Pupillary Reflex

Pupillary reflexes are the visceral reflexes, which alter the size of pupil. Pupillary reflexes are classified into three types:

1. Light reflex

2. Ciliospinal reflex

3. Accommodation reflex.

Pupillary Reflex


LIGHT REFLEX

Light reflex is the reflex in which pupil constricts when light is flashed into the eyes. It is also called pupillary light reflex. Light reflex is of two types:

1. Direct light reflex

2. Indirect light reflex.

DIRECT LIGHT REFLEX

Direct light reflex is the reflex in which there is constriction of pupil in an eye when light is thrown into

that eye. It is also called the direct pupillary light reflex or the direct reaction to light.

INDIRECT LIGHT REFLEX

Indirect light reflex is the reflex that involves constriction of pupil in both eyes when light is thrown into one eye. If light is flashed into one eye, the constriction of pupil occurs in the opposite eye, even though no light rays falls on that eye. It is otherwise called consensual light reflex.

PATHWAY FOR LIGHT REFLEX

Pathway for light reflex is slightly deviated from visual pathway. Fibers of light reflex pathway and the fibers of visual pathway are the same up to optic tract. Beyond that, these two sets of fibers are separated. When light falls on the eye, the visual receptors are stimulated. Afferent (sensory) impulses from the receptors pass through the optic nerve, optic chiasma and optic tract. At the midbrain level, few fibers get separated from optic tract and synapse on the neurons of pretectal nucleus, which lies close to the superior colli culus. Pretectal nucleus of midbrain forms the center for light reflexes. Efferent (motor) impulses from this nucleus are carried by short fibers to Edinger-Westphal nucleus (parasympathetic nucleus) of oculomotor nerve (third cranial nerve). From Edinger-Westphal nucleus, preganglionic fibers pass through oculomotor nerve and reach the ciliary ganglion. Postganglionic fibers arising

from ciliary ganglion pass through short ciliary nerves and reach the eyeball. These fibers cause contraction of constrictor pupillae muscle of iris. Reason for consensual light reflex is that, some

of the fibers from pretectal nucleus of one side cross to the opposite side and end on opposite Edinger-

Westphal nucleus.

CILIOSPINAL REFLEX

Ciliospinal reflex is the dilatation of pupil in eyes caused by painful stimulation of skin over the neck. It is due to the contraction of dilator pupillae muscle. Sensory impulses pass through cutaneous afferent nerve. The center is in first thoracic spinal segment. Efferent impulses pass through sympathetic fibers and reach dilator pupillae.

ACCOMMODATION

DEFINITION

Accommodation is the adjustment of eye to see either near or distant objects clearly. It is the process by

which light rays from near objects or distant objects are brought to a focus on sensitive part of retina. It is

achieved by various adjustments made in the eyeball.

MECHANISM OF ACCOMMODATION

Light rays from distant objects are approximately parallel and are less refracted while getting focused on retina. But, the light rays from near objects are divergent. So, to be focused on retina, these light rays should be refracted (converged) to a greater extent. There are three possible ways by which, accommodation occurs:

1. Retina must be moved towards or away from the lens. It is done by shortening or elongation of eyeball.

So, the divergent, parallel or convergent rays are focused accurately. This mechanism is present only

in some molluscs and not in human beings.

2. Lens must be moved towards or away from the retina. It is done in photography. This mechanism

exists in some fishes.

3. Convexity of lens must be altered, so that the refractory power of lens is altered according to the

need. This mechanism is present in human eye and it was first suggested by Young and later supported

by Helmholtz.

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