The visual system or the optic pathway transmits visual information from the retina within the eyes to the primary visual cortex of the occipital lobe as well as the pretectal nuclei and superior colliculi of the midbrain.
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Gross anatomy
Below the visual pathway is described from distal to proximal in a single hemisphere.
Extracranial course
Bipolar cells are the first-order neurones within the retina that receive signals from rods (80:1 ratio) and cones (1:1 ratio). They synapse with ganglion cells locally.
Ganglion cells are the second-order neurones that lie in the inner membrane of the retina. Their axonal fibres enter via the optic discs to continue as the optic nerve.
The optic nerve (cranial nerve II) contains both nasal and temporal fibres from one eye. Distally, it is covered by meninges. As it passes through the optic canal, the meninges leave its outer coat.
Intracranial course
The optic nerve continues intracranially.
The optic chiasm is a midline junction where nasal fibres of each optic nerve decussate. It forms the anterior part of the floor of the third ventricle. A lesion here results in bitemporal hemianopia.
The optic tract contains fibres transmitting information from the contralateral visual field. A lesion here results in contralateral homonymous hemianopia. The optic tract passes around the cerebral peduncle, high up against the temporal lobe. The tract splits into two. The larger visual fibres head to the lateral geniculate body. The smaller fibres for the light reflex pass between the lateral and medial geniculate bodies to the midbrain.
Visual fibres
The lateral geniculate body of the posterior part of the thalamus contains the third-order neurones of the visual system. Of its 6 layers, layers 2, 3, and 5 receive temporal fibres. Layers 1, 4, and 6 receive nasal fibres.
The optic radiation (geniculocalcarine tract) is the white matter tract from the lateral geniculate body to the primary visual cortex. It travels via the retrolentiform segment of the internal capsule. Some fibres pass directly through the deep part of the parietal lobe to reach the visual cortex of the occipital lobe above the calcarine sulcus. Other fibres form the Meyer loop around the inferior temporal horn, passing deep to the temporal lobe, and reaching the occipital lobe, inferior to the calcarine sulcus. A lesion of the Meyer loop results in contralateral superior quadrantanopia.
The primary visual cortex (Brodmann area 17) receives visual information via the geniculocalcarine tract. It is located on the medial surface of the occipital lobe. The region lies above and below the posterior limb of the calcarine sulcus, and extends anteriorly, below the anterior limb of the calcarine sulcus.
The primary visual cortex registers information from the contralateral side. Upper and lower visual fields are crossed. Central vision is represented more posteriorly (macula is most posterior), while peripheral vision is represented progressively more anteriorly.
The visual association areas are located in Brodmann areas 18 and 19.
Fibres for the light reflexes
The superior brachium contains axonal fibres that bypass the thalamus to project to the tectum of the midbrain.
The superior colliculus receives a signal and mediates the general light reflex.
The pretectal nuclei receive a signal and mediate the pupillary light reflex.
Blood supply
retina and distal 1/3 of optic nerve in orbit: central retinal artery
proximal 2/3 of optic nerve in orbit: ophthalmic artery
intracranial optic nerve and optic chiasma: anterior cerebral artery small branches and superior hypophyseal artery
optic tract: small branches of the anterior choroidal artery and posterior communicating artery
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