The visual pathway consists of the retina, optic nerve, optic chiasm, optic tract, lateral geniculate body, optic radiations, and visual cortex. The retina contains photoreceptors and bipolar and ganglion cells. Ganglion cell axons form the optic nerve, which crosses at the optic chiasm. The optic tract relays signals to the lateral geniculate body before projecting to the primary visual cortex via the optic radiations. Lesions in different parts of the pathway cause specific visual field defects, such as homonymous hemianopia from damage to the optic radiation. The pupillary light reflex and accommodation reflex are mediated by subcortical structures.

1. VISUAL PATHWAY
Dr.GURUDASAN
04.04.13

2. INTRODUCTION
- Most important special
sense in humans.
Components:
A. Retina
B. Optic nerve and optic
chiasm.
C. Optic tract
D. Lateral geniculate body
(of the thalamus).
E. Optic radiations
F. Visual cortex

3. Outer layer: photoreceptors-
rods
(night vision) and cones
(for daylight and colour,
densest just around the
fovea)
Middle layer: bipolar
neurons: horizontal (more
superficial) and amacrine
(deeper). Both perform
lateral interactions, which
enhance visual contrast.
Inner layer: ganglion cells.
Regional Anatomy of the Visual System: Retinal structure

4.  Ganglion cell axons
(which are clear and
unmyelinated, while
running along the inner
surface of the retina)
gather together and
exit at the optic disk,
where they become
myelinated and form
the optic nerve.
OPTIC NERVE AND OPTIC CHIASM

5.  Optic nerves from
both eyes converge
at optic chiasm:
partial cross-over.
 Images in the nasal
hemiretina from both
sides cross over
(temporal stay
ipsilateral).
 This allows for
complete cross-over
of each visual field.
 The right visual field
maps on the left
visual cortex and vice
versa

6. 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
LGB – THALAMIC RELAY STATION FOR
VISUAL IMPULSE

7. OPTIC RADIATIONS AND PROJECTIONS TO THE
PRIMARY VISUAL CORTEX
 Meyer’s Loop: course
anterior for a short
distance in order to move
over the lateral
ventricles.
 1° cortex, in columnar
fissure, is also Brodman’s
Area 17.

9. 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.

10. NEURAL PATHWAY FOR VISION (CONTD…
 The cell bodies of III
order sensory neurons
are located in LGB.
 Their axons form optic
radiation which project
into the visual cortex.

11. 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.

12. PATHWAYS FOR VISUAL REFLEX

13. 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
Accomodation or near
reflex

14. 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.

15.  VISUAL FIELD DEFECTS
DUE TO LESION IN
DIFFERENT PARTS OF
VISUAL PATHWAY:
1. Injury to optic N – Total
blindness in that eye
2. Lesion in central part of
optic chiasma –
Bitemporal heteronymous
hemianopia ( Tunnel
Vision)
3. Lesion in lateral part of
optic chiasma – Unilateral
nasal hemianopia on the
side of lesion

16.  4 &5. Complete destruction of
optic tract(4) & Optic
radiation(5) – Right or left
homonymous hemianopia
 6. Lesion in visual cortex –
blindness in opposite half of
visual field (Homonymous
hemianopia) with intact
macular vision.
 7. Lesion in upper lip of
calcarine sulcus –
Contralateral inferior
quadrant anopia.
 8. Lesion in lower lip of
calcarine sulcus - Contralateral
superior quadrant anopia.

17. ARGYL ROBERTSON PUPIL
 Lesion in pretectal
nucleus in Midbrain
produces loss of pupillary
reflex.
 As a result pupillary
constriction in response
to light is lost.
 But accomodation reflex
is present, as it passes
through frontal eye field
(area 8) via the superior
longitudinal fasciculus.
 Such pupil is known as
ARGYL REBERTSON
PUPIL

18. RIGHT ARGYLL ROBERTSON PUPIL DUE TO
RIGHT TRAUMATIC CAROTICO-CAVERNOUS
FISTULA

19. TO REMEMBER ARGYL ROBERTSON PUPIL

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