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The axons of the second order neurons originating from the lateral geniculate bodies form upper and lower optic radiations on each side. The upper optic radiations contain fibers from the upper hemi retina while the lower optic radiation contains fibers from the lower hemi retina. These tracts pass through the retrolenticular limb of the internal capsule (the posterior limb continues as the retrolenticular limb) and terminate in the upper and lower lip of the calcarine sulcus {the parieto occipital sulcus on the medial side gives rise to a posterio-inferior sulcus-the calcarine sulcus. Above and below the calcarine sulcus each there is an inner band of primary visual area and an outer band of secondary visual area-Snells 290 fig 8-4 B}.    

            
            http://wdict.net/img/optic+radiation.jpg  -open this link to view relevant diagram.

 

Note: the lateral geniculate body has 6 layers. The fibers in an optic tract from the ipsilateral eye go to layer 2,3 & 5 while the fibers from the contralateral eye go to the remaining layers.

 

Look at FIG 1 for the 5 lesions and the visual fields affected on right. The grey side indicates the side of visual field affected.

1.   TOTAL BLINDNESS OF ONE EYE: when there is lesion completely across the optic nerve.

2.   NASAL HEMI-ANOPIA: when there is a lesion across the lateral fibers of the optic nerve. These lateral fibers have come from the temporal hemi retina which in turn has come from the right side of the left visual field and left side of the right visual field. Anopia means blindness.

3.   BITEMPORAL HEMI-ANOPIA: when there is a lesion across the central part of chiasma, the contralateral fibers are affected only. The right side of the right visual field is and the left side of the left visual field is affected. The pituitary gland moves anteriorly towards the chiasma. Pituitary adenoma (tumour) may cause lesion in chiasma.

4.   CONTRALATERAL HOMONYMOUS HEMI-ANOPIA: any lesion after the optic chiasma, i.e. in the optic tract or optic radiation causes blindness in the right sides of both right and left visual field, if the left tract or radiation is affected. Blindness in the left sides of both right and left visual field occurs if the right tract or radiation is affected.

 

                   PUPILLARY SKIN REFLEX:

When your skin is pricked, afferents from the skin synapse with cell-bodies in the lateral grey horn at the thoracic level. From the lateral grey horn, the second order neurons rise up the sympathetic trunk to synapse with the ganglion in the sympathetic trunk at the cervical level. From the sympathetic trunk the third order neurons originate and synapse with the dilator pupillae, to dilate the pupil.