MODULE 5
INTRODUCTION
- - From a clinical point of view, this may be the most important topic we take up in this sequence of Neuroscience labs. We will base our treatment on the model proposed by Lawrence and Kuypers and shown in Figure 5-1. For decades the traditional teaching has been that there is a single descending motor pathway which, when activated, brings about movement. It is called the pyramidal tract, so named because all the descending axons of the tract pass through the pyramids of the medulla. It is also known as the corticospinal tract, because it is the only pathway in which cortical neurons give rise to axons which descend all the way to terminate in the spinal cord. Studies in the 1950s - particularly the clinical reports by a prominent neurosurgeon named Paul Bucy - raised doubts about the concept that the pyramidal tract was the only descending motor pathway and this led to the landmark studies of Lawrence and Kuypers. Their findings suggested that there are, in fact, two descending motor systems. They named one of these systems the lateral system; it includes the pyramidal tract and another pathway that descends through the lateral part of the brainstem - the rubrospinal tract. They called their second group of pathways the medial system. It includes several tracts, all of which pass downward through the medial part of the brainstem. Your job is to trace these pathways on our slides. Your job will be made easier by the fact that Blumenfeld lays each of them out quite clearly in Figure 6.11 and summarizes them clearly in his Table 6.3.
- - The concept of medial and lateral descending pathways seems to be generally accepted, and we stick with it in this module. However there is growing doubt about the role of the rubrospinal tract as a component of the lateral system. Kandel supports a motor role for the tract (see Kandel pages 668-9) and Haines mentions it in a guarded way on page 408, but Fitzgerald says it is a "negligible" pathway. For those who doubt the importance of the rubrospinal tract, the lateral pathway becomes, simply, the lateral corticospinal tract. The problem is, however, that following section of the pyramidal tract within the cerebral peduncle, motor impairment is modest, and mainly involves loss of fine movements in the contralateral hand (Bucy's observation). If the rubrospinal pathway isn't taking over the motor role of the pyramidal tract in this case, as Lawrence and Kuypers postulated, what pathway is doing it? So far, there are no candidates. Stay tuned.
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