Nystagmus; significance and function

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.