Visual Pathway

Visual pathway or optic pathway is the nervous pathway that transmits impulses from retina visual center in cerebral cortex. In binocular vision, the light rays from temporal (outer) half of visual field fall upon the nasal part of corresponding retina. The rays from nasal (inner) half of visual field fall upon the temporal part of retina.

VISUAL RECEPTORS

Rods and cones which are present in the retina of eye form the visual receptors. Fibers from the visual

receptors synapse with dendrites of bipolar cells of inner nuclear layer of the retina.

FIRST ORDER NEURONS

First order neurons (primary neurons) are bipolar cells in the retina. Axons from the bipolar cells synapse with dendrites of ganglionic cells.

SECOND ORDER NEURONS

Second order neurons (secondary neurons) are the ganglionic cells in ganglionic cell layer of retina. Axons of the ganglionic cells form optic nerve. Optic nerve leaves the eye and terminates in lateral geniculate body.

THIRD ORDER NEURONS

Third order neurons are in the lateral geniculate body. Fibers arising from here, reach the visual cortex

CONNECTIONS OF VISUAL RECEPTORS TO OPTIC NERVE

Two pathways exist between the visual receptors and optic nerve:

1. Private pathway

2. Diffuse pathway.

PRIVATE PATHWAY

The individual cones in fovea centralis are connected to separate bipolar cells. Each bipolar cell is connected to separate ganglionic cell, namely midget ganglionic cell. Thus, individual cone is connected to an individual optic nerve fiber. This type of private pathway is responsible for visual acuity and intensity discrimination.

DIFFUSE PATHWAY

A number of cones and rods are connected with a polysynaptic bipolar cell. The bipolar cells are connected to diffused ganglionic cells. So, there is great overlapping. This type of pathway is present outside the fovea.

COURSE OF VISUAL PATHWAY

Visual pathway consists of six components:

1. Optic nerve

2. Optic chiasma

3. Optic tract

4. Lateral geniculate body

5. Optic radiation

6. Visual cortex.

OPTIC NERVE

Optic nerve is formed by the axons of ganglionic cells. Optic nerve leaves the eye through optic

disk. The fibers from temporal part of retina are in lateral part of the nerve and carry the impulses from nasal half of visual field of same eye. The fibers from nasal part of retina are in medial part of the nerve and carry the impulses from temporal half of visual field of same eye.

OPTIC CHIASMA

Medial fibers of each optic nerve cross the midline and join the uncrossed lateral fibers of opposite side, to form pathway This area of crossing of the optic nerve fibers is called optic chiasma.

OPTIC TRACT

Optic tract is formed by uncrossed fibers of optic nerve on the same side and crossed fibers of optic nerve from the opposite side. All the fibers of optic tract run backward, outward and towards the cerebral peduncle. While reaching the peduncle, the fibers pass between tuber cinereum and anterior perforated substance. Then, the fibers turn around the peduncle to reach the lateral geniculate body in thalamus. Here, many fibers synapse while few fibers just pass through this and run towards superior colliculus in midbrain. Fibers from fovea do not enter superior colliculus. Some fibers from fovea of each side pass through the optic tract of same side and others through the optic tract of opposite side. Due to crossing of medial fibers in optic chiasma, the left optic tract carries impulses from temporal part of left retina and nasal part of right retina, i.e. it is responsible for vision in nasal half of left visual field and temporal half of right visual field. The right optic tract contains fibers from nasal half of left retina and temporal half of right retina. It is responsible for vision in temporal half of left visual field and nasal half of right visual field.

LATERAL GENICULATE BODY

Majority of the fibers of optic tract terminate in lateral geniculate body, which forms the subcortical center

for visual sensation. From here, the geniculocalcarine tract or optic radiation arises. This tract is the last

relay of visual pathway. Some of the fibers from optic tract do not synapse in lateral geniculate body, but pass through it and terminate in one of the following centers:

i. Superior colliculus: It is concerned with reflex movements of eyeballs and head, in response

to optic stimulus

ii. Pretectal nucleus: It is concerned with light reflexes

iii. Supraoptic nucleus of hypothalamus: It is concerned with the retinal control of pituitary

in animals. But in human, it does not play any important role.