Functions of Vestibular Apparatus

 FUNCTIONS OF VESTIBULAR APPARATUS

Receptors of semicircular canals give response to rotatory movements or angular acceleration of the head. And receptors of utricle and saccule give  response to linear acceleration of head. Thus, the vestibular apparatus is responsible for detecting the position of head during different movements. It also causes reflex adjustments in the position of eyeball, head and body during postural changes.

FUNCTIONS OF SEMICIRCULAR CANALS

Semicircular canals are concerned with angular (rotatory) acceleration. Semicircular canals sense the rotational movement. Each semicircular canal is sensitive to rotation in a particular plane.

 Superior Semicircular Canal

Superior semicircular canal gives response to rotation in anteroposterior plane (transverse axis), i.e. front to back movements like nodding the head while saying ‘yes – yes’.

Horizontal Semicircular Canal

Horizontal semicircular canal gives response to rotation in horizontal plane (vertical axis), i.e. side to side

movements (left to right or right to left) like shaking the head while saying ‘no – no’.

Posterior Semicircular Canal

Posterior semicircular canal gives response to rotation in the vertical plane (anteroposterior axis) by which head is rotated from shoulder to shoulder.

Mechanism of Stimulation of Receptor

Cells in Semicircular Canal

At the beginning of rotation, receptor cells are stimulated by movement of endolymph inside the

semicircular canals. However, receptors are stimulated only at the beginning and at the stoppage of rotatory movements. And during rotation at a constant speed, these receptors are not stimulated.

When a person rotates in clockwise direction in horizontal plane (vertical axis), horizontal canal moves

in clockwise direction. But there is no corresponding movement of endolymph inside the canal at the beginning of rotation. Because of the inertia, endolymph remains static. This phenomenon causes relative displace ment of endolymph in the direction opposite to that of the rotation of head. That is, the fluid is pushed in anticlockwise direction. Thus, in the right horizontal semicircular canal, the endolymph flows towards the ampulla and in the left canal, the fluid moves away from the ampulla. Movement of endolymph in semicircular canal, in turn causes corresponding movement of gelatinous cupula. Thus, in the right horizontal canal, the cupula moves towards the ampulla. Whereas in left canal cupula moves away from ampulla. In any semicircular canal, when cupula moves towards the ampulla, stereocilia development of mild depolarization in hair cells up to –50 mV. This type of depolarization is called receptor potential. Besides potassium ions, calcium ions also enter the hair cells from endolymph.

Receptor potential in hair cells is non-propagative. But, it causes generation of action potential in nerve

fibers distributed to hair cells. Depolarization of hair cells causes them to release a neurotransmitter, which generates the action potential in the nerve fibers. It is believed that the probable neurotransmitter may be glutamate.

Movement of stereocilia in the opposite direction (away from kinocilium) causes hyperpolarization of

hair cells. Calcium may play a role in the development of hyperpolarization. Hyperpolarization in hair cells

stops generation of action potential in the nerve fibers of hair cells are pushed towards kinocilium leading to stimulation of hair cells. When cupula moves away from ampulla, the stereocilia are pushed away from kinocilium and hair cells are not stimulated. Thus, at the commencement of rotation in clockwise

direction around vertical axis, hair cells at ampulla of horizontal canal in right ear are stimulated. But, the hair cells in horizontal canal of left ear are not stimulated. Because of stimulation, the hair cells in right horizontal canal send information (impulses) through sensory nerve fibers to vestibular, cerebellar and reticular centers. Now, these centers send proper instructions to various muscles of the body to maintain equilibrium of the body during angular acceleration (rotation). On the other hand, rotation in anticlockwise

direction causes stimulation of hair cells in ampulla of horizontal canal in left ear only. Hair cells of horizontal canal in right ear are not stimulated. Stimulation of hair cells in left ear is followed by the process as in the case of clockwise rotation.

Electrical Potential in Hair Cells –

Mechanotransduction

Mechanotransduction is a type of sensory transduction in the hair cell (receptor) by which the mechanical energy (movement of cilia in hair cell) caused by stimulus is converted into action

potentials in the vestibular nerve fiber. Resting membrane potential in hair cells is –60 mV. Movement of stereocilia of hair cells towards kinocilium causes opening of mechanically gated potassium channels . It is followed by influx of potassium ions from endolymph which contains large amount of potassium ions. Potassium ions cause development of mild depolarization in hair cells up to –50 mV. This type of depolarization is called receptor potential. Besides potassium ions, calcium ions also enter the hair cells from endolymph. Receptor potential in hair cells is non-propagative. But, it causes generation of action potential in nerve fibers distributed to hair cells. Depolarization of hair cells causes them to release a neurotransmitter, which generates the action potential in the nerve fibers. It is believed that the probable neurotransmitter may be glutamate. Movement of stereocilia in the opposite direction (away from kinocilium) causes hyperpolarization of hair cells. Calcium may play a role in the development

of hyperpolarization. Hyperpolarization in hair cells stops generation of action potential in the nerve fibers.