Coclia Anatomy and physiology of hearing

Coclia Anatomy and physiology of hearing

7. Give us an in-depth review on the function of the inner and outer hair cells.

From Cummings

Hair Cells

The inner and outer hair cells function as receptor cells that transduce mechanical
movement into an electrochemical signal to stimulate the auditory nerve. Figure 128-3
presents schematic examples of inner and outer hair cells. The apical portion of all hair
cells includes a thickened region called the cuticular plate, which in conjunction with the
supporting cells forms the reticular lamina. Rooted in the cuticular plate of each hair cell
and projecting through the reticular lamina are bundles of actin filaments called
stereocilia, stiff hairlike structures that deflect with mechanical disturbances. Adjacent to
this is a noncuticular region that contains a rudimentary kinocilium.

Figure 128-3. Schematic depictions of inner (left) and outer (right) hair cells. Inner hair
cells are flask-shaped, receive extensive afferent innervation, and receive indirect efferent
innervation. Outer hair cells are cylindrical and receive direct afferent and efferent
innervation.
There are approximately 3500 inner hair cells arranged in one row on the modiolar side
of the tunnel of Corti and 12,000 outer hair cells in three rows on the strial side. Figure
128-4 is a scanning electron micrograph of the reticular lamina of the organ of Corti
showing the stereocilia arrangement for the three rows of outer hair cells and single row
of inner hair cells. The stereocilia on outer hair cells form a “V” or “W” shape with the
bottom facing away from the modiolus. The inner hair cell stereocilia form a shallow “U”
that opens toward the modiolus. On each hair cell, there are rows of stereocilia, two or
more on inner hair cells, and three or more on outer hair cells. For both types of hair
cells, the stereocilia lengths are graduated from longest on the strial side to shortest on the
modiolar side. The longest stereocilia on the outer hair cells contact the tectorial
membrane, which results in deflection of the stereocilia with basilar membrane
movement. The stereocilia are connected to each other by filamentous links laterally by
cross-links and from the tips of shorter stereocilia to the sides of the taller ones by tip-
links. These ensure that the connected stereocilia move as a unit when the longer
stereocilia are deflected.

Figure 128-4. Scanning electron micrograph showing reticular lamina of organ of Corti.
Note inner border cells (IBC), inner hair cells (IHC), inner pillar head plate (IPC), three
rows of outer hair cells (OHC), three rows of Deiters’ cell phalangeal processes (D),

outer pillar cell phalangeal process (OP), and Hensen's cells (HC). Two phalangeal scars
(circled) are present in row 3, which indicates loss of two outer hair cells. Bar = 10 ?m.

The structural aspects of the inner and outer hair cell bodies themselves differ
significantly and are reflective of their functional differences. The inner hair cells are
flask-shaped, wide at the bottom and narrower at the top, and contain high concentrations
of organelles that are involved in metabolic activity, particularly Golgi bodies and
mitochondria. Although highly metabolic, inner hair cells are considered to be passive
transducers in the auditory system. Outer hair cells are cylindrical in shape and contain
microfilaments and microtubules along the length of the cell that give rise to motile
activity.[7] The motile properties have been shown empirically to result in highly tuned,
frequency-specific contractile activity even when stimulated in isolation from the basilar
membrane.[8] All hair cells have synaptic bars at afferent synapses, which serve as sites
for presynaptic vesicle docking and release for subsequent stimulation of auditory nerve
fibers.

Innervation

Afferent innervation of the cochlear hair cells consists of approximately 30,000 auditory
nerve fibers in humans, and is responsible for providing ascending information from the
cochlea to the central auditory system. The cell bodies of afferent fibers make up the
spiral ganglion that resides in Rosenthal's canal within the modiolus. Nerve fibers reach
the hair cells by traveling through the modiolus, and into the osseous spiral lamina, where
they pass through holes in the lamina referred to as the habenulae perforatae. The nerve
fibers are classified as type I and type II. Type I fibers are bipolar, of large diameter, and
myelinated, and they constitute nearly 95% of the total number of fibers. Each type I
fiber has a direct and independent synapse on the body of a single inner hair cell, and
each inner hair cell is innervated by approximately 20 such fibers.[9,10] Type II fibers
constitute the remaining 5%, and are smaller and may be myelinated or unmyelinated.
Type II fibers synapse directly on the outer hair cells, with a single fiber diverging to
form branches that synapse with multiple other outer hair cells.

The efferent auditory pathway originates from the olivocochlear bundle and provides
central inhibitory modulation of hair cell activity via descending information. The
olivocochlear bundle has about 1600 fibers, and these constitute the uncrossed
olivocochlear bundle and crossed olivocochlear bundle.[11-15] These pathways originate
from the medial superior olive (MSO) and lateral superior olive (LSO) regions on both
sides. For the uncrossed olivocochlear bundle, the LSO projects many small-diameter,
unmyelinated efferent fibers toward the ipsilateral inner hair cells, where they synapse
with the afferent fibers. The MSO projects fewer myelinated fibers ipsilaterally that
synapse directly on the outer hair cells. For the crossed olivocochlear bundle, the MSO
projects large-diameter myelinated fibers to the contralateral outer hair cells, and the LSO
projects a few unmyelinated fibers to the contralateral inner hair cell afferents. The fibers
from the crossed olivocochlear bundle cross midline at the level of the fourth ventricle.
To illustrate the pathways, Figure 128-5 shows a schematic example of the uncrossed

olivocochlear bundle and crossed olivocochlear bundle innervation for the left organ of
Corti.

Figure 128-5. Illustration of the efferent uncrossed and crossed olivocochlear bundle
pathways to the hair cells in the left organ of Corti. Yellow entities represent the efferent
fibers and synapses. The arrow widths represent the relative amount of innervation.
Uncrossed pathway: 1, small-diameter fibers from left lateral superior olive (LSO; green)
synapsing on left inner hair cell afferents; 2, myelinated fibers from left medial superior
olive (MSO; pink) to left outer hair cells. Crossed pathway: 3, large-diameter myelinated
fibers from right MSO to left outer hair cells; 4, unmyelinated fibers from right LSO to
left inner hair cell afferents. DMPO, dorsomedial periolivary nucleus; MNTB, medial
nucleus of the trapezoid body; VNTB, ventral nucleus of the trapezoid body.
Cochlear Nerve

The cochlear nerve, a trunk of the cochleovestibular or eighth cranial nerve, contains
afferent fibers transmitting auditory information from inner and outer hair cells to the
brainstem. The cell bodies of these afferent neurons are located within the spiral ganglion
of the cochlea. Spiral ganglion neurons are bipolar, with one process extending toward
inner and outer hair cells and the other projecting to the brainstem. Approximately 90%
to 95% of the traversing axons are large myelinated fibers, and the remaining 5% to 10%
are thinner, unmyelinated axons.[16,17] The larger neurons are type I ganglion cells, which
project from the inner hair cells. The smaller neurons are type II ganglion cells, which
contact the outer hair cells. Both these cell types project to the cochlear nucleus, and their
fibers form the cochlear nerve within the internal auditory canal and cerebellopontine
angle.

8. Review the basilar membrane and traveling wave.

The Traveling Wave

The hydromechanical consequences of introducing a pressure pulse, or wave, into
perilymphatic fluids is a traveling wave that propagates along the organ of Corti from the
base to the apex (see Fig. 146-2). The traveling wave propagates basoapically, because
the stiffness of the cochlear partition decreases longitudinally from the base to the apex
(see Fig. 146-3F) as its mass increases. The motion and direction of the wave that travels
along the basilar membrane are independent of the location of the vibratory source. von
B?k?sy[586] observed this feature of cochlear mechanics by introducing a pressure pulse
through an artificial window carved into the bony labyrinth near the cochlear apex.
Traveling waves created under these conditions are essentially indistinguishable from
traveling waves triggered by ossicular vibration. The basoapical propagation of the
traveling wave is an outcome of the end organ's physical makeup, with the stiffer and less
massive elements forming the base of the organ of Corti being set into motion by an
instantaneous pressure difference between the major scalae before the more compliant
and massive sections of the apical cochlea. The resultant traveling wave reflects the more
or less independent, yet orderly, sequential movement of loosely coupled basilar
membrane segments that form a frequency decomposition system generally thought of as
the space-frequency or tonotopic map (see Fig. 146-3G).

Figure 146-2. A, Schematic illustration of the inner ear with the cochlea uncoiled
revealing the basilar membrane (BM) and a traveling wave at one instant in time in
response to a pure tone. The eardrum (tympanic membrane) and middle ear ossicles also
are shown in relation to the semicircular canals. The oval window opens into scala
vestibuli (SV) and the round window to scala tympani (ST). B, Drawing of the inner ear
with a segment of a cochlear turn extracted to expose the scalae. C, Diagram of a section
through the cochlea showing the location of inner and outer hair cells (IHC and OHCs,
respectively) in relation to the BM and TM and the direction of wave propagation. SM,
scala media. D, The ratio of pressures measured within the SV and ST to the pressure at
the eardrum produced in response to tone bursts are plotted as a function of stimulus
frequency; responses were recorded from a cat. The difference between SV and ST
pressures is responsible for displacement of the basilar membrane. E, BM displacements
produced in a cadaveric human cochlea in response to 200 Hz at four separate points in
time. The envelope of the traveling wave also is indicated. F, Envelopes of traveling
waves measured along the BM in a cadaveric human cochlea in response to four stimulus
frequencies, showing the relationship between the location of peak displacement and
frequency (higher frequencies produce peaks at locations progressively closer to the
base).

Figure 146-3. A, Photograph of the cochlea of a guinea pig after the outer bony shell has
been thinned. B, Scanning electron micrograph (SEM) of a chinchilla cochlea after the
bone on one side has been removed. The bony modiolus (M) is shown in the center, and
the fluid-filled scala tympani (ST) and scala vestibuli (SV) are indicated in the basal turn.
The helicotrema (H) is shown at the apex, and the arrows indicate the osseous spiral
lamina to which the cochlear partition is attached at its medial extent. The round window
(RW) and the stapes (S) are indicated, and the footplate of the stapes has been pulled
away slightly, exposing the oval window (OW). C, Schematic illustration of the variation
in thickness of the basilar membrane (dark blue area) and the associated mesothelial
layer (light blue area) and the width of the basilar membrane (horizontal axis) at six
locations along the cochlear spiral of the cat; specified locations are relative to the base.
The demarcation between the arcuate and pectinate zones is indicated by the left arrow at
each location; the arcuate zone (pars tecta) extends from the lip of the osseous spiral
lamina to the foot of the outer pillar cell (OPC) and the pectinate zone from the OPC to
the basilar crest of the spiral ligament (right arrows). D, Scaled representation of the
ribbon-like nature of the human basilar membrane indicating its width relative to its
length. E, Quantitative relationship between the width of the basilar membrane (BM) and
the distance from the base for the human, cat, guinea pig, and gerbil. F, Estimates of the
stiffness of the basilar membrane in the guinea pig and gerbil as a function of location. G,
Maps of the characteristic frequency (CF) of a cochlear location (i.e., peak of the
envelope of the traveling wave) as a function of position along the basilar membrane for
several species. Note that the lengths of the basilar membrane for each species can be
estimated by the distance that each curve occupies along the x-axis. Frequency-position
maps were based on a formulation developed by Greenwood using empirical fits to data
for the species shown here, except for rat, which was developed by Muller.

As the traveling wave propagates toward the apex, traveling initially at a high rate of
speed, the velocity of the wave decreases, with production of progressively shorter
wavelengths for any given input frequency as a consequence (see Fig. 146-2E). As the
amplitude of the propagating wave steadily increases as it passes toward the apex,
partition displacement reaches a maximum value at its characteristic place—where
intrinsic resonance meets, and nearly matches, the vibratory frequency of the triggering
acoustic event (see Fig. 146-2F). As pointed out previously, the stiffness of the basilar
membrane decreases exponentially with distance from the stapes,[205,394,395,586] so it is not
surprising that the resonant frequency also decreases exponentially as a function of
distance from the stapes, because the characteristic place is determined by the stiffness of
the cochlear partition. Thus, the aforementioned space-frequency, or tonotopic, map is
created, whereby the peak in vibration amplitude is located at the base of the cochlea for
high frequencies and at progressively apical regions for lower frequencies.

Just as stiffness dominates the physics of the traveling wave as it travels toward its
characteristic place, basilar membrane motion at and just above the characteristic place is
dominated by the mass of the system. It is important to point out that the actual peak of
the traveling wave occurs at a location just basal to the true resonant place, characterized
by a match between intrinsic resonance and the triggering vibratory energy. At the place
of peak displacement, the traveling wave slows to an essential standstill (i.e., it stalls)
before remaining energy is completely dissipated, and the partition is completely and
rapidly restored to its resting position slightly apical to the resonant place. Near the
characteristic place for a given stimulus frequency, the pressure wave returns to the basal
end of the cochlea through the fluids of the scala tympani, where the flexible round
window membrane is displaced in an equal but opposite direction to that of the volume
displacement of the stapes footplate.

At the apical end of the cochlea, the endolymphatic space terminates just short of the
bony labyrinthine wall and maintains a firm physical boundary between endolymph and
perilymph, allowing the fluids in the scala vestibuli and scala tympani to communicate
directly by way of a small duct called the helicotrema (see Fig. 146-3B). The helicotrema
acts as an acoustic shunt across the cochlear partition that reduces the pressure difference
between the scalae produced by very-low-frequency stimulation.[96] The size of the
helicotrema is thought to determine the low frequency cutoff of the system; that is, large
openings shunt pressure waves extending to higher frequencies than those associated with
small helicotrema. The shunting effect of helicotrema reduces the pressure differential
across the cochlear partition, and it is this aspect of cochlear design that is thought to
decrease damage to the cochlea produced by intense low-frequency pressure
fluctuations.[403]

Active Cochlear Mechanics

Von B?k?sy recognized more than half a century ago that the envelope of the traveling
wave measured in cochleae from human cadavers is broadly tuned—so broadly tuned as
to be inconsistent with a living person's ability to discriminate closely spaced frequencies.
Numerous groups of investigators have since confirmed the findings of von B?k?sy and

have shown with clarity that sharp tuning and high sensitivity are lost in physiologically
compromised cochleae or cochleae from cadavers.[432,494] Only after mechanical
measurements were made in vivo with nonhuman species did it become clear that the
peak of the traveling wave in living animals is, in fact, very sharply tuned at low levels of
stimulation and exhibits nonlinear growth as sound levels increase.[274,314,433,454,494] The
seminal study of Rhode,[433] which showed the amplitude of basilar membrane vibration
relative to stapes displacement to be as much as two to three orders of magnitude greater
under low-stimulus-level conditions than at higher levels in normal animals, led most
directly to the contemporary understanding of cochlear mechanics as a nonlinear
phenomenon (i.e., high sensitivity to changes in stimulus pressure at low levels, marked
compression in the midlevel range, and linear growth at high levels)

Figure 146-4. A, Schematic representation of traveling waves propagating along the
basilar membrane (BM) in response to a pure tone. When the cochlear amplifier is
functioning, outer hair cells (OHCs) within a restricted region of the traveling wave peak
exert a force that enhances BM motion relative to that observed during passive mechanics
alone for low-level stimuli. B, Similar to A, except note that the peak of the traveling
wave produced with cochlear amplification is much narrower than the peak produced
during passive mechanics alone, which is important for frequency discrimination. Also
note that during active amplification, the peak of the traveling wave occurs slightly more
basal than that produced during passive mechanics. C, Schematic relationship between
stimulus level and BM displacement. When OHCs are normal, stimulus level increments
of less than 40 dB sound pressure level (SPL) produce a linear increase in BM

displacement. Between approximately 40 and 80 dB SPL, OHC responses saturate,
producing a compressed response and nonlinear growth. Above 80 dB SPL, growth
becomes linear again. When OHCs are damaged, displacements occur only to higher
level stimuli and exhibit linear growth. D, The relationship between stimulus frequency
(measured in octaves relative to the characteristic frequency [oct re CF] at that location)
and the level required to elicit a criterion or “threshold” response at a given point along
the BM (e.g., tuning curves). When the cochlear amplifier is operational, thresholds may
be 40 to 60 dB lower and frequency selectivity greatly enhanced compared with a passive
cochlea.

Active mechanics associated with basilar membrane amplification is a metabolically
labile, energy-consuming process that is, consequently, physiologically vulnerable. It is
of particular importance to recognize that the active mechanical event underlying
amplification is highly localized. That is, the principle of active mechanics applies to a
circumscribed portion of the cochlear partition that is limited to the immediate vicinity of
the characteristic place for a given stimulus frequency (see Fig. 146-4A and B). At
frequencies removed from the characteristic place, the properties of cochlear partition
vibration in a living animal follow linear rules of growth, as in cadaverous tissue (Fig.
146-5).[179] Another important point is that the traveling wave peaks at increasingly apical
regions of the cochlear partition with increasing stimulus level, shifting the cochlear
place-frequency map toward lower frequencies at any given place.

Figure 146-5. Measurements of basilar membrane (BM) motion in a healthy chinchilla
cochlea at one location near the base in response to tone bursts. A, Measurement of BM
velocity in response to tone bursts of different frequencies (abscissa) at various levels
(parameter). The characteristic frequency (CF) at this location is 9 kHz. SPL, sound
pressure level. B, Tuning curves (i.e., stimulus levels required to generate a criterion
response as a function of stimulus frequency) are shown for a criterion BM velocity of
0.1 mm/sec and for a criterion displacement amplitude of 1.77 nm. Data were taken from
A. A tuning curve from an auditory nerve fiber that innervates this BM location is also
shown. C, BM velocities as a function of stimulus level for various frequencies. A line
showing a linear relationship between velocity and level also is shown. D, The ratio
between BM velocity and stapes velocity (i.e., gain) as a function of stimulus frequency.
Note the high gain (i.e., amplification) at low stimulus levels near the CF location that is
reduced as a stimulus level increases.