ANATOMY OF THE VISUAL PATHWAY
what does visual pathways mean ?
The visual pathway starting from retina consists of
optic nerves, optic chiasma, optic tracts, lateral
geniculate bodies, optic radiations and the visual
cortex
Optic nerve
Each optic nerve (second cranial nerve) 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 axons originating from the ganglion cells. It
also contains the afferent fibres of the pupillary light
reflex.
Morphologically and embryologically, the optic
nerve is comparable to a sensory tract. Unlike
peripheral nerves it is not covered by neurilemma (so
it does not regenerate when cut). The fibres of optic
nerve, numbering about a million, are very fine (2-10
µm in diameter as compared to 20 µm of sensory
nerves).
Parts of optic nerve.
The optic nerve is about 47-50
mm in length, and can be divided into 4 parts:
intraocular (1 mm), intraorbital (30 mm), intracanalicular (6-9 mm) and intracranial (10 mm).
1. Intraocular part passes through sclera
(converting it into a sieve-like structure—the
lamina cribrosa), choroid and finally appears inside
the eye as optic disc.
2. Intraorbital part extends from back of the eyeball to the optic foramina. This part is slightly
sinuous to give play for the eye movements.
Posteriorly, near the optic foramina, it is closely
surrounded by the annulus of Zinn and the
origin of the four rectus muscles. Some fibres of superior rectus muscle are adherent to its sheath
here, and accounts for the painful ocular
movements seen in retrobulbar neuritis. Anteriorly, the nerve is separated from the ocular
muscles by the orbital fat.
3. Intracanalicular part is closely related to the
ophthalmic artery which lies inferolateral to it and
crosses obliquely over it, as it enters the orbit, to
lie on its medial side. Sphenoid and posterior
ethmoidal sinuses lie medial to it and are separated
by a thin bony lamina. This relation accounts for
retrobulbar neuritis following infection of the
sinuses.
4. Intracranial part of the optic nerve lies above
the cavernous sinus and converges with its fellow
(over the diaphragma sellae) to form the optic
chiasma.
Meningeal sheaths.
Pia mater, arachnoid and dura
covering the brain are continuous over the optic
nerves. In the optic canal the dura is firmly adherent
with the surrounding bone. The subarachnoid and
subdural spaces around the optic nerve are also
continuous with those of the brain.
Optic chiasma
It is a flattened structure measuring 12 mm
(horizontally) and 8 mm (anterioposteriorly). It lies
over the tuberculum and diaphragma sellae. Fibres
originating from the nasal halves of the retina
decussate at the chiasma.
Optic tracts
These are cylindrical bundles of nerve fibres running
outwards and backwards from the posterolateral
aspect of the optic chiasma. Each optic tract consists
of fibres from the temporal half of the retina of the
same eye and the nasal half of the opposite eye.
Posteriorly each optic tract ends in the lateral
geniculate body. The pupillary reflex fibres pass on
to pretectal nucleus in the midbrain through the
superior brachium. some fibres terminate in the
superior colliculus.
Lateral geniculate bodies
These are oval structures situated at the posterior
termination of the optic tracts. Each geniculate body
consists of six layers of neurons (grey matter)
alternating with white matter (formed by optic fibres).
The fibres of second-order neurons coming via optic
tracts relay in these neurons.
Optic radiations
These extend from the lateral geniculate bodies to
the visual cortex and consist of the axons of third order neurons of visual pathway.
Visual cortex
It is located on the medial aspect of the occipital lobe,
above and below the calcarine fissure. It is subdivided
into the visuosensory area that
receives the fibres of the radiations, and the
surrounding visuopsychic area.
Blood supply of the visual pathway
The visual pathway is mainly supplied by pial network
of vessels except the orbital part of optic nerve which
is also supplied by an axial system derived from the
central artery of retina. The pial plexus around
different parts of the visual pathway gets contribution
from different arteries
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