1. The visual pathway begins in the retina and passes through the optic nerve, optic chiasm, optic tract, lateral geniculate body, optic radiations, and visual cortex. 2. Lesions in different parts of the visual pathway cause specific visual field defects such as hemianopia. For example, a lesion in the optic chiasm causes bitemporal hemianopia. 3. Pupillary reflexes can localize lesions, such as Wernicke's hemianopic pupil indicating an optic tract lesion.

1. Visual Pathway
Dr Prashanth Reddy
DNB PGT in Paediatrics,
CSI Hospital, Bangalore.

2. Visual pathway starting from retina consists of—
Components:
A. Optic nerve
B. Optic chiasma.
C. Optic tract
D. Lateral geniculate body (of the thalamus).
E. Optic radiations
F. Visual cortex

4. Optic Nerve
• 2nd cranial nerve.
• 47-50 mm in length.
• Starts from optic disc & extends upto optic chiasma where
the two nerves meet.
• Backward continuation of nerve fibre layer of retina which
consist of axons originating from ganglion cells.
• Contains the afferent fibres of light reflex
• Has 4 parts : 1)intraocular (1mm)
2)intraorbital (30mm)
3)intra canalicular (6-9mm)
4)intracranial (10mm)

5. • Optic nerve is an outgrowth
of brain.
• Not covered by neurilemma
so does not regenerate
when cut.
• Fibres of optic nerve are
very thin(2-10 um in
diameter)& are million in
number.
• surrounded by meninges
unlike other peripheral
nerves.
• Both primary & secondary
neurons are in retina

6. Intraocular part:
• Starts from the optic disc, pierces the choroid
and sclera (converting into a sieve like
structure- lamina cribrosa).
• At the back of eye ball it becomes continuous
with intraorbital part.

7. Intraorbital part :
• Extends from back of eyeball to optic foramina.
• This part slightly sinuous to give play for the eye
movements.
• Here optic nerve is surrounded by all 3 layers of meninges
& subarachnoid space.
• The central retinal artery along with enters the
subarachnoid space to enter the nerve on its inferomedial
aspect.
• Near optic foramina, optic nerve is closely surrounded by
annulus of zinn & the origin of four recti muscles.
• Some fibres of superior & medial rectus are adherent to its
sheath & account for painful occular movements in
retrobulbar neuritis.

8. Intracanalicular part
• This part is closely related to ophthalmic
artery.
• it crosses the nerve from medial to lateral side
in dural sheath.
• Sphenoid & posterior ethmoidal sinuses lie
medial to it & separated by thin bony lamina,
this relation accounts for retrobulbar neuritis
following infection of sinuses

9. Intracranial part:
• About 10mm
• Lies above cavernous sinus & converges with
its fellow to form chiasma.
• Ensheathed in piamater.
• Internal carotid artery runs below then lateral
to it.

10. Optic chiasma
• Flattened structure,12 mm horizontally & 8mm
anteroposteriorly.
• Ensheathed by pia & surrounded by CSF.
• Lies over diaphragma sellae so visual field
defects seen in patient with pituitary tumor
having suprasellar extension.
• Posteriorly chiasma continuous with the optic
tracts & form the anterior wall of 3rd ventricle.
• Nerve fibres arising from nasal half of two retina
decussate at the chiasma.

11. Optic tract
• Cylindrical bundle of nerve fibres.
• Run outwards & backwards from
posterolateral aspect of optic chiasma.
• Fibres from temporal half of retina of same
eye & nasal half of opposite eye.
• Posteriorly each ends in Lateral Geniculate
Body.

12. Lateral Geniculate body
-A nucleus in the
thalamus, which projects
to the 1° visual cortex
and serves visual
perception.
- This body is the site of
termination of all optic
nerve fibres except few
which reach and relay in
the pretectal region and
superior colliculus of
Midbrain.
Consists of 6 lamina.
Contralateral retina – 1,4,6
Ipsilateral retina – 2,3,5

13. 0ptic radiation
• From LGB to the occipital cortex.
• Pass forwards then laterally through the area of wernicke
as optic peduncles.
• Anterior to lateral ventricle ,traversing the retrolenticular
part of internal capsule, medial to auditory tract.
• Its fibres then spread out fanwise to form medullary optic
lamina.
• Inferior fibres subserve upper visual fields & sweep
anteroinferiorly in meyer’s loop & temporal lobe to visual
cortex.
• Superior fibres subserve inferior visual field proceed
posteriorly through parietal lobe to visual cortex.

15. Visual cortex
• It is located on the medial aspect of occipital
lobe, above and below the calcarine fissure.
Visual cortex
Visuopsychic
area
Peristriate
area 18
Parastriate
area 19
Visuosensory
area
Striate area
17

16. feature Visual sensation Somatic sensation
Sensory end organ Rods and Cones Nerve endings in the skin
Neurons of 1st order Lie in bipolar layer of
retina
Lies in posterior root
ganglion
Neurons of 2nd order Lies in ganglion cells of
the retina
Lies in nucleus gracilis
and nucleus cuneatus
Neurons of 3rd order Lie in geniculate body Lie in geniculte body

17. Neural pathway for vision
• Ist order sensory neurons –
Arise from the bipolar cells
of the retina.
• II nd order neurons are the
Multipolar neurons whose
axons run along the optic
nerve to the optic chiasma
• Nasal fibres – Cross to
opposite side and terminate
in LGB of opposite side.
• Temporal fibres – Doesn’t
cross and terminates in
ipsilateral LGB.

18. • The cell bodies of III
order sensory neurons
are located in LGB.
• Their axons form optic
radiation which project
into the visual cortex.

19. Visual reflexes
• Light reflex or pupillary
reflex:
When light is shown to one
eye, normally the pupils of
both eyes constrict.
- Direct light reflex:
The constriction of pupils
upon which light is shown
is called direct light reflex.
- Indirect or consensual:
The constriction of pupil on
the other eye even though
no light is shown

21. ACCCOMODATION REFLEX
• When the eyes are
focussed from a distant to
near object, three reactions
take place
• 1. Constriction of pupils
• 2. thickening of lens due to
contraction of ciliary
muscles
• 3. Convergence of both eye
balls
These three reactions
together constitute
Accommodation or near
reflex

22. Clinical correlation
• Loss of vision in one half
of the visual field (Rt or
Lt) is termed as
hemianopia.
• Homonymous
hemianopia: Loss of
vision in the same halves
of the visual field.
• Heteronymous
Hemianopia: Loss of
vision in the different
halves of the visual field.

23. Lesions of the visual pathway
Lesions of the optic nerve:
• Characterised by marked loss of vision or
complete blindness on the affected side
associated with abolition of direct light reflex on
the ipsilateral side and consensual on
contralateral side.
Causes:
optic atrophy
 indirect optic neuropathy
acute optic neuritis
traumatic avulsion of optic nerve
Eg : right optic nerve involvement

24. 2)Lesions through proximal part of optic nerve :
ipsilateral blindness.
 contralateral hemianopia
 abolition of direct light reflex on affected side
& concensual light reflex on contralateral side.
 near reflex intact.
Rt optic nerve
Involvement in
Proximal part

25. Central lesions of chiasma (sagittal):
Characterised by:
 Bitemporal hemianopia
 Bitemporal hemianopic
paralysis of pupillary reflex. (usually lead to partial
descending optic atrophy)
causes:
 suprasellar aneurysm
 tumors of pituitary gland
 craniopharyngioma
 suprasellar meningioma & glioma of 3rd ventricle.
 third ventricular dilatation due to obstructive
hydrocephalus.
 chronic chiasmal arachnoiditis

26. Lateral chiasmal lesions :
Characterised by
• Binasal hemianopia
• Binasal hemianopic
parallysis of pupillary reflex (usually lead to partial
descending optic atrophy)
causes:
• Distension of 3rd ventricle causing pressure on
each side of optic chiasma
• Atheroma of carotids & posterior communicating
artery.

27. Lesions of optic tract :
Characterised by :
• Incongruous homonymous hemianopia with C/L
hemianopic pupillary reaction( wernicke’s
reaction)
• These lesions usually lead to partial descending
optic atrophy & may be associated with C/L 3rd
nerve paralysis & ipsilateral hemiplegia.
Causes:
Syphilitic meningitis/ gumma.
Tuberculosis
 Tumors of optic thalamus
Aneurysm of superior cerebellar or posterior
cerebral arteries.

28. Lesions of lateral geniculate body :
leads to homonymous hemianopia with
sparing of pupillary
reflexes & may end in
partial optic atrophy

29. Lesions of optic radiations :
Causes:
Vascular occlusion
Primary & secondary tumors
Trauma
Characterised by :
TOTAL OPTIC RADIATION
INVOLVEMENT
COMPLETE
HOMONYMOUS
HEMIANOPIA(
sometimes sparing
macula)

30. LESIONS OF PARIETAL LOBE
(involving superior fibres of
optic radiations)
INFERIOR
QUADRANTIC
HEMIANOPIA( PIE
ON THE FLOOR)
LESIONS OF
TEMPORAL LOBE
(involving inferior
fibres of optic
radiations)
SUPERIOR
QUADRANTIC
HEMIANOPIA( PIE
ON THE ROOF)

31. • Pupillary reactions are normal as fibres of light
reflex leave the optic tracts to synapse in the
superior colliculi.
• Lesions of optic radiations do not produce
optic atrophy as the 1st order neurons (optic
nerve fibres) synapse in LGB.

32. Lesions of visual cortex: pupillary light reflex is
normal & optic atrophy does not occur
following visual cortex lesions.
Congruous
homonymous
hemianopia(sparing
macula)
Occlusion of posterior
cerebral artery
supplyin anterior part
of occipiatl cortex
Congruous
homonymous macular
defect
Head injury/gun shot
injury leading to
lesions of tip of
occipital cortex+

33. Abnormalities of pupillary reactions
Amaurotic light eye reflex: absence of direct
light reflex on affected side and absence of
consensual light reflex on the normal side.
• Indicates lesions of the optic nerve or retina
on the affected side leading to complete
blindness.
• In diffuse illumination both pupils are of equal
size.

34. Efferent pathway defect: absence of both direct
and consensual reflex on the affected side and
presence of both on the normal side.
• Near reflex is also absent on the affected side.
Causes :
• Effect of parasympatholytic drugs.
• Internal opthalmoplegia.
• Third nerve paralysis.

35. Wernick's hemianopic pupil:
• It indicates lesion of the optic tract.
• Light reflex (ipsilateral direct and contralateral
consensual) is absent when light is thrown on
temporal half of retina on the affected side
and nasal half of the opposite side
- while it is present when light is thrown
on nasal half of the affected side and temporal
half of the opposite side.

36. Marcus gun pupil :
• It is the paradoxical response to light in the
presence of a relative affarent pathway defect(
RAPD).
• It is tested by swinging flash light test.
• It is the earliest indication of optic nerve
disease even in the presence of normal visual
activity.

37. Argyll Robertson Pupil(ARP):
• Here the pupil is slightly small in size and
reaction to near reflex is present but light
reflex is absent, i.e, there is light near
dissociation.
• Both pupils are involved and dilate poorly with
mydriatics.
• It is usually caused by a lesion ( usually
neurosyphilis ) in the region of tectum.

39. The Adie's tonic pupil:
• Reaction to light is absent and to near reflex is
very slow and tonic.
• The affected pupil is large( anisocoria).
• It is caused by postganglionic parasympathetic
pupillomotor damage.
• It is usually unilateral, associated with absent
knee jerk and occurs more often in young
women.

40. 15 sq.Km of rain forest disappear every minute