Visual Pathway

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

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

Parts of optic nerve

  • Intraocular part: 1 mm
  • Intraorbital part: 25 – 30 mm
  • Intracanalicular part: 5 – 9 mm
  • Intracranial part: 10 – 16 mm

 Lesions of visual pathway

No

Site of lesion

Causes

Features

1

Normal

2

Optic nerve

Optic neuritis

Optic atrophy

Sarcoidosis

LHON

Traumatic avulsion

Complete blindness in affected eye

Loss of light reflex on the affected side and

consensual reflex loss on the other side

Near reflex present

Central/paracentral scotoma

3

Central chiasma

Pituitary tumors (MCC)

Suprasellar aneurysm,

Craniopharyngioma,

Glioma of Ill ventricle

Bitemporal hemianopia

Bitemporal hemianopic paralysis of

pupillary reflexes

3a

Lateral chiasma

Distension of 3rd  ventricle,

Atheroma of post.

communicating A

Binasal hemianopia

Binasal hemianopic paralysis of pupillary

reflexes

4

Optic tract

Geniculate body

Syphilitic meningitis,

gumma, tuberculosis,

tumors of optic thalamus

Incongruous (contralateral) homonymous

hemianopia

Contralateral Wernicke’s pupil (optic tract)

5

Optic radiations in

temporal lobe

(inferior fibres)

 

Superior quadrantic hemianopia (pie in the sky)

Normal pupillary reactions

5a

Optic radiations in

parietal lobe

(Superior fibres)

 

Inferior quadrantic hemianopia (pie on the floor)

Normal pupillary reactions

6

Optic radiations

 

Homonymous hemianopia

7

Anterior occipital

(visual) cortex

Occlusion of posterior

cerebral artery

Congruous homonymous hemianopia sparing macula

8

Tip of occipital cortex

 

Congruous homonymous macular defect

  • Optic atrophy (partial, descending) – occurs in lesions of chiasma, optic tract and geniculate body
  • Optic atrophy does not occur in lesions of optic radiations and visual cortex
Don’t Forget to Solve all the previous Year Question asked on VISUAL PATHWAY

Module Below Start Quiz

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