Synaptic organization of the cerebello-thalamo-cerebral pathway in the cat. III. Cerebellar input to corticofugal neurons destined for different subcortical nuclei in areas 4 and 6

Abstract

To analyze the cerebellar effects on corticofugal neurons destined for different subcortical nuclei, intracellular recordings were made from corticofugal neurons in areas 4 and 6 of the cat. Corticonuclear neurons to the red nucleus (RN) and the pontine nucleus (PN), and pyramidal tract neurons (PTNs) with collaterals to these nuclei were identified by their antidromic responses to the stimulation of these nuclei and the pyramid. Three types of RN-projecting neurons (corticorubral neurons (CRNs), corticopontine neurons (CPNs) with a collateral to the RN and PTNs with a collateral to the RN) and two types of PN-projecting neurons (CPNs and PTNs with a collateral to the PN) were differentiated. Furthermore, these corticofugal neurons were classified as fast and slow neurons on the basis of a critical axonal conduction velocity of 20 m/s. About 80% of 98 RN-projecting neurons in area 4 were PTNs, and among the rest, CPNs were more common than CRNs. A similar tendency of the frequency distribution of 37 RN-projecting neurons was also observed in area 6. In area 4, about 70% of 158 PN-projecting neurons were PTNs (80 fast and 30 slow PTNs) and the rest were CPNs, while in area 6, only 35% of 99 PN-projecting neurons were PTNs (10 fast and 25 slow PTNs). Among the CPNs in areas 4 and 6, slow CPNs were more frequently encountered. Cerebellar effects on these identified corticofugal neurons were investigated, using electrical stimulation of the brachium conjunctivum (BC). In both areas 4 and 6, a substantial number of fast conducting CRNs, CPNs and PTNs projecting to the RN or the PN received short-latency (pre-dominantly disynaptic), large-amplitude EPSPs from the BC, and a considerable number of slow conducting neurons to the RN and/or the PN received longer-latency, smaller-amplitude EPSPs from the BC.