Auditory System I: The Ear And Cochlea
The auditory system is
responsible for sound perception. The receptive end-organ is the cochlea of the inner ear, which converts
sound waves into electrical signals by mechanotransduction. The electrical
signal generated in response to a sound is passed (together with information
from the vestibular system; see Chapter 29), via the eighth cranial nerve
(vestibulocochlear nerve) to the brainstem where it synapses in the cochlear
nuclear complex (see Chapter 28). Although the auditory system as a whole
performs many functions, the primary site responsible for frequency
discrimination is at the level of
the cochlea.
Properties of sound waves
A sound wave is
characterized by:
•
Amplitude
or loudness (measured
in decibels [db]);
•
Frequency
or pitch (measured in
hertz [Hz]);
•
Waveform;
•
Phase and
•
Quality
or timbre.
The intensity of sound can vary
enormously but in general we can discriminate changes in intensity of around
1–2 dB. The arrival of a sound at the head creates phase and intensity
differences between the two ears unless the sound originates from the midline.
The degree of delay and intensity change between the two ears as a result of
their physical separation is useful but probably not necessary for the
localization of sounds (see Chapter 28).
External and middle ear
On reaching the ear the sound
passes down the external auditory meatus to the tympanic membrane or
eardrum, which vibrates at a frequency and strength determined by the
impinging sound. This causes the three ear ossicles in the middle ear
to move, displacing fluid within the cochlea as the stapedial foot
process moves within the oval window of the cochlea. This process is essential
in reducing the acoustic impedance of the system and in enhancing the response
to sound, because a sound hitting a fluid directly is largely reflected.
There are two small muscles
associated with the ear ossicles, which protect them from damage by loud noises
as well as modifying the movement of the stapedial foot process in the oval
window. Damage to the ear ossicles (e.g. otosclerosis), middle
ear (e.g. infection or otitis media) or external auditory meatus
(e.g. blockage by wax) all lead to a reduction in hearing or deafness that
is conductive in nature.
Inner ear and cochlea
The displacement of the stapedial
foot process in the oval window generates waves in the perilymph-filled scala
vestibuli and tympani of the cochlea. These two scalae are in
communication at the apical end of
the cochlea, the helicotrema, but are separated for the rest
of their length by the scala media,
which contains the transduction apparatus in the organ of Corti.
The organ of Cortisits on the floor
of the scala media on a structure known as the basilar membrane (BM),
the width of which increases with distance from the stapedial end. This
increase in width coupled to a decrease in stiffness of the BM means that
sounds of high frequency maximally displace the BM at the stapedial end of the
cochlea while low-frequency sounds maximally activate the apical end of the BM.
Thus, frequency tuning is, in part, a function of the BM although it is greatly
enhanced and made more selective by the hair cells of the organ of Corti that
lie on this membrane.
The organ of Corti is a
complex structure that contains the cells of auditory transduction, the hair
cells (see Chapter 23), which are of two types in this structure:
• a single
row of inner hair cells (IHCs) – which provide most of the signal in the
eighth cranial nerve;
•
3–4 rows
of outer hair cells (OHCs) – which have a role in modulating the
response of IHCs to a given sound.
These two types of hair cell are morphologically
and electrophysiologically distinct:
• While the
IHCs receive little input from the brainstem, the OHCs do so from the superior
olivary complex, which has the effect of modifying the shape and response
properties of these cells.
•
Some of
the OHCs make direct contact with the overlying tectorial membrane (TM)
in the organ of Corti which may be important in modifying the response of the
IHCs to sound, as these cells do not contact the TM but provide 93% of the
afferent input of the cochlear nerve.
• One
afferent fibre receives from many OHCs, but a single IHC is associated with
many afferent fibres.
In addition to these differences
between OHCs and IHCs, there are subtle alterations in the hair cells
themselves with distance along the scala media. These alterations in shape
modify their tuning characteristics, which adds a degree of refinement to frequency
tuning beyond that imparted by the resonance properties of the BM.
Deafness
Damage to the cochlea, hair cells or
cochlear part of the vestibulocochlear nerve leads to deafness that
is described as being sensorineural in nature. Trauma, ischaemia and
tumours of the eighth cranial nerve can lead to this. Certain hereditary causes
of deafness have been associated recently with defects in the proteins found in
the stereocilia of hair cells (see also
Chapter 23).
