Medial reticular and perihypoglossal neurons projecting to cerebellum.

Almost 10% of neurons in the medial reticular nucleus or adjacent thereto were invaded antidromically in response to stimulation of the fastigial and interpositus nuclei. The fraction was 77/835 for the bulbar and caudal pontine levels, but 0/167 for rostral pontine levels. The mahority, 49, of the neurons projecting to the cerebellum were superficially located in the region of the perihypoglossal nucleus, but 23 were scattered through the medial reticular nucleus, being 2.5-5.0 mm below the bulbopontine dorsum. Both classes of cerebellopetal neurons had a similar range of antidromic latencies, usually from 0.8 to 2.0 ms, but some were ober 3 ms. Both classes responded to volleys from limb nerves and inputs from cutaneous mechanoreceptors, with ranges of excitatory and inhibitory latencies that were similar to those for other medial reticular neurons. It is conjectured that the axonal projection is primarily to the cerebellar cortex and that the branches to the nuclei are often slender, hence the long antidromic latencies; 31 of 59 neurons tested projected to cerebellar nuclei on both sides, often with a considerable latency differential. Rarely, there were also axonal branches projecting up the central tegmental tract. The experimental findings are in very good accord with the anatomical descriptions of Brodal and associates (4, 5, 8, 19). It is suggested that the paramedian reticular and the perihypoglossal nuclei may provide a background excitatory input to the interpositus nuclei.