Nystagmus
Nystagmus is the rhythmic
oscillatory involuntary movements of eyeball. It is common during rotation. It
is due to the natural stimulatory effect of vestibular apparatus during
rotational acceleration. Nystagmus occurs both in physiological and
pathological conditions.
Vestibulo-ocular reflex and nystagmus
Nystagmus is a reflex phenomenon that occurs in order to maintain the visual fixation. Since the movements of eyeballs
occur in response to stimulation of vestibular apparatus
this reflex is called vestibulo-ocular reflex.
Movement of eyeball during nystagmus
Nystagmus has two
components of movement, which occur alternately:
1. Slow component
2. Quick component.
1. Slow component
At the
beginning of rotation, since eyes are fixed at a particular object
(point), eyeballs rotate slowly in
the direction opposite to
that of rotation of the head. It is called slow component of nystagmus. It is
due
to
vestibulo-ocular reflex. This reflex is because of labyrinthine
impulses reaching the ocular muscles via
vestibular nuclei and III,
IV and V cranial nerves.
2. Quick component
When the slow movement of
eyeballs is limited, the eyeballs move to a new fixation point
in the direction of rotation of head. This movement to a
new fixation point occurs with a jerk. So, it is called the quick component. Quick
component of nystagmus is due to the activation of some centers in brainstem.
Postrotatory nystagmus
Nystagmus that occurs immediately
after stoppage of rotation is called postrotatory nystagmus. It is due to movement
of cupula in opposite direction caused by the endolymph, when rotation is
stopped. Postrotatory nystagmus can be demonstrated by Barany chair.
Postrotatory
Reactions
After the end of rotatory
movement, two reactions occur:
1. Feeling of rotation in
opposite direction
2. Postrotatory nystagmus.
1. Feeling of rotation in the opposite direction
When rotation in clockwise
direction is stopped suddenly, endolymph moves in the direction of rotation in
right horizontal semicircular canal although the semicircular canal stops moving.
So, cupula moves away from utricle. However, in the case of left horizontal
semicircular canal, endolymph moves into ampulla. There, it pushes cupula
towards the utricle and stimulates the hair cells in crista of left canal. It
causes feeling of rotation in opposite direction when the rotation is stopped.
2. Postrotatory nystagmus
It is already explained
above.
Nystagmus in
Pathological Conditions
Nystagmus is very common
in lesions of cerebellum and lesions of brainstem involving vestibular nuclei
or vestibular nerve. It also occurs due to the damage of labyrinth.
FUNCTION OF OTOLITH
ORGAN
Otolith organ is concerned
with linear acceleration and detects acceleration in both horizontal and
vertical
planes.
Utricle responds during horizontal acceleration and saccule responds
during vertical acceleration.
Function of Utricle
Position of hair cells of macula
helps utricle to respond to horizontal acceleration. In utricle, the macula is
situated in horizontal
plane with the hair cells in vertical plane. While moving horizontally, because
of inertia the otoconia
move in opposite direction and pull the cilia of hair cells resulting in
stimulation of hair cells.
For example, when the body
moves forward, the otoconia fall back in otolith membrane and pull the cilia
of hair cells backward.
Pulling of cilia causes stimulation of hair cells. Hair cells send information
(impulses) to vestibular, cerebellar and reticular centers. These centers in
turn send instructions to various muscles to maintain equilibrium of the body
during the forward movement.
Function of Saccule
Macula of saccule is situated
in vertical plane with the cilia of hair cells in horizontal plane. While
moving
vertically, as in the case
of utricle, otoconia of saccule move in opposite direction and pull the cilia
resulting in stimulation of hair cells.
For example, while
climbing up, the otoconia move down by pulling the cilia downwards. It
stimulates
the hair cells, which in
turn send information to the brain centers. And the action follows as in the
case of
movement in horizontal
plane.
Role of Otolith Organ
in Resting Position
During resting conditions
(in the absence of head movement), hair cells are stimulated continuously
because of the pulling of otoconia by
gravitational force. Stimulation of hair cells produces
reflex movements of head and limbs for the maintenance of posture in relation to
gravity. Because of this function, the receptors of otolith organ are called gravity
receptors.
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