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Optic tract

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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To learn more about the differences between the optic nerve and the optic tract, check out the study unit below:

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