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.
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