The Auditory System
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and nuclei II | Main Anatomy Index | The diencephalon
Last updated 30 March 2006
The Auditory System
Click here to go the "The Ear" under 1st year Gross
Anatomy, Head and Neck (ANAT1006)
The auditory system faces a basic mechanical problem.
Sound vibrations are propagated in air, whereas the auditory receptor cells live in a fluid-filled
- Nearly all (99.9%) of the sound energy incident on an air-water
interface is reflected.
- One major task of the air-filled outer and middle ears is to transfer sound
as efficiently as possible to the fluid-filled
The Conduction of Sound Energy
The outer ear is basically a complicated
It consists of the auricle (or pinna) and the external auditory meatus.
It conducts sound to the tympanic membrane.
Sound-induced vibrations are transferred along the
chain of three ossicles that traverse the middle ear cavity.
- The handle of the malleus
is directly attached to the medial
surface of the tympanic membrane.
- The malleus in turn is attached to the incus, which is
attached to the stapes.
- The sound-induced vibrations eventually reach the oval-shaped
footplate of the stapes.
- The footplate of the stapes
occupies the oval window (or fenestra vestibuli).
- The other side of the oval window is the perilymph-filled vestibule of the bony
- The vestibule leads directly to cochlea,
which contains the organ of Corti.
- Thus vibration of the tympanic
membrane ultimately results in vibration of
the fluids of the inner ear.
Muscles of the Ossicles
Tensor Tympani Muscle
This is attached to the handle of the malleus.
When it contracts, it increases
the tension on the tympanic membrane.
This decreases the transmission of vibrations through
the ossicular chain.
- Its innervation is by the nerve to medial pterygoid, a
branch of the mandibular nerve (CN V3).
This is attached to the neck of the stapes.
It too decreases the transmission of vibrations when it
- Its innervation is by the nerve to stapedius, a branch
of the facial
nerve (CN VII).
The Auditory Part of the Inner Ear
This consists of a portion of the endolymph-filled
membranous labyrinth suspended within a portion
of the perilymph-filled bony labyrinth.
Click here for a diagram of a cross-section
through the cochlea.
- The bony part is the cochlea (L. snail).
- It coils through 2 and a half turns from its relatively broad base to its apex.
- The cochlea lies on its side in the temporal bone, with
its base facing medially and posteriorly.
- The bony core of the cochlea is the modiolus.
- From here the osseous spiral lamina projects like the threads of a screw.
- Within the winding cavity of the modiolus
is the spiral ganglion.
- This contains the cell bodies of the primary
auditory afferent fibres.
- The central processes of these cells collect at the base of the cochlea.
- Here they form the cochlear division of the vestibulocochlear nerve (CN VIII).
- The peripheral processes pass in bundles
through a series of canals in the osseous
spiral lamina to innervate the auditory receptors.
The Cochlear Duct
This is the auditory portion of the membranous
labyrinth and is firmly anchored to the bony labyrinth.
The duct is triangular in cross
The space enclosed by the cochlear duct is filled with endolymph
and is called the scala media.
- One corner of the triangle is attached
to the edge of the osseous spiral
- The other two corners are attached
to the outer wall of the bony cochlea.
- The result is that the cochlear duct and osseous spiral lamina act as a partition
separating two perilymphatic spaces from each other.
- The exception is at the apex of
- Here, perilymph can pass from one space to the other
through a small opening called the helicotrema.
- The perilymphatic space above the cochlear
duct is called the scala vestibuli (it is
directly continuous with the perilymph of the vestibule).
- The space below the cochlear duct
is called the scala tympani (it ends blindly at the secondary
tympanic membrane or round window membrane).
- As perilymph is incompressible (a fluid), the round window membrane is required for vibrations to enter the
- Small quantities of perilymph oscillate within the cochlea.
- Most of this vibratory energy passes directly from the scala vestibuli to the scala tympani,
deforming the cochlear duct.
- The cochlear duct contains the auditory
receptors, and this deformation stimulates
some of them.
- Each of the 3 walls of the cochlear
duct has a different structure:
- The vestibular membrane (or Reissner's membrane)
borders the scala vestibuli and serves mainly as a barrier between the endolymph
- The stria vascularis adheres to the outer
wall of the bony cochlea and is a specialised area that is rich in
capillaries, producing most of the endolymph;
- The basilar membrane separates the scala
media from the scala tympani.
- Passing from the base to the apex
of the cochlea, the osseous spiral lamina
- At the same time, the basilar membrane becomes broader.
- This change in its width and progressive
changes in its mechanical properties, the basilar membrane is vibrated most
- By higher frequencies at the base
of the cochlea;
- And by lower frequencies at the apex
of the cochlea.
- Since the organ of Corti rests on the basilar
membrane, different receptor cells respond
best to sounds of different frequencies.
- This is the beginning of a tonotopic organisation
within the auditory system, which is analogous to the somatotopic organisation of the somatosensory
The Organ of Corti
Click here for a diagram of a cross-section
through the Organ of Corti.
- This is a long strip of hair
cells and supporting cells that rest on the basilar membrane.
- The hair cells are arranged
in 2 groups:
- A single row of inner hair cells
near the osseous spiral lamina;
- And a band of outer hair cells
3-5 cells wide.
- The 2 groups are separated
by a space called the tunnel of Corti.
- Here, the peripheral processes of auditory
afferents must pass on their way to the outer hair cells.
- The sensory hairs are stereocilia
of the outer hair cells.
- These are inserted into the gelatinous
- Vibration of the basilar membrane --> oscillations of the hairs
--> oscillation of the membrane
potentials of the hair cells.
Role of Inner and Outer Hair Cells
There are about 12,000 outer hair cells and 3,500 inner hair cells per cochlea.
However, most of the auditory
information in the cochlear nerve is carried by fibres that innervate the less
numerous inner hair cells.
- About 90% of all auditory
afferents receive their entire input from single inner hair cell.
- One inner hair cell may make synapses on as many as 20 different
- In contrast the remaining 10% of the auditory
afferents branch repeatedly and innervate multiple outer
- The outer hair cells seem to contribute
substantially to the sensitivity of the inner hair cells.
- One possible mechanism suggested by recent work
indicates that the outer hair cells are contractile.
Vibrations of basilar membrane
--> oscillations of membrane
potential --> vibration of the cells themselves
- This in turn increases the vibratory
stimulation of the inner hair cells, thereby increasing their sensitivity.
Click here for a diagram of the auditory
- The cell bodies of the auditory
primary afferents are located in the spiral ganglion
of the modiolus.
- They enter the brainstem
at the pontomedullary junction.
- There each fibre bifurcates and sends
one branch to:
- The dorsal cochlear nucleus;
- And the ventral cochlear nucleus.
- These cochlear nuclei form a continuous
band of cells that covers the dorsal and lateral aspects of
the inferior cerebellar peduncle.
Some fibres from the cochlear nuclei, mainly the dorsal cochlear nucleus, loop over
the top of the inferior cerebellar
These fibres cross the midline with a rostral inclination, and join the lateral
This is the major ascending auditory pathway of the brainstem.
- The lateral lemniscus is somewhat diffuse as it forms in the caudal pons.
- In the rostral pons it
forms a flattened band (L. lemniscus, ribbon) on
the lateral surface of the tegmentum.
- A smaller number of fibres from the cochlear
nuclei do not cross the midline.
- They instead join the ipsilateral lateral lemniscus.
- Thus, each lateral lemniscus carries some information
from both ears.
Inferior Colliculus and Medial
Nearly all fibres of the lateral lemniscus terminate in
the inferior colliculus.
- The inferior colliculus then gives rise to the inferior brachium (L. brachium, arm).
- This assumes a superficial position and terminates in the medial geniculate
- This is a portion of the thalamus that protrudes in a posterior direction,
overlapping the midbrain.
- Fibres from the medial geniculate nucleus project to
the primary auditory cortex (Brodman's area 41 &
42 or the transverse gyri of Heschl).
- This is located in a portion of the superior temporal gyrus buried in
the lateral sulcus.
- Low frequencies are represented anterolaterally
while high frequencies are represented posteromedially.
- A few of these ascending auditory fibres, representing
the contralateral ear, may proceed
directly to the medial geniculate nucleus
without stopping at the inferior colliculus.
Superior Olivary Nucleus
A much larger number of efferents
from the ventral cochlear nucleus, pass beneath the inferior cerebellar peduncle.
Some join the lateral lemniscus of each
side and proceed to the inferior colliculus.
- Many, however, are involved in sound localisation and
end in the superior olivary nucleus.
- This is located at the rostral end of the facial motor
Strategies of Sound Localisation
There are 2 general strategies and the superior olivary complex contains a medial
and a lateral subnucleus correspondingly.
- Sound localisation can be accomplished by:
- Comparison of the time-of-arrival of the
- And the intensity of a sound
at the two ears.
- The time-of-arrival comparison is begun in the medial superior olive.
- This form of sound localisation is more effective for terrestrial
animals with relatively large heads.
- Humans therefore have a large medial superior olive and
a small lateral superior olive.
- Fibres from the ventral cochlear nuclei of both sides converge on the medial
superior olive of each side.
- Crossing from one cochlear nucleus to the contralateral superior
olivary nucleus occurs in the trapezoid body.
- This is a large collection of second-order
fibres that pass through and ventral to the medial lemnisci.
Clinical Significance of the
Contraction of the stapedius stiffens the ossicular chain and hampers
the transmission of vibrations.
When a loud sound enters one ear, both
stapedius muscles contract in a reflex fashion.
An individual with a damaged facial nerve may complain that sounds are too loud in the ipsilateral
Pathway Involved in Stapedius Reflex
One ventral cochlear nucleus --> both
superior olivary nuclei --> both facial motor nuclei.
- It is possible to test this reflex arc in a useful clinical procedure.
- When the stapedius contracts, less
sound energy incident on the eardrum is transferred along the ossicular chain, and more is reflected.
- By measuring changes in reflected
test sounds back from one eardrum when a loud
sound is introduced into the contralateral ear, the
stapedius reflex can be analysed quantitatively.
Changes in Frequency Sensitivity
||20 Hz - 20 kHz
||50 Hz - 8 kHz
- Although as one ages, the range of frequencies able to be heard is significantly
decreased, this doesn't pose much of a problem as human
speech generally falls in the range of 300 Hz to 3 kHz.
- Also, how loud a sound seems is a function of both the amplitude and frequency of the
This results from:
- Blockage of the external acoustic meatus;
- Impairment of the movement of the ossicles (due to bony overgrowth of the stapes);
- Infection of the middle ear;
- Or physical trauma.
May be caused by lesions of the:
- Cochlea (noise, old age, drugs);
- Cochlear nerve (tumour);
- Or cochlear nuclei (vascular lesion).
- These result in ipsilateral hearing loss.
- This sensorineural deafness may be restricted to certain frequencies associated with the portion of the cochlea
that was damaged.
- Damage to the auditory pathway at any level rostral to the cochlear nuclei does not
cause deafness in either ear.
- Rather, it causes problems with localisation
of sound from the contralateral side and may cause some high-frequency hearing loss in the contralateral
Tests to Distinguish Between Conduction and
Ganong, William F. (1997) Review of Medical Physiology 18th Ed.
Appleton & Lange, Stanford, Connecticut, USA. Table 9-1. p.172
||Base of vibrating tuning fork placed on the vertex of
||Base of vibrating tuning fork placed on mastoid process
until subject no longer hears it, then held in air next to ear
||Bone conduction of patient compared with that of a normal
||Hears equally on both sides
||Hears vibrations in air after bone conduction is over
||Sound louder in affected ear because of masking effect of
environmental noise is absent on diseased side
||Vibrations in air not heard after bone conduction is over
||Bone conduction better than normal (conduction defect
excludes masking noise)
||Sound louder in normal ear
||Vibration hear in air after bone conduction is over, as
long as nerve deafness is partial
||Bone conduction worse than normal