The cerebello-thalamo-cortical pathway. Topographical investigation at the unitary level in the cat.

1. The spatial organization of the cerebello-thalamo-cortical relations has been studied in semi-chronic cats. VL neurons were recorded during stimulation of two cerebellar nuclei, the nucleus interpositus posterior (NIP) and the dentate nucleus (ND). 2. Latency histograms of thalamic responses induced from the stimulation of each of two cerebellar nuclei are comparable and suggest that the cerebello-thalamic fibres have similar calibers whether they originate in the NIP or ND. 3. The anatomical localisation of recording sites indicates that the short latency (less than 1.5 msec) thalamic responses (transmitted by a monosynaptic pathway) are confined to a restricted VL region while the long latency responses are dispersed throughout the nucleus. The monosynaptic responses induced by NIP stimulation are localized in the ventrolateral part of the VL. This VL region is connected to the motor cortical area controlling the forelimb. The ND sends projections into a more medial and dorsal region which is principally connected to the motor area controlling the axial musculature and the shoulder. 4. In some cases it has been observed that the same thalamic cell cen be excited from both cerebellar nuclei. The fact that thalamic neurones are not tied specifically to a given afferent pathway is discussed. 5. The cerebellothalamic well as the thalamocortical relations have a divergent pattern. The efferents from each cerebellar site can excite a large number of VL neurons which can be sometimes several millimeters apart. 6. Cerebellar projections on small cortical foci have also been found. The functional interpretation of these two characteristics of the pathway is discussed.