Lab 7 - Visual System

Central Visual Pathways - Cortical Structures

In Figure 1 locate the lateral geniculate body, which is next to the medial geniculate body. The axons of the lateral geniculate nucleus form the optic radiations (geniculocalcarine fibers) in the sublenticular and retrolenticular segments of the internal capsule. Note that the sublenticular optic radiation forms a major component of the external sagittal striatum while the retrolenticular optic radiation forms part of the posterior thalamic radiations. The retrolenticular fibers travel caudally to terminate in the superior lip of the calcarine fissure. The sublenticular fibers take an indirect course to the calcarine cortex: the sublenticular optic radiations pass around the rostral pole of the temporal horn and swing caudally in Meyer's loop to end in the inferior lip of the calcarine fissure.

Figure 2 is a Nissl stained section illustrating the laminar organization of the lateral geniculate nucleus (LGN). Note that the LGN consists of 6 layers, which are folded about the middle to form a dome-like structure with its concave surface lying at the inferior surface of the nucleus. The two inferior, thinner, layers are the magnocellular layers of the LGB (mLGB) while the four superior, thicker, layers are the parvocellular layers (pLGB).

Figure 3 is a Nissl stained section through the visual cortex. The striate area of the calcarine (primary visual) cortex (Area 17) is differentiated from other cortical areas by the layered appearance of its Nissl-stained cell soma. Recall that the axons of the pLGB and mLGB neurons terminate on stellate cells in sublayers of Layer IV (the internal granular layer) and the major outflow of the primary visual cortex arise from pyramidal cells in Layers III, V, and VI.