The Optic Tract
The axons of the retinal ganglion cells converge to form the optic nerve, which after a partial decussation at the optic chiasm forms the optic tract. The optic tract is an important part of the visual pathway. Almost all of the axons of the left and right and left optic tracts synapse with the cells of the ipsilateral lateral geniculate nucleus. The efferent fibres emerge as the optic radiation and ultimately travel to the primary visual cortex.
Optic chiasmThe optic tract arises from the optic chiasm, which lies in the subarachnoid space above the pituitary gland. The optic nerve just behind the eyeball, contains the temporal and nasal fibres which are situated on their respective sides. Near the optic chiasma the macular fibres are central, becoming superoposterior within the chiasm. The nasal fibres cross and join the axons of the contralateral hemitemporal retina in the chiasma. The crossed fibres are in a greater number (53%) compared to the uncrossed fibres (47%).
Each optic tract contains the fibres from the ipsilateral temporal and contralateral nasal retina. Thus, the right optic tract contains fibres from the right halves of the right and left retinae, and the left optic tract from the left halves of the right and left retinae of the eyeballs. In other words, all optic nerve fibres carrying impulses relating to the left half of the field of vision are brought together in the right optic tract and vice versa, ensuring bilateral vision.
The fibres from the lower retinal quadrants lie laterally, while upper retinal fibres go medially. The macular fibres are positioned dorsolaterally in the optic tract. Most of the optic tract fibres synapse in the lateral geniculate body, while a few of the fibres concerned with the light reflexes pass to the pretectal nucleus and superior colliculus. There is a retinotopic organization in the lateral geniculate nucleus. Fibres from the superior and inferior parts of the retina go to the medial and lateral horns respectively, while macular vision is represented in the hilum. There are six neuronal layers in the lateral geniculate nucleus, uncrossed fibres (temporal retina) synapse in layers 2,3 and 5, while crossed axons synapse in neuronal layers 1,4 and 6. Optic radiation and visual impulses commence here. They are ultimately interpreted in the primary visual areas of the cerebral cortex and an appropriate response is relayed. The optic radiation, also known as the geniculo-calcarine tract, is a relay centre receiving about 80% of the optic tract. The remaining fibres of the optic tract terminate in the pretectal nuclei and superior colliculus.
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.
Optic tract lesions
The nasal field is also referred to as the medial field, while the temporal field is the lateral field of vision. Loss of vision in one half (right or left) of the visual field is called hemianopia. Hemianopia is defined in relation to the visual field and not to the retina. It may be noted that in lesions of the visual pathway, macular vision is often spared. This is 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. Injury to the optic nerve will produce total blindness in the eye concerned. Damage to the central part of the optic chiasma (e.g., by pressure from an enlarged hypophysis) interrupts the crossing fibres derived from the nasal halves of the two retinae, resulting in bitemporal heteronymous hemianopia. When the lateral part of the chiasma is affected, a nasal hemianopia results. This may be unilateral or bilateral. However, 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 quadrantic anopia.