The visual pathway consists of four neurons connected in tandem:
! 1st neuron, the photoreceptors
! 2nd neuron, the bipolar neurons of the ret-ina, which transmit the impulses from the rods and cones to the large ganglion cells of the retina
! 3rd neuron, the large ganglion cells, the axons of which combine to form the optic nerve and extend to the primary visual centers (lateral geniculate nucleus)
! 4th neuron, the geniculate cells, the axons of which project as the optic radiation to the visual cortex (striate area)
The optic nerve (A1) enters the cranial cavity through the optic canal. At the base of the diencephalon, together with the con-tralateral optic nerve, it forms the optic chi-asm(A2). The fiber bundle starting from thechiasm is known as the optic tract (A3). The two tracts run around the cerebral peduncles to the two lateral geniculate bod-ies (A4). Before reaching these, each tract divides into a lateral root (A5) and a medialroot (A6). Whereas most of the fibers runthrough the lateral root to the lateral geniculate body, the medial fibers continue below the medial geniculate body (A7) to the superior colliculi. They contain visual reflex pathways. The optic nerve fibers are thought to give off collater-als to the pulvinar of the thalamus (A8) prior to terminating in the lateral geniculate body. The optic radiation (radiation of Gra-tiolet) (B9) begins at thelateral geniculatebody and extends as a broad fiber plate tothe calcarine sulcus at the medial aspect of the occipital lobe and, while doing so, forms the outward-arching temporal genu (B10) . Numerous fibers bend ros-trally (occipital genu) (B11) in the occipital lobe to reach the anterior areas of the visual cortex.
The optic nerve fibers originating from the nasal halves (B12) of the retina cross in theoptic chiasm. The fibers from the temporal halves (B13) do not cross but continue on the ipsilateral side. Hence, the right tract contains the fibers from the temporal half of the right eye and from the nasal half of the left eye. The left tract contains fibers from the temporal half of the left eye and from the nasal half of the right eye. In a cross section of the tract, the crossed fibers lie mostly ventrolaterally, and the uncrossed fibers dorsomedially; in between, the fibers are mixed.
The crossed and uncrossed fibers of the optic tract extend to different cell layers of the lateral geniculate body. The number of geniculate cells, approximately one million, corresponds to the number of optic nerve fibers. However, the optic nerve fibers usually end on five to six cells located in different cell layers. Cor-ticofugal fibers of the occipital cortex also end in the lateral geniculate body. They probably control the input of impulses, as suggested by the pres-ence of axo-axonal synapses characteristic for presynaptic inhibition.
The axons of the geniculate cells form the optic radiation. Its fibers are arranged according to the different regions of the retina. The fibers for the lower half of the retina, especially those for the periphery of the retina, arch most rostrally in the temporal genu. The fibers for the upper half of the retina and for the central region of the mac-ula arch only slightly in the temporal lobe.
In the striate area (B14) of the right hemi-sphere terminate the fibers for the right halves of the retinae; hence, it receives sensory input from the left halves of the visual fields. In the striate area of the left hemisphere there terminate the fibers of the left halves of the retinae with input from the right halves of the visual fields. The right hand and the right visual field are therefore both represented in the left hemisphere, which dominates in right-handed persons.
B15 Visual fields.