MODULE 3
THE DORSAL COLUMN - MEDIAL LEMNISCUS (DC-ML) PATHWAY
THE THIRD ORDER NEURON
- - As you know, ascending fibers of the medial lemniscus pathway terminate on third order neurons within the nucleus ventralis posterolateralis (hereafter, VPL) of the thalamus. The nucleus is organized in a precise somatotopic manner, as shown in our views of slides 26, 27 and 28; call up Figure 3-5 to confirm that sensory information from the lower part of the body is relayed through the lateral part of VPL, whereas sensory information from the upper part of the body is relayed through the more medial part of the nucleus.
Third order neurons in VPL send their axons laterally into the internal capsule, to ascend toward the cerebral cortex of the parietal lobe. As shown in Figure 1-46, these fibers are initially "squeezed" between the thalamus medially, and the putamen and globus pallidus laterally. As the fibers escape above the edge of the putamen they fan out, as seen in a lateral view such as Figure 1-51. This part of the pathway is known as the corona radiata. The lateral views in Figures 3-6 and 3-7 try to depict this part of the pathway. In Figure 3-6 the putamen and globus pallidus have been removed; in Figure 3-7 they are in place and the fibers of the internal capsule are "hidden" behind (medial to) the putamen.
Hopefully, it is obvious from these figures that the internal capsule/corona radiata is a massive, fan-shaped array of fibers. Through it EVERYTHING passes to and from the cerebral cortex. The pathway we have been studying occupies only a small part of this structure - as Figures 3-6 and 3-7 try to demonstrate. While views of this sort give the best picture of the internal capsule, the neuropathologists and the neuroradiologists are more likely to view it by making horizontal cuts through the brain. When cut in this way, as in our horizontal section #8 (Figure 3-8) the internal capsule has a V shaped appearance. The anterior half of the V is known as the anterior limb of the internal capsule, and the posterior half is called the posterior limb. So we say that the pathway from VPL to the cortex passes through the posterior part of the posterior limb of the internal capsule.
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