Auditory & Vestibular System

Auditory System:-

Peripheral Receptors:

1.Organ of Corti- the sense organ of hearing, situated on the basilar membrane

-tunnel of corti- formed by inner & outer rods, contains a fluid called cortilymph

-hair cells- transduce sound into electrical energy.inner cells-single row,outer cells-3-4 rows

inner cells-richly supplied by afferent cochlear fibres, more important in transmission of auditory impulses.flask shaped,develop earlier, more resistant
outer cells mainly receive efferent innervation from the olivary complex & are concerned with modulating the function of inner hair cells.cylindrical, develop late, modulate function of inner hair cells-easily damaged by ototoxic drugs & high intensity noises

-supporting cells-Deiters' cells are situated between the outer hair cells & provide support.Cells of Hansen lie outside the Deiter's cells.

-tectorial membrane-consists of gelatinous matrix with delicate fibres,overlies the organ of Corti.The shearing force between the hair cells & tectorial membrane produces the stimulus to hair cells

-cells of claudius-on the basilar membrane, outer to the organ of corti

Reissner's Membrane-partition between Scala Vestibuli(uppermost) & Scala Media

Basilar Membrane-partition between Scala Tympani(lowermost) & Scala media

Striae Vascularis- outer lining of Scala Media

Limbus & Osseous Spiral Lamina- situated inner to Organ of Corti

Nerve Supply of Hair Cells:

95% of afferent fibres of spiral ganglion supply the inner hair cells while only 5% supply to the outer hair cells. Efferent fibres to the hair cells come from the olivocochlear bundle. Their cell bodies are situated in superior olivary complex. Each cochlea sends innervation to both sides of the brain

Auditory Neural Pathways & Their Nuclei:

Hair cells are innervated by dendrites of bipolar cells of spiral ganglion which is situated in Rosenthal's Canal(canal running along osseous spiral lamina).Axons of these bipolar cells form the cochlear division of CN VIII & end in the cochlear nuclei; the dorsal & ventral, on each side of the medulla. Further course of auditory pathway is complex. From cochlear nuclei, the main nucleus centres in the ascending auditory pathways are

a-superior olivary complex

b-nucleus of lateral leminiscus

c-inferior colliculus

d.medial geniculate body

e.auditory cortex

The auditory pathways travel via the ipsilateral & contralateral routes to have multiple decussation points. Thus, each ear is represented in both cerebral hemispheres. The area of cortex, concerned with hearing is situated in the superior temporal gyrus (Broadman's area 41)

ECOLI,MA

Vestibular System:-

Peripheral Receptors

1.Cristae-located in ampulated ends of 3 semicircular ducts, respond to angular accleration

2.Maculae-located in otolith organs;(utricles & saccules). Macula of the utricle lies in its floor in a horizontal plane. Macula of saccule lies in the medial wall in a vertical plane. They sense position of head in response to gravity & linear acceleration.

Cristae: structure:-

A crest like mound of connective tissues over which sensory epithelial cells lie.The cilia of the sensory epithelial cells project into the cupula which is a gelatinous mass extending from the surface of the crista to the ceiling of the ampulla & form a water tight partition, only to be displaced to one or the other side like a swing door, with movements of endolymph, the gelatinous mass of cupula consists of polysaccharide & contain canals into which projects cilia of sensory cells.

Hair cells are of two types.

Type-I cells are flask shaped witha single large cup-like nerve terminal surrounding the base

Type-II- cylindrical with multiple nerve terminals @ base

From the upper surface of each cell, project a single hair- the kinocilium & a number of other cilia-the stereocilia. The kinocilium is thicker & is located on the edge of the cell. The kinocilium is thicker & is located on the edge of the cell. Sensory cells are surrounded by supporting cells which show microvilli on their upper ends.

Structure of Macula:

A macula consists mainly of two parts:

a-a sensory neuroepithelium, made up of type I & type II cells, similar to those in crista

b-an otolithic membrane, which is made up of a gelatinous mass on the top, the crystals of calcium carbonate called otoliths/otoconia. The cilia of hair cells project into the gelatinous layer. The linear, gravitational & head tilt movements cause displacement of otolithic membrane & thus stimulate the hair cells which lie in different planes.

Vestibular Nerve:

Vestibular/Scarpa's ganglion is situated in the the lateral part of the internal acoustic meatus. It contains bipolar cells. The distal processes of bipolar cells innervate the sensory epithelium of the labyrinth while its central processes aggregate to form the vestibular nerve.

Central Vestibular Connections:

The fibres of vestibular nerve end in vestibular nuclei & some go to the cerebellum directly.Vestibular nuclei are four in number, superior,medial,lateral & descending. Afferents to these nuclei come from:

1.peripheral vestibular receptors(semicircular canals, utricle & saccule)

2.cerebellum

3.reticular formation

4.spinal cord

5.contralateral vestibular nuclei

information received by the labyrinthine receptors is integrated with information from other somatosensory systems.

Efferents from vestibular nuclei go to-

1.nuclei of III, IV & VI via medial longitudinal bundle-pathway for vestibulo-ocular reflexes & this explains the genesis of nystagmus

2.motor part of spinal cord (vestibulospinal nerves)-coordinates the movements of head, neck & body in the maintenance of balance

3.Cerebellum-vestibulocelebellar fibres-it helps to co-ordinate input information to maintain the body balance

4.autonomic nervous system-this explains nausea, vomiting, palpitation, sweating & pallor seen in vestibular disorders like Meniere's disease

5.Vestibular nuclei of the opposite side

6.Cerebral Cortex-temporal lobe-this is responsible for subjective awareness of motion