Page 379 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

!
brane (! D1) give against the pressure wave cell cilia at the site of maximum reaction to the
(migrating wave, ! B and C). It can therefore sound frequency (! D4), resulting in opening
take a “short cut” to reach the round window of transduction channels and depolarization of
without crossing the helicotrema. Since the the cells (sensor potential). This leads to trans-
cochlear duct is deformed in waves, Reissner’s mitter release (glutamate coupling to AMPA re-
membrane and the basilar membrane vibrate ceptors; ! p. 55 F) by internal hair cells and
alternately towards the scala vestibuli and the subsequent conduction of impulses to the
CNS.
scala tympani (! D1,2). The velocity and
Central Nervous System and Senses (! B), while their amplitude increases to a can be measured by placing a microphone in front of
wavelength of the migrating wave that started
Vibrations in the internal ear set off an outward emis-
at the oval window decrease continuously
sion of sound. These evoked otoacoustic emissions
maximum and then quickly subsides (! B, en-
the tympanic membrane, e.g., to test internal ear
function in infants and other individuals incapable of
velope curve). (The wave velocity is not equal
reporting their hearing sensations.
to the velocity of sound, but is much slower.)
The site of the maximum excursion of the
Inner ear potentials (! p. 369 C). On the cilia
duct
is
characteristic
of
the
cochlear
side, the hair cells border with the endolymph-
wavelength of the stimulating sound. The
filled space, which has a potential difference
higher the frequency of the sound, the closer
(endocochlear potential) of ca. + 80 to + 110 mV
the site is to the stapes (! C).
Outer hair cells. Vibration of the cochlear
tial difference is maintained by an active trans-
port mechanism in the stria vascularis. Since
0.3 nm) of the tectorial membrane against the
the cell potential of outer (inner) hair cells is
12 duct causes a discrete shearing (of roughly relative to perilymph (! p. 369 C). This poten-
basilar membrane, causing bending of the cilia
of the outer hair cells (! D3). This exerts also a – 70 mV (– 40 mV), a potential difference of
roughly 150–180 mV (120–150 mV) prevails
shearing force between the rows of cilia of the across the cell membrane occupied by cilia
individual external hair cell. Probably via the (cell interior negative). Since the K conc. in the
+
”tip links” (! p. 342), cation channels in the endolymph and hair cells are roughly equal
ciliary membranes open (mechanosensitive (! 140 mmol/L), the prevailing K equilibrium
+
transduction channels), allowing cations (K , + potential is ca. 0 mV (! p. 32). These high
+
2+
Na , Ca ) to enter and depolarize the outer potentials provide the driving forces for the in-
hair cells. This causes the outer hair cells to flux not only of Ca 2+ and Na , but also of K , +
+
shorten in sync with stimulation (! D3). The prerequisites for provoking the sensor poten-
successive shearing force on the cilia bends tial.
them in the opposite direction. This leads to
+
hyperpolarization (opening of K channels) Hearing tests are performed using an audiometer.
and extension of the outer hair cells. The patient is presented sounds of various frequen-
cies and routes of conduction (bone, air). The sound
The mechanism for this extremely fast electromotil- pressure is initially set at a level under the threshold
ity (up to 20 kHz or 2 · 10 times per second) is un- of hearing and is raised in increments until the
4
clear, but it seems to be related to the high turgor of patient is able to hear the presented sound (thresh-
outer hair cells (128 mmHg) and the unusual struc- old audiogram). If the patient is unable to hear the
ture of their cell walls. sounds at normal levels, he or she has an hearing
loss, which is quantitated in decibels (dB). In
These outer hair cell electromotility con- audiometry, all frequencies at the normal threshold
tributes to the cochlear amplification (ca. 100- of hearing are assigned the value of 0 dB (unlike the
fold or 40 dB amplification), which occurs diagram on p. 363 B, green curve). Hearing loss can
before sound waves reach the actual sound be caused by presbycusis (! p. 362), middle ear in-
sensors, i.e. inner hair cells. This explains the fection (impaired air conduction), and damage to
very low threshold within the very narrow lo- the internal ear (impaired air and bone conduction)
cation (0.5 nm) and thus within a very small caused, for example, by prolonged exposure to ex-
cessive sound pressure (! 90 dB, e.g. disco music,
frequency range. The electromotility causes pneumatic drill etc.).
366 endolymph waves in the subtectorial space
which exert shearing forces on the inner hair
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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