Physiology Of Hearing

PHYSIOLOGY OF HEARING
Any vibrating object causes waves of compression and
rarefaction and is capable of producing sound. In the air,
at 20°C and at sea level, sound travels at a speed of
344 metre- (1120 feet) per second. It travels faster in liquids
and solids than in the air. Also, when sound energy has
to pass from air to liquid medium, most of it is reflected
because of the impedance offered by the liquid.

Mechanism of Hearing

A sound signal in the environment is collected by the
pinna, passes through external auditory canal and strikes
the tympanic membrane_ Vibrations of the tympanic
membrane are transmitted to stapes footplate through
a chain of ossicles coupled to the tympanic membrane_
Movements of stapes footplate cause pressure changes in
the labyrinthine flu ids which move the basilar membrane.
This stimulates the hair cells of the organ of Corti.
It is these hair cells which act as transducers and convert
the mechanical energy into electrical impulses which
travel along the auditory nerve. Thus, the mechanism of
hearing can be broadly di vided into:
l. Mechanical conduction of sound (conductive
apparatus) .
2. Transduction of mechanical energy to electrical
impulses (sensory system of cochlea).
3. Conduct ion of electrical impulses to the brain (neural
pathways).
I. Conduction of Sound
A person under water cannot h ea r any sound made in
the air because 99.9% of the sound energy is reflected
away from the surface of water because of the impedance
offered by it. A similar situation exists in the ear when
air-conducted sound has to travel to cochlear fluids. Nature
has compensated for this loss of sound energy by interposing
the middle ear which converts sound of greater amplitude
, but lesser force, to that of lesser amplitude but greater
force. This function of the middle ear is called impedance
matching mechanism or the transformer action.
It is accomplished by:
(a) Lever action of the ossicles. Handle of malleus is 1.3
times longer than long process of the incus, providing
a mechanical advantage of 1.3.
(b) Hydraulic action of tympanic membrane_ The area of
tympanic membrane is much larger than the area
of stapes footplate, the average ratio between the
two being 21:1. As the effective vibratory area of
tympanic membrane is only two-thirds, the effective
areal ratio is reduced to 14: 1, and this is the
mechanical advantage provided by the tympanic
membrane.
The product of areal ratio and lever action of ossicles
is 18: l.
According to some workers (Wever & Lawrence)
out of a total of 90 mm2 area of human tympanic
membrane, only 55 mm2 is functional and given the
area of stapes footplate (3.2 mm2), the areal ratio is
17: 1 and total transformer ratio (17 X l.3) is 22.1

(c) Curved membrane effect. Movements of tympanic
membrane are more at the periphery than at the
centre where malleus handle is attached. This too
provides some leverage.
Phase differential between oval and round windows.
Sound waves striking the tympanic membrane do not
reach the oval and round windows simultaneously. There is
a preferential pathway to the oval window because of the
ossicular chain. Thus, when oval window is receiving wave
of compression, the round window is at the phase of rarefaction.
If the sound waves were to strike both the windows
simultaneously, they would cancel each other's effect with
no movement of the perilymph and no hearing. This
acoustic separation of windows is achieved by the presence
of intact tympanic membrane and a cushion of air in the
middle ear around [he round window. Phase differential
between the windows contributes 4 dB when tympanic
membrane is intact.
Natural resonance of external and middle ear. Inherent
anatomic and physiologic properties of the external
and middle ear allow certain frequencies of sound to pass
more easily to [he inner ear due to their natural resonances.
Natural resonance of external ear canal is 3000 Hz and that
of middle ear 800 Hz. Frequencies most efficiently transmitted
by ossicular chain are between 500 and 2000 Hz
while that by tympanic membrane is 800- 1600 Hz. Thus
greatest sensitivity of the sound transmiss ion is between
500 and 3000 Hz and these are the frequencies most
important to man in day to day conversation

2. Transduction of Mechanical Energy to
Electrical Impulses
Movements of the stapes footplate, transmitted to the
cochlear fluids, move the basilar membrane, setting up
shearing force between the tectorial membrane and the
hair cells. The distort ion of hair cells gives rise to cochlear
microphonics which trigger the nerve impulse.
A sound wave, depending on its frequency, reaches
maximum amplitude on a particular place on the basilar
membrane and stimulates that segment (travelling wave
theory of von Bekesy). Higher frequencies are represented in
the basal turn of the cochlea and the progressively 10IVer
ones towards the apex.
3. Neural Pathways
Hair cells get innervation from the bipolar cells of spiral
ganglion. Central axons of these cells collect to form
cochlear nerve which goes to ventral and dorsal cochlear
nuclei . From there, both crossed and uncrossed fibres
travel [0 the superior olivary nucleus, lateral lemniscus,
inferior colliculus, medial geniculate body and finally
reach the auditory cortex of the temporal lobe.
Electrical Potentials of Cochlea and CN VIII
Four types of potentials have been recorded; three from
the cochlea and one from CN VIII fibres. They are:
1. Endocochlear potential
2. Cochlear microphonic from cochlea
3. Summating potential
4. Compound action potential- from nerve fibres
1. Endocochlear potential. It is a direct current
(DC) potential recorded from scala media. It is +80 mV
and is generated from the stria vascularis by Na + /K+ATPase
pump and provides source of energy for cochlear
transduction. It is present at rest and does not require
sound stimulus. This potential provides 8 sort of "battery"
to dri ve the current through hair cells when they
move in response t0 a sound stimulus.
2. Cochlear microphonic (CM). When basilar
membrane moves in response to sound stimulus, electrical
resistance at the tips of hair cells changes allowing
flow of K+ through hair cells and produces voltage fluctuations
called cochlear microphonic. It is an alternating
current (AC) potential.
3. Summating potential (SP). It is a DC potential
and follows "envelope" of stimulating sound. It is produced
by hair cells. It may be negative or positive. SF has
been used in diagnosis of Meniere's disease. It is superimposed
on VIII nerve action potential.
Both CM and SF are receptor potentials as seen in
other sensory end-organs. They differ from action potentials
in that (a) they are graded rather than all or none
phenomenon, (b) have no latency, (c) are not propagated
and (d) have no post-response refractory period .
4. Compound action potential. It is an all or none
response of auditory nerve fibres.