Visual Pathway

VISUAL PATHWAY

• Visual pathway starting from retina.

Consists of:

1. Optic nerve
2. Optic chiasma
3. Optic tracts
4. Lateral geniculate bodies
5. Optic radiations (geniculocalcarine pathway)
6. Visual cortex
7. 

OPTIC NERVE

• Also known as cranial nerve II.
• Paired nerve that transmits visual information from the retina to the brain. 
• Starts from the optic disc and extends up to optic chiasma, where the two nerves meet. 
• It is the backward continuation of the nerve fibre layer of the retina, which consists of the second order neurons i.e. axon originating from the ganglion cells. 
• Also contains the afferent fibres of the pupillary light reflex.
• The fibres of optic nerve, numbering about a million.
• The optic nerve  is about 47-50mm in length may be divided in the four parts :

1. Intraocular part (1 mm):

• Begins at optic disc (optic nerve head) and exits the nerve through a hole in sclera that is occupied by a mesh like structure called the lamina cribrosa.
• The nerve fibres from the retina leave the eye through pores (holes) in lamina cribrosa, a sieve – like structure made up of collagen meshwork.

2. Intraorbital part (30 mm):

• Extends from back of the eyeball (at lamina cribrosa) to the optic foramina.

3. Intracanalicular part (6-9 mm):

• It lies within the optic canal and closely related to ophthalmic artery which crosses obliquely over it.

4. Intracranial part (10 mm):

• It lies above the cavernous sinus and converges with its fellow from contralateral side to form optic chiasma.

OPTIC CHIASMA

• Both sides optic nerves converge with each other to form optic chiasma.
• Posteriorly, chiasma is continuous with the optic tract.
• Nerve fibres arising from the nasal halves of two retinae
• Decussate at chiasma & go to the contralateral visual cortex through optic tracts.
• While nerve fibres from temporal halves of the retinae & go to the same side of visual cortex.

OPTIC TRACTS

• Cylindrical bundle of nerve fibres
• Running outwards & backwards from posterolateral aspect of optic chiasma.
• Each optic tract consists of fibres from the temporal half of the retina of the same eye & nasal half of the contralateral eye.

LATERAL GENICULATE BODY

• Optic tracts terminate in the lateral geniculate body.
• Fibres of second order neurons coming via optic tracts relay in neurons of lateral geniculate body.
• Third order neuron originate from lateral geniculate body & extend into optic radiation.

OPTIC RADIATIONS

• The optic radiations (geniculocalcarine pathway).
• Extends from the lateral geniculate body to the visual cortex.
• Inferor fibres of optic radiations subserve upper visual fields
• Superior fibres subserve inferior visual fields.

VISUAL CORTEX

• Located on the medial aspect of occipital lobe

In visual pathway
Sensory organs	Photoreceptors (Rods & cones)
Neurons of first order	Axon of bipolar cells (in Retina)
Neurons of second order	Axons of ganglionic cell (Retina i.e optic disc, Optic nerve, optic chiasma, optic tracts)
Neurons of third order	Axons from nerve cells in lateral geniculate body (optic radiation)

Exam Important

Don’t Forget to Solve all the previous Year Question asked on VISUAL PATHWAY