loud noise being followed by signs and symptoms of shock.

3. Protective function:

Explosive noises may damage the very fine structures of the inner ear. Within a matter of 15 to 17 milliseconds (the latent period), the two small muscles in the middle ear contract. The tympanic membrane is pulled inwards and the foot plate of the stapes is drawn outwards. This results in decreased amount of sound waves reaching the inner ear.

This protects the finer structures present in the cochlea. This reflex is known as the tympanic reflex. This reflex can be initiated even by the ticking sounds of a time piece. In paralysis of the facial nerve, the stapedius muscle is paralyzed. Hence, the protective mechanism is lost and these patients complain of painful hearing—hyperacusis.

4. It acts as a physiological filter:

It allows the trans­mission of speech frequency and prevents the transmission of noise frequency. The axis of rotation of the foot plate of the stapes gets changed and it prevents the transmission of noises.

5. Preferential route of conduction:

There are two routes through which the sound waves can be conducted to the inner ear. One of the routes will be through the bone conduction and the other being the ossicular conduction (air conduction). Since, impedance matching is available only for ossicular conduction, this route of conduction forms the preferential route of conduction.

The Inner Ear:

This part lodges two important structures, namely the cochlea and the vestibular apparatus. The cochlea is the hearing part of the inner ear (Fig. 10.24).

The cochlea is a coiled structure about two and a half circle. The cochlea is divided into three compartments by two membranes namely the basilar membrane and the Reissner’s membrane.

The upper compartment is scala vestibuli, the middle is scala media and the lower scala tympani. The scala vestibuli and scala tympani contain perilymph, the composition of this fluid resembles that of ECF and the scala media contains endolymph, the composition of which resembles that of ICF.

The receptors for hearing are the organ of Corti (hair cells) present on the basilar membrane. There are two types of hair cells namely, the outer row of hair cells, arranged in three rows and a single row of inner hair cells. The outer row of hair cells is test tube­like, whereas the inner row of cells is flask-like (Fig. 10. 25).

Signals produced by these receptors are carried by the cochlear division of the eighth cranial nerve. These receptors also receive efferent nerve supply. These fibers take origin from the olivary nucleus (olivocochlear bundle of nerve fibers). Overlying the hair cells is the tectorial membrane. The hairs on the hair cells are actually embedded in the substance of the tectorial membrane.

Cochlea:

The cochlea is the hearing part of the inner ear. The cochlea is a coiled structure about two and a half circle. The cochlea is divided into three compartments by two membranes, namely the basilar membrane and the Reissner’s membrane.

The upper compartment is scala vestibuli, the middle scala media and the lower scala tympani (Fig. 10.26). The scala vestibuli and scala tympani contain perilymph, the composition of this fluid resembles that of extracellular fluid and the scala media contains endolymph, the composition of which resembles that of intracellular fluid.

Resistance offered by Reissner’s membrane is extremely small as it is a thin delicate membrane. Reissner’s membrane stretches from the upper surface of the spiral lamina to the bony wall of the canal a little above the attachment of the basilar membrane.

Basilar Membrane:

Basilar membrane is attached to the spinal lamina to the outer wall of the canal. There is no tension in the fibers maintaining the basilar membrane.

1. If a cut is made in the basilar membrane, no gaping is seen in the membrane showing the fibres are not taut or kept under tension.

2. Basal part of basilar membrane is narrow and width is gradually increased upwards to the apex. Basilar membrane is about 32 mm long.

3. Rods of Corti form the supporting pillars. The height of these rods are increased from base to apex, and the rods of Corti are present on the basement membrane.

There are certain differences between the base and apical part of cochlea (Fig. 10.27).

They are with respect to:

c. Response to frequencies

The receptors for hearing are the organ of Corti (hair cells) present on the basilar membrane. There are two types of hair cells namely, the outer row of hair cells, arranged in three rows and a single row of inner hair cells. The outer row of hair cells is test tube­like, whereas the inner row of cells is flask-like.

Signals produced by these receptors are carried by the cochlear division of the 8th cranial nerve (Fig. 10.28). These receptors also receive efferent nerve supply. These fibers take origin from the olivary nucleus (olivocochlear bundle of nerve fibers).

Overlying the hair cells is the tectorial membrane. The hairs on the hair cells are embedded in the substance of the tectorial membrane. The hairs of the hair cells are bathed in endolymph present in scala media.

When the sound vibrations are transmitted through the foot plate of the stapes to the inner ear, the fluid medium is set into motion (Fig. 10.29). This in turn moves the basilar membrane, which later on moves the tectorial membrane. The shearing motion of the tectorial membrane bends the hairs of the receptor cells.

Mechanism of Stimulation of Receptors in Cochlea:

1. Movement of oval window.

2. Disturbance of fluid in scala vestibuli.

3. Movement of Reissner’s membrane.

4. Disturbance of fluid in scala media.

5. Movement of tectorial membrane.

6. Shear motion on the hair of hair cells due to movement of tectorial membrane

7. Stimulation of receptor cells (Fig. 10.29).

This brings about the production of receptor potentials known as cochlear microphonic potentials. The amplitude of the microphonic potentials depends on the intensity of the impinging sound waves. Greater the intensity, greater is the amplitude of the microphonic potentials.

The cochlear microphonic potentials are nothing but the local potentials and hence have almost all the properties of local potential. These cochlear microphonic potentials in turn bring about the development of action potentials in the auditory nerve fibers.

1. The disturbance of fluid in the scala media also brings about movement of basilar membrane.

2. Leads to disturbance of fluid present in scala tympani

3. Movement of round window

There should be movement of the round window in an appropriate direction when the oval window moves. This is essential because, in the cochlea the fluid is present and this fluid is incompressible. If fluid is unable to get disturbed, there will not be scope for the stimulation of receptors since the receptors for hearing are nothing but mechanoceptors.

Theories of Hearing:

The basilar membrane is about 31 mm long and its width increases gradually from the base to the apex. Depending on the frequency of the sound waves, different parts of the membrane is displaced to varying extent. For low frequency, the apical portion of the membrane gets displaced to a greater extent stimulating those receptors.

For higher frequency sounds, the basal part of the membrane gets displaced stimulating those receptors. Whenever there is disturbance in the fluid medium of cochlea, a wave of disturbance originates from the base of cochlea irrespective of the pitch of the sound.

This wave as it traverses from the base towards the apex, the amplitude of wave goes on increasing till it comes across a point on the basilar membrane which is tuned to respond maximally for that particular frequency (Fig. 10.30).

Beyond the area of maximal disturbance, the wave dies out. Hence the receptors present at the site of maximal disturbance get stimulated. This fact is proved by recording microphonic potentials from different parts of the basilar membrane and also directly observing the movement of the membrane.

Frequency analysis of the sound waves is, therefore, partly made at this level itself. Further analysis is made by the auditory cortex when these impulses reach the cortex.

Auditory Pathway (Fig. 10.31):

The cochlear afferent nerve fibers from the receptors reach the spiral ganglion. From the ganglia, the fibers reach the anterior and posterior cochlear nuclei present in the brainstem and synapse. From the posterior and anterior cochlear nuclei, nerve fibers take origin and synapse in the superior olivary nucleus and posterior nucleus of trapezoid body of same side as well as on the opposite side.

From these structures, nerve fibers taking origin reach the medial geniculate body through any of the following pathways:

a. Some of the fibers directly reach the medial geniculate body and synapse.

b. Some fibers synapse in the inferior colliculus and from there reaches the medial geniculate body. The crossing of the fibers to the opposite side can occur even at inferior colliculus.

c. Some other fibers synapse in the nucleus of lateral leminscus. From here, the fibers reach the inferior colliculus and synapse and finally reach medial geniculate body.

The whole bundle of nerve fibers taking origin from the superior olivary nucleus and posterior nucleus of trapezoid body is known as lateral lemniscus. The lateral lemniscus gives out collaterals that feed information to the reticular formation present in the brainstem.

From the medial geniculate body, fibers taking origin are called as auditory radiation fibers. Auditory radiation fibers pass through the posterior limb of internal capsule to reach the auditory cortex present in the superior temporal gyrus.

Auditory Cortex:

In the auditory cortex (superior transverse temporal gyrus), there are two important areas:

i. Primary auditory area (area no. 41, 42)

ii. Association auditory area (area no. 21, 22)

The primary auditory area is connected to medial geniculate body. The association area is connected to the primary auditory area. Fibers from primary auditory area convey information to the association area. The association area also receives fibers directly from the thalamus. The individual tone and frequency is represented in the auditory cortex that has tonotopic representation.

Intensity of sound discrimination:

It is similar to intensity discrimination in general sensory physiology.

Intensity of sound discrimination can be explained by:

1. Recruitment of receptors

Direction Analysis:

The laterality of the sound can be discriminated by:

1. Time lag in the stimulation of receptors present in two different ears. In the ear which is directed towards the source of sound, there will be stimulation of receptors few milliseconds earlier than the stimulation of receptors present in the opposite ear.

2. Decrease in the amplitude of the sound in the opposite ear as the sound waves while reaching the opposite ear will strike against the hard bones of the cranium and would lose some amount of sound energy because of this.

Types of Deafness:

1. Conductive type—due to:

a. Accumulation of wax in the auditory meatus.

b. Damage to tympanic membrane.

2. Perceptive type—due to:

a. Site of lesion mainly the receptors, e.g. prolonged listening of rock music.

b. May be due to tumor arising from the auditory nerve fibers compressing the other fibers.

c. Toxicity of certain drugs (anti-malarial drugs), quinine and streptomycin (anti-TB drugs).

3. Central type—very rare.

Tests Employed to Detect Hearing Impairment:

The recording is called audiogram.

Ear phones are placed over the subject’s ear and one ear is tested at a time. Subject is connected to instrument. Gradually, there will be increased frequency of sound. The intensity of the sound applied corresponds to the standard intensity this is reported as normal or represented as 0 db.

If the findings of the study are graphically represented and is around zero line, the subject is supposed to be normal.

Conductive and perceptive types of deafness can be differentiated by the audiometry.

Gross difference between bone conduction and ossicular conduction:

If ossicular conduction is affected to a greater extent, it means that it is a conductive type of deafness and in such person bone conduction is better than ossicular conduction. In perceptive deafness, both bone and ossicular conduction are affected to the same extent.

Audiometry enables to ascertain the:

1. Type of deafness—conductive or perceptive

Tests Conducted to Ascertain the Type of Deafness:

Place the vibrating tuning fork on the mastoid process and ask the subject if he can hear. For accurate result, do not allow the subject to move. Subject is asked to tell when he is unable to hear. When he is unable to hear, transfer the tuning fork from mastoid process to the front of the ear and if subject is able to hear it means that ossicular conduction is better than bone conduction.

Strike a tuning fork and place the vibrating tuning fork on the forehead of the patient. Subject must be able to hear equally in both the ears.

If he hears better in the right ear, it may be due to:

a. Conductive type of deafness in right ear

b. Perceptive type of deafness in left ear

In conductive type of deafness, when Weber’s test performed, the subject is able to hear better on the affected side. In perceptive type of deafness, subject is able to hear better on the normal side.

Presbyacusis is the hearing loss that is due to old age. In aged people, the ability to hear higher frequencies decline.

Chemical Senses:

Taste Receptors and Olfactory Receptors:

Activity in these receptors concerned with visceral function, i.e. concerned with food intake thus they are classified under visceral receptors. They can be also termed as chemoreceptors as they respond to chemical changes.

Differences between Taste and Smell Sensations:

1. The pathway involved in olfaction does not pass through the thalamus. All the other sensory pathways pass through the thalamus.

2. The olfaction sensation has no neocortical projection—it is a very primitive type of sensation. These two sensations play a vital role in food intake.

In lower animals, the olfactory receptors also play other important roles in:


ABSTRACT

To report the cochlear morphology and electrophysiology of Chinese experimental miniature pigs. Twenty Chinese experimental miniature pigs were used in this study. Auditory brainstem responses (ABR), cochlear endolymphatic potentials (EP), and the potassium concentrations of cochlear endolymph were recorded. Hair cell morphology was examined using electron microscopy. The capsule of cochlea of the miniature pig has three and one-half turns which contains a 39-mm long membranous labyrinth. The organ of Corti in the labyrinth encompasses three rows of outer hair cells and one row of inner hair cells. The stereocilia of the hair cells in the apical turn of the cochlea were significantly longer than those in the basal turn. The vestibular apparatus consists of three semicircular canals and the otolith organs. The average threshold of the ABR was 35–45 dB SPL (n = 20) from 4 to 32 kHz. There was no significant difference in the threshold or latency of the ABR between 1-day-old and 30-day-old miniature pigs. The average EP value was 77.3 ± 14 mV (n = 9) and the average potassium concentration was 147.1 ± 13 mM (n = 5) recorded from the second turn of the cochlea. These studies on the cochlear morphology and electrophysiology of the miniature pigs help to establish the Chinese experimental miniature pig as an animal model for future studies in otology and audiology. Anat Rec, 298:494–500, 2015. © 2014 Wiley Periodicals, Inc.

Animal models are essential for basic and clinical investigations of effective ways to prevent and treat human diseases. Rodents, including rats, mice, guinea pigs, and chinchillas, are the most commonly used laboratory animals. Because the cochlear organs of rodents are much smaller than those of humans, it is difficult to use them for cochlear and middle ear implantation studies. An animal model that has a cochlea larger than that of rodents is desirable for otologic and audiologic studies.

Pigs have been used for many biomedical experiments and studies of artificial organs (Ferraz et al., 2008 , Petersen et al., 2009 ). The anatomy of the external ear and middle ear of pigs has been studied and the temporal bones of pigs have also been used in otologic surgical education (Gurr et al., 2010 ). Previous studies have found that the temporal bone of pigs has a different appearance compared to humans in the length and location of the external ear canal. However, the middle ear of pigs is very similar to that of humans (Pracy et al., 1998 , Gurr et al., 2010 ). Lovell and Harper also found that the morphology of the cochlear hair cells of domestic pigs is similar to that of humans (Lovell and Harper, 2007 ). Heffner reported that the behavioral hearing range of Sus scrofa (wild boars) is from 42 Hz to 40.5 kHz, with the best hearing sensitivity at 250 Hz to 16 kHz, similar to humans (Heffner and Heffner, 1990 ). Hansen et al. studied the effects of hyperbilirubinemia on the amplitude of auditory evoked potentials using newborn piglets (2- to 9-days old). They recorded clear auditory brainstem responses (ABRs) with well segregated waves I–V in newborn piglets, suggesting that pigs are a precocial species, as are humans (Hansen et al., 1992 ). Studies by Strain et al. also found mature ABRs in juvenile Vietnamese pot-bellied pigs (Strain et al., 2006 ). Their results suggest that pigs may be a good animal model for otologic and audiologic studies, such as cochlear implant. However, since normal pigs can weigh more than 100 kg, the use of pigs requires more equipment and space for laboratory use and housing. In addition, the costs of purchasing, delivery and housing of a normal-size pig are also much more expensive than small size animals. This affects the wide-ranging use of pigs in many laboratories and animal facilities.

Miniature pigs, which normally have one-fifth of a normal pig's weight and size, have also been used for many biomedical experiments (Van Dorp et al., 1998 , Polejaeva et al., 2000 , Screaton et al., 2003 , Hoffstetter et al., 2011 ). The micro-dissection of the temporal bone of the miniature pig has been verified and the morphology of its ear is similar to humans (Van Dorp et al., 1998 , Polejaeva et al., 2000 , Screaton et al., 2003 , Hoffstetter et al., 2011 ). Because the cost of using miniature pigs is much lower than the cost of using normal-sized pigs, miniature pigs are potentially suitable animal models used for middle ear and inner ear surgical experiments. Chinese experimental miniature pigs were recently derived from small-sized swine species from Guizhou Province in China and have been used in gene therapy, organ transplantation and stem cell studies (Cao et al., 2012 , Cao et al., 2014 , Yuan et al., 2014 ). These miniature pigs are known as Xiao-Xiang Zhu (good smelling mini-pig) by the local farmers due to the delicious flavor of the meat. An adult miniature pig weighs between 20 and 30 kg and their body size is about 50-cm long and 20-cm tall. They have rapid breeding cycles and show early sexual maturity at about 4-months old (Yu et al., 2003 ). A sow can produce about six to eight piglets. To establish the miniature pig as an animal model for future studies in otology and audiology, we have characterized in this article the cochlear function and morphology of Chinese experimental miniature pigs.


The Vestibular System

Figure 22-1. A cross section of the outer, middle, and inner ear.

Figure 22-2. The membranous labyrinth and associated vessels and nerves. The approximate configuration of the receptor sites in the ampulla, utricle, and saccule are shown in green. The detail shows the relationship between bony and membranous labyrinths.

Between the membranous labyrinth and bony labyrinth is a space containing fluid called perilymph , which is similar to cerebrospinal fluid. Perilymph has a high sodium content (150 mM) and a low potassium content (7 mM), and it bathes the vestibular portion of the eighth cranial nerve.

The membranous labyrinth is filled with a different type of fluid, called endolymph , which covers the specialized sensory receptors of both the vestibular and the auditory systems. Endolymph has a high concentration of potassium (150 mM) and a low concentration of sodium (16 mM). It is important to note the differences in these two fluids because both are involved in the normal functioning of the vestibular system. Disturbances in the distribution or ionic content of endolymph often lead to vestibular disease.

Vestibular Receptor Organs

The five vestibular receptor organs in the inner ear complement each other in function. The semicircular canals (horizontal, anterior, and posterior) transduce rotational head movements (angular accelerations). The otolith organs (utricle and saccule) respond to translational head movements (linear accelerations) or to the orientation of the head relative to gravity. Each semicircular canal and otolith organ is spatially aligned to be most sensitive to movements in specific planes in three-dimensional space.

In humans, the horizontal semicircular canal and the utricle both lie in a plane that is slightly tilted anterodorsally relative to the nasooccipital plane (Fig. 22-3). When a person walks or runs, the head is normally declined (pitched downward) by approximately 30 degrees, so that the line of sight is directed a few meters in front of the feet. This orientation causes the plane of the horizontal canal and utricle to be parallel with the earth and perpendicular to gravity. The anterior and posterior semicircular canals and the saccule are arranged vertically in the head, orthogonal to the horizontal semicircular canal and utricle (Fig. 22-3). The two vertical canals in each ear are positioned orthogonal to each other, whereas the plane of the anterior canal on one side of the head is coplanar with the plane of the contralateral posterior canal (Fig. 22-3).

Figure 22-3. Orientation of the vestibular receptors. In the lateral view ( A ), the horizontal semicircular canal and the utricle lie in a plane that is tilted relative to the nasooccipital plane. In the axial view ( B ), the vertical semicircular canals lie at right angles to each other.

The receptor cells in each vestibular organ are innervated by primary afferent fibers that join with those from the cochlea to comprise the vestibulocochlear ( eighth ) cranial nerve . The cell bodies of these bipolar vestibular afferent neurons are in the vestibular ganglion ( Scarpa ganglion ), which lies in the internal acoustic meatus (Fig. 22-4). The central processes of these bipolar cells enter the brainstem and terminate in the ipsilateral vestibular nuclei and cerebellum.

Figure 22-4. Computed tomography scans of the human temporal bone. The horizontal ( A , arrowhead ) and anterior and posterior ( B , arrowheads ) semicircular canals, utricle ( A , small arrow ), and internal acoustic canal ( A , large arrow ) are visible.

The blood supply to the labyrinth is primarily via the labyrinthine artery , usually a branch of the anterior inferior cerebellar artery. This vessel enters the temporal bone through the internal auditory meatus. Although it is not as important as the labyrinthine artery, the stylomastoid artery also provides branches to the labyrinth, mainly to the semicircular canals. An interruption of blood supply to the labyrinth will compromise vestibular (and cochlear) function, resulting in labyrinth-associated symptoms, such as vertigo or oscillopsia, and clinical signs, such as nystagmus or unstable gait.

Membranous Labyrinth

The membranous labyrinth is supported inside the bony labyrinth by connective tissue. The three ducts of the semicircular canals connect to the utricle, and each duct ends with a single prominent enlargement, the ampulla (Fig. 22-2). Sensory receptors for the semicircular canals reside in a neuroepithelium at the base of each ampulla. The receptors in the utricle are oriented longitudinally along its base, and in the saccule they are oriented vertically along the medial wall (Fig. 22-2). Endolymph in the labyrinth is drained into the endolymphatic sinus via small ducts. In turn, this sinus communicates through the endolymphatic duct with the endolymphatic sac , which is located adjacent to the dura mater (Fig. 22-2). The saccule is also connected to the cochlea by the ductus reuniens .

Meniere Disease

The balance between the ionic contents of endolymph and perilymph is maintained by specialized secretory cells in the membranous labyrinth and the endolymphatic sac. In cases of advanced Meniere disease, there is disruption of normal endolymph volume, resulting in endolymphatic hydrops (an abnormal distention of the membranous labyrinth). Symptoms of Meniere disease include severe vertigo (a sense of spinning in space), positional nystagmus, and nausea. Affected persons often have unpredictable attacks of auditory and vestibular symptoms, including vomiting, tinnitus (ringing in the ears), and a complete inability to make head movements or even to stand passively. For patients with frequent debilitating attacks, the first course of treatment is often administration of a diuretic (e.g., hydrochlorothiazide) and a salt-restricted diet to reduce the hydrops. If persistent symptoms of Meniere disease continue, second treatment options include either the implantation of a small shunt into the abnormally swollen endolymphatic sac or the delivery of a vestibulotoxic agent such as gentamicin into the perilymph.

Semicircular Canal Dehiscence

On occasion, a condition may develop in which a portion of the temporal bone overlying either the anterior or the posterior semicircular canal thins so much that an opening (dehiscence) is created next to the dura (Fig. 22-5). In affected patients, the canal dehiscence exposes the normally closed bony labyrinth to the extradural space. Symptoms can include vertigo and oscillopsia (a sense that objects are moving to and fro, oscillating, in the visual fields) in response to loud sounds (the Tullio phenomenon ) or in response to maneuvers that change middle ear or intracranial pressure. The eye movements evoked by these stimuli (nystagmus) align with the plane of the dehiscent superior canal. Surgical closure of the defect by bone replacement is often performed.

Figure 22-5. Computed tomography scan of the temporal bone projected into the plane of the left superior canal in a patient with superior canal dehiscence syndrome. The patient had vertigo, oscillopsia, and eye movements in the plane of the left superior canal in response to loud noises and pressure in the left ear. A dehiscence is noted overlying the left superior canal ( arrowhead ).

VESTIBULAR SENSORY RECEPTORS

Hair Cell Morphology

The sensory receptor cells in the vestibular system, like those in the auditory system, are called hair cells because of the stereocilia that project from the apical surface of the cell (Fig. 22-6 A ). Each hair cell contains 60 to 100 hexagonally arranged stereocilia and a single longer kinocilium . The stereocilia are oriented in rows of ascending height, with the tallest lying next to the lone kinocilium. The stereocilia arise from a region of dense actin, the cuticular plate , located at the apical end of the hair cell. The cuticular plate acts as an elastic spring to return the stereocilia to the normal upright position after bending. Each stereocilium is connected to its neighbor by small filaments.

Figure 22-6. The receptor cells ( A , type I and type II hair cells) of the vestibular system. The relation of these cells to the crista and cupula ( B ) in the ampullae and to the macula and otolith membrane ( C ) of the otolith organs is shown.

There are two types of hair cells, and they differ in their pattern of innervation by fibers of the eighth cranial nerve (Fig. 22-6 A ). Type I hair cells are chalice shaped and typically are surrounded by an afferent terminal that forms a nerve calyx . Type II hair cells are cylindric and are innervated by simple synaptic boutons. Excitatory amino acids such as aspartate and glutamate are the neurotransmitters at the receptor cell–afferent fiber synapses. Both types of hair cells, or their afferents, receive synapses from vestibular efferent fibers that control the sensitivity of the receptor. These efferent fibers contain acetylcholine and calcitonin gene–related peptide as neurotransmitters. Efferent cell bodies are located in the brainstem just rostral to the vestibular nuclei and lateral to the abducens nucleus. They are activated by behaviorally arousing stimuli or by trigeminal stimulation.

Within each ampulla, the hair cells and their supporting cells lie embedded in a saddle-shaped neuroepithelial ridge, the crista , which extends across the base of the ampulla (Fig. 22-6 B ). Type I hair cells are concentrated in central regions of the crista, and type II hair cells are more numerous in peripheral areas. Arising from the crista and completely enveloping the stereocilia of the hair cells is a gelatinous structure, the cupula . The cupula attaches to the roof and walls of the ampulla, forming a fluid-tight partition that has the same specific density as that of endolymph. Rotational head movements produce angular accelerations that cause the endolymph in the membranous ducts to be displaced so that the cupula is pushed to one side or the other like the skin of a drum. These cupular movements displace the stereocilia (and kinocilium) of the hair cells in the same direction.

For the otolith organs, a structure analogous to the crista, the macula , contains the receptor hair cells (Fig. 22-6 C ). The hair cell stereocilia of otolith organs extend into a gelatinous coating called the otolith membrane , which is covered by calcium carbonate crystals called otoconia (from the Greek, meaning “ear stones”). Otoconia are about three times as dense as the surrounding endolymph, and they are not displaced by normal endolymph movements. Instead, changes in head position relative to gravity or linear accelerations (forward-backward, upward-downward) produce displacements of the otoconia, resulting in bending of the underlying hair cell stereocilia.

Hair Cell Transduction

The response of hair cells to deflection of their stereocilia is highly polarized (Figs. 22-7 and 22-8 A ). Movements of the stereocilia toward the kinocilium cause the hair cell membranes to depolarize , which results in an increased rate of firing in the vestibular afferent fibers. If the stereocilia are deflected away from the kinocilium , however, the hair cell is hyperpolarized and the afferent firing rate decreases.

Figure 22-7. Physiologic responses of vestibular hair cells and their vestibular afferent fibers. Asp , aspartate Glu , glutamate.

Figure 22-8. Morphologic polarization of vestibular receptor cells showing polarity of stereocilia and kinocilia ( A ) and the orientation of receptors in the ampullae ( B ) and maculae ( C ).

The mechanisms underlying the depolarization and hyperpolarization of vestibular hair cells depend, respectively, on the potassium-rich character of endolymph and the potassium-poor character of the perilymph that bathes the basal and lateral portions of the hair cells. Deflection of the stereocilia toward the kinocilium causes potassium channels in the apical portions of the stereocilia to open. Potassium flows into the cell from the endolymph, depolarizing the cell membrane (Fig. 22-7). This depolarization in turn causes voltage-gated calcium channels at the base of the hair cells to open, allowing calcium to enter the cell. The influx of calcium causes synaptic vesicles to release their transmitter (aspartate or glutamate) into the synaptic clefts, and the afferent fibers respond by undergoing depolarization and increasing their rate of firing. When the stimulus subsides, the stereocilia and kinocilium return to their resting position, allowing most calcium channels to close and voltage-gated potassium channels at the base of the cell to open. Potassium efflux returns the hair cell membrane to its resting potential (Fig. 22-7).

Deflection of the stereocilia away from the kinocilium causes potassium channels in the basolateral portions of the hair cell to open, allowing potassium to flow out from the cell into the interstitial space. The resulting hyperpolarization of the cell membrane decreases the rate at which the neurotransmitter is released by the hair cells and consequently decreases the firing rate of afferent fibers.

Almost all vestibular primary afferent fibers have a moderate spontaneous firing rate at rest (approximately 90 spikes per second). Therefore it is likely that some hair cell calcium channels are open at all times, causing a slow, constant release of neurotransmitter. The ototoxic effects of some aminoglycoside antibiotics (e.g., streptomycin, gentamicin) may be due to direct reduction of the transduction currents of hair cells.

Morphologic Polarization of Hair Cells

Given that deflections of the stereocilia toward and away from the kinocilium cause opposing physiologic responses, it is clear that the directional orientation of the hair cells in the vestibular organs will play an essential role in signaling the direction of movement. On the cristae of the horizontal semicircular canal, the hair cells are all arranged with their kinocilium on the side closer to the utricle (Fig. 22-8 B ). Thus movement of endolymph toward the ampulla in the horizontal canal causes the stereocilia to be deflected toward the kinocilium, resulting in depolarization of the hair cell. In the vertical semicircular canals, the hair cells are arranged with their kinocilium on the side farther from the utricle (closest to the endolymphatic duct). Thus the hair cells of the vertical canals are hyperpolarized by movement of endolymph toward the ampulla (ampullipetal movement) and are depolarized by movement away from the ampulla (ampullifugal movement).

In both the utricle and the saccule, the otolith membrane overlying the hair cells contains a small, curving depression, the striola , that roughly bisects the underlying macula (Fig. 22-8 C ). Hair cells on the utricular macula are polarized so that the kinocilium is always on the side toward the striola (Figs. 22-6 C and 22-8 C ), which effectively splits the receptors into two morphologically opposed groups. In contrast, the kinocilia of saccular hair cells are oriented on the side away from the striola. Because the striola curves through the macula, otolith hair cells are polarized in many different directions (Fig. 22-8 C ). In this way, utricular and saccular hair cells are directionally sensitive to a wide variety of head positions and linear movements.

SEMICIRCULAR CANALS AND OTOLITH ORGANS

As stated previously, the vestibular receptors transduce movement and position stimuli into neural signals that are sent to the brain. The semicircular canals are responsive to rotational acceleration resulting from turns of the head or body. The otolith organs are responsive to linear accelerations . The most prominent linear acceleration on earth is the constant force of gravity. Linear motion, such as experienced during swinging on a swing or flying in an airplane through turbulence, couples with gravity to change the direction and amplitude of the resultant gravitoinertial acceleration


Watch the video: Inner Ear Fluids Perilymph u0026 Endolymph (November 2022).