The Auditory System

Cranial nerves and nuclei II | Main Anatomy Index | The diencephalon

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 environment.

• 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 inner ear.

• The outer ear is basically a complicated funnel.
• 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 labyrinth.
• 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.

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• 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 contracts.

• Its innervation is by the nerve to stapedius, a branch of the facial nerve (CN VII).

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• 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.

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• This is the auditory portion of the membranous labyrinth and is firmly anchored to the bony labyrinth.
• The duct is triangular in cross section.
• 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 lamina.
• 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 the cochlea.
• 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 perilymph.
• 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:

1. The vestibular membrane (or Reissner's membrane) borders the scala vestibuli and serves mainly as a barrier between the endolymph and perilymph;
2. 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;
3. 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 becomes narrower.
• 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 efficiently:

1. By higher frequencies at the base of the cochlea;
2. 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 system.

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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:

1. A single row of inner hair cells near the osseous spiral lamina;
2. 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 tectorial membrane.
• Vibration of the basilar membrane --> oscillations of the hairs --> oscillation of the membrane potentials of the 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 auditory afferents.

• In contrast the remaining 10% of the auditory afferents branch repeatedly and innervate multiple outer hair cells.

• 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.

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Click here for a diagram of the auditory pathways.

• 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:

1. The dorsal cochlear nucleus;
2. 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 peduncle.
• These fibres cross the midline with a rostral inclination, and join the lateral lemniscus.
• 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.

• 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 nucleus.
• 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.

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• 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 nucleus.

• There are 2 general strategies and the superior olivary complex contains a medial and a lateral subnucleus correspondingly.

• Sound localisation can be accomplished by:

1. Comparison of the time-of-arrival of the sound;
2. 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.

• Each superior olivary nucleus then projects through the lateral lemniscus to the ipsilateral inferior colliculus.

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• 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 ear (hyperacusis).

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.

• 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 sound.

• This results from:

1. Blockage of the external acoustic meatus;
2. Impairment of the movement of the ossicles (due to bony overgrowth of the stapes);
3. Infection of the middle ear;
4. Or physical trauma.

• May be caused by lesions of the:

1. Cochlea (noise, old age, drugs);
2. Cochlear nerve (tumour);
3. 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 ear.

Ganong, William F. (1997) Review of Medical Physiology 18^th Ed. Appleton & Lange, Stanford, Connecticut, USA. Table 9-1. p.172

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