The optic tract is a large bundle of nerve fibers of the visual pathway. It is a paired structure located in both left and right sides of the brain. The origin of the optic tract is the optic chiasm, an X-shaped structure located just above the pituitary gland (or hypophysis), in which optic nerve fibers partly decussate (meaning they cross to the other side, intersecting one another). The optic tract then extends caudally and laterally to end in the lateral geniculate bodies of the thalamus.
Almost all axons of the left and right optic tracts synapse with the cells of their corresponding lateral geniculate nucleus (LGN). However, some fibers of the optic tract, particularly the fibers concerned with the light reflexes, bypass the lateral geniculate nucleus and continue to the pretectal nucleus and superior colliculus of the midbrain.
The organization of the fibers that carry visual information in the optic tract and the lateral geniculate nucleus is specific, which is known as retinotopy:
- In the optic tract, the fibers from the superior retinal quadrants lie laterally, while inferior retinal fibers run medially.
- The two lateral geniculate nuclei receive input from both eyes. However, each nucleus only receives information from one half of the visual field, due to the decussation of fibers in the chiasm. The axons of the ganglion cells from the outer half of the retina (temporal side/field) remain on the same side of the brain, and the axons from the inner half (nasal side/field) cross over to the contralateral side. Within one nucleus, the visual information is divided among the various layers.
The efferent fibers from the lateral geniculate nucleus emerge as the optic radiation and travel to the primary visual cortex located in the occipital cortex of the brain (Brodmann's area 17) where the visual processing continues.
The blood supply to the optic tract is variable, but typically arises from anastomotic branches of the posterior communicating and anterior choroidal arteries, together with branches from the middle cerebral arteries.
The venous drainage is from the superior aspect through the anterior cerebral veins and from the inferior aspect through the basal vein.
|Terminology||English: Optic tract
Latin: Tractus opticus
|Definition||A bundle of nerve fibers that extends from the optic chiasm to the left and right lateral geniculate bodies|
|Function||Major part of the visual pathway; transmits the visual stimuli from the optic chiasm|
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Optic pathway lesions
Loss of vision in one half (right or left) of the visual field is called hemianopsia. Hemianopsia is defined in relation to the visual field and not to the retina. In lesions of the visual pathway, macular vision is often spared due to the large size of the macular area, and because some areas have a double blood supply (from posterior and middle cerebral arteries).
If the same half of the visual field is lost in both eyes the defect is said to be homonymous, and if different halves are lost, the defect is said to be heteronymous. The injuries of the different parts of the visual pathway will produce different symptoms:
- Injury to the optic nerve will produce total blindness in the eye concerned.
- Damage to the central part of the optic chiasm (e.g. by pressure from an enlarged hypophysis) interrupts the crossing fibers derived from the nasal halves of the two retinae, resulting in bitemporal heteronymous hemianopsia.
- When the lateral part of the optic chiasm is affected, a nasal hemianopia results. This may be unilateral or bilateral.
- Complete destruction of the optic tract, the lateral geniculate body, the optic radiation, or the visual cortex of one side, results in loss of the opposite half of the field of vision.
A lesion on the right side of the optic tract may lead to left homonymous hemianopia. Partial injury to the optic tract may affect only one quadrant of the visual field. The resulting condition is called quadrantanopsia.