MODULE 6
THE SPINO-CEREBELLAR LOOP
- - The next step in this sequence is that the motor cortex, guided in part by information that has been refined by processing in the lateral part of the cerebellar hemisphere, sends motor commands to the spinal cord (and motor nuclei in the brainstem). As shown in Figure 6-9 and Figure 6-10, the descending axons of this pathway give off collaterals as they pass through the basilar pons and these fibers synapse on neurons which in turn project through the middle cerebellar peduncle to terminate in the vermis and intermediate region of the cerebellar hemisphere (on the side contralateral to the one where the motor commands originated). Thus, these regions of the cerebellar cortex receive a "copy" of the motor commands being sent to the spinal cord and motor nuclei of the brainstem.
- - Once the motor commands have reached the spinal cord and movement has ensued, large diameter afferent fibers from muscle spindles and joint receptors measure the displacement that has actually taken place. This proprioceptive information is relayed to the spinocerebellum (intermediate region and vermis) by the spinocerebellar pathways shown in Figure 6-11. This part of the cerebellum makes a comparison between the intended and actual movement. If there is a discrepancy between the two, corrective instructions are sent to the spinal cord.
- - The output of the intermediate region projects to the magnocellular division of the red nucleus by way of the superior cerebellar peduncle. Most of the fibers terminate there. Cells within this division of the nucleus then relay this information down to the cord in the rubrospinal tract. As Blumenfeld notes in his Figure 15.9B, some of the fibers in the superior cerebellar peduncle pass through the red nucleus and continue on, to terminate in the the thalamus. So the information concerned with the adjustment of ongoing motor activity is passed "up" to the cortex - see Figure 6-17 - as well as "down" to the spinal cord. The output of the vermis projects to the vestibular nuclei, and on to the cord in the vestibulospinal tracts, as shown in Blumenfeld's Figure 15.9C.
- - Thinking back to Module 5 you will recognize that the intermediate zone of the cerebellum exerts control over the distal parts of the extremities by acting through the lateral motor system (the rubrospinal tract). In contrast, the vermis exerts control over the proximal parts of the extremities and axial muscle by acting through the medial motor system (the vestibulospinal tracts).
- - The above version of events, regarding the output from the intermediate zone of the cerebellum, is based largely on the one in Blumenfeld. To a slight extent, it relies on the Lawrence and Kuypers model discussed in Module 5 because it assumes the intermediate zone acts on the spinal cord, in part, by way of the rubrospinal tract. So it is informative to compare it with the versions proposed by Fitzgerald, in his Figure 25.10 and Haines, in his Figure 27-18. Haines includes the rubrospinal pathway, as part of the output, but Fitzgerald omits it. Since all versions - Blumenfeld's, Fitzgerald's, Haines' and ours (see Figure 6-17) - stress the involvement of the cortex, as a part of the output from the intermediate zone, the differences are not that great.
Figure 6-12 is included just to remind you that the cerebellum also receives a major input from the contralateral inferior olive. These fibers enter by way of the inferior cerebellar peduncle and terminate by making extensive synaptic contact with purkinje cells. Because of the appearance of these terminals, they are called climbing fibers. All parts of the cerebellar cortex receive this input. The inferior olive, itself, receives extensive inputs from both ascending and descending pathways.
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