Rods and cones present in retina of eye are the visual receptors.

Fibers from visual receptors synapse with dendrites of bipolar cells of inner nuclear layer of retina.

First Order Neurons

First order neurons are bipolar cells in the retina.

Their dendrites synapse with fibers of rods and cones.

Axons from bipolar cells synapse with dendrites of ganglionic cells.

Second Order Neurons

Second order neurons are the ganglionic cells in ganglion cell layer of retina.

Axons of ganglionic cells form optic nerve.

Optic nerve leaves the eye and terminates in lateral geniculate body.

Third Order Neurons

Third order neurons are in lateral geniculate body.

Fibers arising from here reach the visual cortex.

Two pathways exist between visual receptors and optic nerve:

Visual pathway consists of six components:

1. Optic Nerve

Optic nerve is formed by axons of ganglion cells.

Optic nerve leaves the eye through optic disk.

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

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

2. Optic Chiasma

Medial fibers of each optic nerve cross the midline and join uncrossed lateral fibers of opposite side to form optic tract.

Area of crossing of optic nerve fibers is called optic chiasma.

3. Optic Tract

Optic tract is formed by un-crossed fibers of optic nerve on same side and crossed fibers of optic nerve of opposite side.

All the fibers of optic tract run backward and outward, and terminate in lateral geniculate body in thalamus.

Few fibers just pass through lateral geniculate body and run towards superior colliculus.

4. Lateral Geniculate Body — Subcortical Center

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

From here, geniculocalcarine tract or optic radiation arises. This tract is the last relay of visual pathway.

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

1. Superior colliculus of midbrain which is concerned with reflex movements of eyeballs and head in response to optic stimulus.
2. Pretectal nucleus of midbrain which is concerned with light reflexes.
3. Supraoptic nucleus of hypothalamus which is concerned with the retinal control of pituitary.

5. Optic Radiation

Fibers from lateral geniculate body pass through internal capsule and form optic radiation.

Optic radiation ends in visual cortex.

6. Visual Cortex — Cortical Center

Cortical center for vision is called visual cortex that is located on medial surface of occipital lobe.

It forms the walls and lips of calcarine fissure in medial surface of occipital lobe.

There is definite localization of retinal projections upon visual cortex.

In fact, the point to point projection of peripheral retina upon visual cortex is well established.

Peripheral retina representation occupies the anterior part of visual cortex.

Macular representation occupies the posterior part of visual cortex near the occipital pole.

Areas of Visual Cortex and Their Function

Visual cortex has three areas:

1. Primary visual area (area 17) which is concerned with perception of visual impulses.
2. Association area (area 18) which is concerned with interpretation of visual impulses.
3. Occipital eye field (area 19) which is concerned with movement of eyes.

Anopia

Anopia or blindness is the loss of vision in one visual field.

Hemianopia

Hemianopia is the loss of vision in one half of visual field.

Hemianopia is classified into two types:

1. Homonymous Hemianopia
2. Heteronymous Hemianopia

Homonymous Hemianopia

Homonymous hemianopia means loss of vision in same halves of both the visual fields.

Loss of vision in right half of visual field of both eyes is known as right homonymous hemianopia.

Similarly, left homonymous hemianopia means loss of vision in left half of visual field of both eyes.

Heteronymous Hemianopia

Heteronymous hemianopia means loss of vision in opposite halves of visual fields.

For example, binasal heteronymous hemianopia means loss of vision in nasal half of left visual field and right visual field.

It may also involve the temporal parts as described under bitemporal heteronymous hemianopia.

1. Effects of Lesion of Optic Nerve

Lesion in one optic nerve causes total blindness or anopia in the corresponding visual field.

Lesion occurs due to increased intracranial pressure.

2. Effects of Lesion of Optic Chiasma

Nature of defect depends upon fibers involved.

Pressure on Uncrossed Lateral Fibers

Pressure on uncrossed lateral fibers by aneurysmal dilation of carotid artery causes blindness in the temporal part of retina of same side, i.e., the retina cannot receive light stimulus from the objects in nasal half of same visual field.

So, the hemianopia developed is called left or right nasal hemianopia.

Pressure on Lateral Fibers of Both Sides

If lateral fibers of both sides are affected, the vision is lost in nasal half of both visual fields, causing binasal hemianopia.

It occurs due to dilated third ventricle which forces the angle of chiasma against carotid arteries.

It also occurs due to dilatation of carotid artery on both sides.

Compression of Nasal Fibers

Compression of nasal fibers, i.e., crossed fibers by pituitary tumor causes bitemporal hemianopia.

3. Effects of Lesion of Optic Tract, Lateral Geniculate Body and Optic Radiation

Lesion of optic tract or lateral geniculate body or optic radiation causes homonymous hemianopia.

In right-sided lesion of these structures, optic impulses do not reach the right half of retina in both eyes.

So, there is loss of vision in left half of both visual fields.

Hence, it is called left homonymous hemianopia.

In left-sided lesion, optic impulses do not reach left half of retina in both eyes.

So, there is loss of vision on right half of both visual fields.

Hence, it is called right homonymous hemianopia.

4. Effects of Lesion of Visual Cortex

Lesion of upper or lower part of visual cortex leads to inferior or superior homonymous hemianopia.

Significance of Macular Sparing

Because of macular sparing, total blindness does not occur in conditions of hemianopia.

Causes for Macular Sparing

Fibers from macula project into the visual cortex of both sides.

Fibers from macular region are projected into both anterior and posterior parts of each visual cortex.

Only bilateral lesion of visual cortex causes total blindness.