Special Senses: Hearing
And Balance
The young healthy human can detect
sound wave frequencies of between 40 Hz and 20 kHz, but the upper
frequency limit declines with age. When sound waves reach the ear, they pass
down the external auditory meatus (the external ear) to the tympanic
membrane that vibrates at a frequency and strength determined by the
magnitude and pitch of the sound. The vibration of the membrane causes three
ear ossicles (malleus, incus and stapes) in the middle
ear (an air-filled cavity) to move, which, in turn, displaces fluid within
the cochlea (the inner ear) as the foot of the stapes moves the oval
window at the base of the cochlea. This mechanical link prevents the
incoming sound energy from being reflected back, and the ossicles improve the efficiency
with which the sound energy is transferred from the air to the fluid. Small
muscles are attached to the ossicles and contract reflexly in response to loud
sounds, thereby dampening the vibration and attenuating the transmission of the
sound (Fig. 58a).
The inner ear includes the cochlea
and also the vestibular organs responsible for balance (see later).
The receptors involved in both hearing and balance are specialized
mechanoreceptors called hair cells. Projecting from the apical surface
of the hair cell is a bundle of over 100 small hair-like structures called stereocilia
and a larger stere- ocilium called the kinocilium. Deflection of the
stereocilia towards the kinocilium leads to a potential change in the cell
(depolarization), the release of a transmitter substance from the base of the
hair cell, and activation of the nerve fibres that convey impulses to the
higher centres of the brain.
The cochlea comprises a coiled tube
of about 3 cm in length (Fig. 58b), with three tubular canals running parallel
to one another (scala vestibuli, scala media and scala tympani).
The scala vestibuli and the scala
tympani contain perilymph (which is similar to extracellular fluid in
composition), and the scala media contains endolymph (similar in
composition to intracellular fluid). The scala vestibuli and scala tympani are
joined at the tip of the coil (the helicotrema); at the base of the
scala vestibuli is the oval window and at the base of the scala tympani
is the round window, separating the fluid of the inner ear from the air
in the middle ear.
The scala media lies between the
two perilymph-filled canals; the boundary between it and the scala vestibuli is
called Reissner’s mem- brane, and the boundary between it and the scala
tympani is called the basilar membrane. On top of the basilar membrane
sits the organ of Corti in which the hair cells are situated. There are
around 15 000 hair cells distributed in rows along the basilar membrane. There
are two types of hair cell: the inner hair cells which form a single row
and the more numerous outer hair cells arranged in three rows. The hair
cells are ideally placed to detect small amounts of movement of the basilar
membrane. Because of the changing width of the basilar membrane, high-frequency
sounds maximally displace the membrane at the base of the cochlea and
low-frequency sounds maximally dis- place the membrane at the apical end of the
cochlea.
The auditory signals are relayed
through a complex series of nuclei in
the brain stem and the thalamus, eventually reaching the primary auditory
cortex in the temporal lobe of the cerebral cortex.
Balance
The system associated with balance
is called the vestibular system and is not only involved with balance,
but also postural reflexes and eye movements.
As mentioned earlier, the receptors
involved in the vestibular system are hair cells. These hair cells are found in
the inner ear in close proximity to the cochlea in two otolith organs called
the utricle and saccule, and in a structure called the ampulla
found in the three semicircular canals. The otolith
organs are primarily involved in the detection of linear motion and static
head position, and the semicircular canals in the detection of rotational
movements of the head.
The four otolith organs (two on
each side) each contain a structure called the macula which comprises a
number of hair cells (Fig. 58c). With the head erect, the macula in each
utricle is orientated horizontally and that in each saccule is orientated
vertically. The base of each macula contains hair cells whose stereocilia
project into a gelatinous mass called the otolith membrane. When the
head is tilted, the force of gravity displaces the otolith membrane, thereby
bending the stereocilia. The nerve fibres innervating the hair cells are
spontaneously active: displacement in one direction increases firing and
displacement in the opposite direction decreases firing of the neurones. The utricle
sends signals representing forwards and backwards movements and the saccule
conveys information about vertical movements.
The semicircular canals each
contain an organ called the ampulla (Fig. 58d). They respond to rotational
movement of the head, and the plane of each canal is perpendicular to the
other two, so that, between all six (three on each side), they provide
information relating to the rotational acceleration of the head during movement
around any axis. Each canal contains endolymph and the ampulla comprises hair
cells in which the stereocilia project into a gelatinous mass, with the same
specific gravity as the endolymph, called the cupula. During
acceleration in the plane of a particular canal, the endolymph tends to remain
stationary because of inertia. The movement displaces the stereocilia and
stimulation of the associated nerve fibres occurs. Again, movement in one
direction increases firing of the nerves and movement in the opposite direction
causes a decrease in firing. Vestibular afferent fibres from the auditory
(VIII) nerve have their cell bodies in the vestibular ganglion and
terminate in one of four vestibular nuclei in the medulla. These nuclei
also receive inputs from neck muscle receptors and the visual system. They then
project to a number of areas of the central nervous system, including the spinal
cord, thalamus, cerebellum and oculomotor nuclei,
where they are in-volved in posture, gait and eye movements. They also project
to the primary somatosensory cortex and to the posterior parietal
cortex.
