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