Effects of peripheral, rubral, and fastigial stimulation on neurons of the lateral reticular nucleus of the cat

Responses of single neurons of the lateral reticular nucleus (LRN) were investigated after stimulation of the red nucleus, of the fastigial nucleus, and of peripheral nerves in 42 cats anesthetized with chloralose. After rubral and fastigial stimulation, latencies shorter than 2 msec and driving at stimulus rates of 250–700/sec have been observed. This demonstrates the existence of monosynaptic excitatory pathways from both nuclei to the LRN which possibly end in two different groups of LRN neurons. Stimulation of the forepaws elicited excitatory and, occasionally, inhibitory responses with a mean latency of about 20 msec. Some of the LRN neurons were identified by their excitability to antidromic stimulation of the ipsilateral cerebellar cortex. The direct rubroreticular and fastigioreticular pathways are limbs of two feedback loops running from the LRN to the cerebellar cortex and from there via the fastigial nucleus or the interposite and red nuclei back to the LRN. Because of the inhibitory function of the Purkinje cells, these loops might mediate the delayed inhibition observed after stimulation of afferent fibers to the LRN.