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.
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