Functional organization of the parvocellular red nucleus in the cat

The experiments were performed on cats under pentobarbital anesthesia. The following results were obtained. (1) Most of the neurons in the rostral part of the red nucleus (RN) were activated by stimulation of the parietal association cortex (P) and the lateral cerebellar nucleus (CN). A number of the neurons were regarded as rubro-olivary since they responded antidromically to inferior olive (IO) stimulation. (2) Stimulation of the P as well as the frontal motor cortex activated IO neurons. A longer and more variable latency in P-induced responses indicated the existence of an indirect connection from the P to the IO. The recording sites for P-induced responses were located in the rostral region of the IO, giving off projection fibers to the cerebellar hemispheral part. After horseradish peroxidase (HRP) injection through recording microelectrodes into the IO where P-induced responses were recorded, small cells labeled retrogradely with HRP were found distributed in the rostral part of the RN. (3) Effects of stimulus to or lesion of the rostral part of the RN revealed that the climbing fiber responses to P stimulation were conveyed through the rubro-olivary pathway originating in the rostral part of the RN. (4) Following HRP injection through recording electrodes into the rostral part of the RN where P-induced responses were recorded, retrogradely labeled cells were seen and located in the P and CN. From these findings, the rostral part of the RN investigated in this study could be regarded as the rubral parvocellular part. The present study suggests that the latero (dentato-)-rubro-olivo-cerebellar circuit forms an internal feedback loop and the P acts on this loop.