Cerebellar activation of cortical motor regions: comparisons across mammals

This chapter discusses the nature of mammalian cerebello-thalamo-cortical projections. These play an important role in motor control, but with species differences evident in the innervation patterns of thalamo-cortical (T-C) fibers relaying cerebellar inputs. A phylogenetic comparison of the mode of cerebellar activation of cortical motor regions reveals that cerebellar inputs are relayed by the deep and superficial T-C projections in the cat, but predominately by the latter in rat and monkey. Another difference across mammals is the nature of cerebellar activation of cortical neurons. Fast-conducting pyramidal tract neurons in the cat routinely receive fast-rising EPSPs from the deep cerebellar nuclei (CN). This tendency is not observed in rat and monkey, however. These findings suggest that the responsiveness of cortical output neurons also shows species differences, these having bearing on the development of species-specific motor skills. Common to all three species, however, are fast-rising and large CN-EPSP responses of layer III pyramidal neurons to CN input. It is argued that layer III pyramidal neurons modulate cerebellar input to layer V pyramidal neurons, which latter cells provide command signals from the motor cortex to the lower centers.