VISUAL SYSTEM PATHWAYS

A. Optic nerve -- Optic Tract -- Lateral Geniculate -- Primary Visual Cortex (==>)

1. Optic nerve and tract

a. ganglion cell axons form optic nerve

b. optic nerve axons from the nasal half of each retina cross to the opposite side in the optic chiasm, while axons from the temporal half of each retina do not cross

Note: after passing the chiasm, the ganglion cell axons are called the optic tract (same axons, different name)

c. thus, the right visual field is conveyed in the left optic tract and projects to the left lateral geniculate; the left visual field is conveyed in the right optic tract and projects to the right lateral geniculate

d. visual field relations are preserved in the lateral geniculate and its projections to the visual cortex

Note: Receptive field in the visual system is the referred to the external world, and is termed the Visual Field (e.g. left, right, nasal, temporal)

Note: Know the effects of the loss of vision due to pathway lesions

Note:  So much of the optic projection represents the macula that macular vision is often retained even when peripheral vision is lost through lesions;  termed macular sparing

2. Lateral geniculate nucleus (==>)

a. each nucleus receives input from opposite visual field of both eyes

b. organization preserves spatial relations of retinal image

c. organization preserves separate input from the right and left eyes

d. cells arranged in six layers conveying separate submodalities from the two eyes

Magnocellular layers (large soma; large receptive fields, movement sensitive), layers 1-2
layer 1: M cells from contralateral eye
layer 2: M cells from ipsilateral eye

Parvocellular (smaller soma; small receptive fields, detail and color sensitive), layers 3-6
layers 4 & 6: P cells from contralateral eye
layers 3 & 5: P cells from ipsilateral eye

e. each layer topographically organized by visual field

f. output: to primary visual cortex

g. M cells have large receptive fields and respond particularly to movement and changes in illumination; P cells have smaller receptive fields and are involved in perception of detailed shape (form) and color

h. for the most part, information to the retina is relayed to the visual cortex relatively unchanged

i. the lateral geniculate also has inputs from other brain regions; their function is unknown but may modulate visual input to the cortex

j. note that in the projections from the thalamus to the cortex, the separation of the two eyes, the properties of the visual message (M or P), and the topographical mapping are maintained