Download Cranial nerves III, IV,VI and Visual Pathway

Cranial nerves II,III, IV,VI and
Visual Pathway
Optic Nerve
The optic nerve consists
of more than 1 million
axons that originate in
the ganglion cell layer of
the retina and extend
towards the occipital
cortex
Type: Special sensory.
Function: Vision.
Optic Nerve
• Axons of retinal
ganglion cells converge
at the optic disc and
pass into optic nerve.
• Then the nerve passes
posteromedially in the
orbit, exits through the
optic canal to enter the
middle cranial fossa to
joins the optic
chiasma.
Optic Nerve
Lesion results in:
visual field defects
and loss of visual
acuity, a defect of
vision is called
anopsia.
Optic Chiasma
• The chiasma measures
approximately 12 mm
wide, 8 mm long in the
anteroposterior direction,
and 4 mm thick.
• The exact location of the
chiasma with respect to the
sella is variable. Most of
the time it is directly
superior
Optic Chiasma
• Fibers from the nasal (medial)
half of retina decussate within
the chiasma and join uncrossed
fibers from the temporal
(lateral) half of the retina to
form the optic tract.
• The decussation of nerve fibers
in the chiasma results in the
right optic tract conveying
impulses from the LEFT visual
field and vice versa.
• The partial crossing of optic
nerve fibers in the optic
chiasma is a requirement for
binocular vision.
Damage to the optic tract or more
central parts of the pathway tend
to cause comparable visual
deficits in both eyes
Optic Tract
• Each optic tract contains
ipsilateral temporal and
contralateral nasal fibers
from the optic nerves .
Optic Tracts
• Fibers in the optic
tracts:
 Mainly terminate
in the lateral
geniculate bodies
of the thalamus
 A few fibers
terminate in
pretectal area
and superior
colliculus. These
fibers are relatedLateral geniculate
body
to light reflexes
Optic radiations
• The optic radiations
connect the lateral
geniculate body with the
cortex of the occipital lobe
• The fibers of the optic
radiations leave the lateral
geniculate body and run
around the temporal horn
of the lateral ventricle,
approaching the anterior tip
of the temporal lobe (the
so-called loop of Meyer)
Optic Radiation
• Optic radiation or
thalamocortical or
geniculocalcarine
fibers originating in
the lateral geniculate
nucleus.
• Passes through the
retrolenticular part
of the internal
capsule to the visual
cortex above and
below the calcarine
sulcus.
Visual Cortex
• The primary visual cortex (area 17 of Brodmann's)
occupies the upper and lower lips of the calcarine
sulcus on the medial surface of the cerebral
hemisphere.
• The visual cortex, the thinnest area of the human
cerebral cortex
Visual Cortex
The visual association
cortex is extensive,
including the whole of the
occipital lobe, the adjacent
posterior part of the
parietal lobe. This cortex
is involved in recognition
of objects and perception
of color, depth, motion,
and other aspects of
vision.
Visual Pathway
1.
2.
3.
4.
5.
6.
7.
8.
Cones
Bipolar neurons
Ganglion cell’s axon
forms the optic
nerve
Optic nerve to the
Optic Chiasm
Optic tract
Lateral geniculate
nuclei of the
thalamus
Optic Radiations
Primary visual areas
of the occipital
lobes
Visual Pathway
 Photoreceptors:
Rods & Cones of retina
 3 neuron pathway
 1st order neurons:
Bipolar cells of retina.
 2nd order neurons:
Ganglion cells of
retina. Their axons form
the optic nerve
 3rd order neurons:
Neurons in the lateral
geniculate body. Their
axons terminate in
primary visual cortex.
Visual Field Deficits
Cut at level :1. A lesion of the
right optic nerve causes a total loss of
vision (blindness) in the right eye
Cut at level :2. A lesion of the
optic chiasma causes a loss of
vision in the temporal half of
both visual fields:
bitemporal hemianopsia.
Cut at level: 3 & 4. A lesion of
the right optic tract & right
optic radiation just after the
LGN causes a loss of vision in
the left hemifield:
contralateral homonymous
hemianopsia.
A lesion of both visual cortices
causes a complete blindness.
Occulomotor nerve
Motor for most of extraocular
muscles.
Also carries preganglionic
parasympathetic fibers for pupillary
constrictor and ciliary muscle.
Has two nuclei:
1- Main occulomotor nucleus;
Lies in the mid brain, at the level of
superior colliculus
2- Accessory nucleus (EdingerWestphal nucleus);
Lies dorsal to the main motor
nucleus,
Its cells are Preganglionic
Parasympathetic Neurons.
It receives; Corticonuclear fibers for
the accommodation reflex, and from
the pretectal nucleus for the direct and
 Axons from the oculomotor
nucleus curve ventrally
through the tegmentum and
the red nucleus.
 The nerve emerges on the
anterior surface of the
midbrain in the
interpeduncular fossa.
 Then it passes forward
between posterior cerebral
and superior cerebellar
arteries.
 In the middle cranial fossa
it runs in the lateral wall of
the cavernous sinus, then it
divides into superior and
inferior divisions which
enter the orbit through the
superior orbital fissure.
 Axons from the
Edinger-Westphal
nucleus accompany
the oculomotor nerve
fibers to the orbit,
where they terminate
in the ciliary
ganglion.
 Postganglionic
Fibers pass through
the short ciliary
nerves to the eyeball,
where they supply:
 Constrictor pupillae
muscle of the iris and
ciliary muscle.
• CN III usually divides into
superior and inferior divisions
after passing through the
annulus of Zinn in the orbit
Occulomotor nerve supplies:
 Motor to:
• Superior division:
Superior rectus .
Levator pelpabrae superioris.
• Inferior division:
Inferior rectus.
Medial rectus.
Inferior oblique.
Occulomotor nerve
supplies:
 Parasympathetic fibers
to :
1- Constrictor pupillae
and
2- Ciliary muscles.
It is responsible for;
Elevation of upper eyelid.
Turning the eye upward,
downwards and medially,
Constricting the pupil.
Accommodating reflex of
the eyes.
Occulomotor nerve lesion
• Lesion results in:
– Lateral squint.(MR)
– Ptosis.(LPS)
– Diplopia.
– Pupillary dilatation.(SP)
– Loss of
accommodation.(CM)
– Impaired downward
(IR)& extorsion
(IO)movement of the eye
ball on the damaged side.
Trochlear Nerve
 Type: motor
 Small motor nucleus
located in the
periaqueductal grey
matter at the level of
inferior colliculus.
 Fibers curve
backwards and
decussate
 The nerve emerges
immediately caudal
to the inferior
colliculus, on the
dorsal surface of
brain stem.
 It passes forward through
middle cranial fossa in the
lateral wall of the cavernous
sinus.
 The nerve then enters the
orbit through the superior
orbital fissure.
It supplies;
 Superior oblique muscle,
(only one muscle).
Its function;
Primarily rotates the tip of the
eye towards the nose (Intorsion)
Secondarily moves the eye
downwards (depression)
Tertiarily moves the eye
outwards ( abduction)
Rotates the eye ball downwards
and laterally
Trochlear Nerve
lesion
 Lesion results in
diplopia &
 Inability to rotate the
eye infero-laterally.
 So, the eye deviates;
upward and slightly
inward.
 This person has
difficulty in walking
downstairs.
Abducent Nerve
 Only one motor nucleus.
 Lies in caudal pons in
the floor of the 4th
ventricle.
 It forms the facial
colliculus with the fibers
of facial nerve looping
around the nucleus.
 It emerges from the
ventral aspect, at the
junction of the pons and
the pyramid of the
medulla.
Abducent Nerve
 It passes through
cavernous sinus, lying
below and lateral to the
internal carotid
artery
 Then it enters the
orbit through the
superior orbital
fissure.
 It supplies; the lateral
rectus muscle which
rotates the eye ball
laterally ; (abduction).
Abducent nerve
lesion
Lesion results in:
 Inability to direct the
affected eye laterally.
(medial squint).
 A nuclear lesion may
also involve the nearby
facial nucleus or axons
of the facial nerve,
causing paralysis of all
the ipsilateral facial
muscles.
Arterial system
Internal carotid artery
Ophthalmic artery
Central retinal A
Short post ciliary A
Long post ciliary A
Anterior ciliary A
Lacrimal A
Superior muscular A
Inferior muscular A
Posterior ethmoidal A
Anterior ethmoidal A
Supraorbital A
Supratrochlear A
Venous system
Central retinal V
Superior vortex V
Superior episcleral plexus
Inferior vortex V
Inferior episcleral plexus
Pterygoid plexus
Superior
ophthalmic V
Cavernous
sinus
Jugular v