Comparison of response properties of cerebellar- and thalamic-projecting interpolaris neurons.

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Abstract
Intermingled within the [rat] interpolaris (IP) subnucleus of the medullary trigeminal complex are neurons projecting to the cerebellum and others that project to the ventrobasal thalamus. Antidromic stimulation was used to identify vibrissae-activated IP neurons projecting to folium 9a of the cerebellum and other IP neurons projecting to the ventrobasal thalamus. The response properties of the groups of neurons projecting to these 2 loci were compared. The groups of neurons are referred to as cerebellar projecting (C neurons) and thalamic projecting (T neurons), although the uvula and ventrobasal thalamus represent only 2 of several discrete cerebellar and thalamic targets of IP. IP neurons might have branching axons reaching both the cerebellum and thalamus. The antidromic collision techniques revealed that only 1 of the 80 IP neurons that were studied, projected to both structures. If branching of IP projection axons does occur, it is between cerebellar and thalamic loci other than the 2 studied, which receive dense projections from IP. A wide range of measurements was made on each neuron, including its responses to precisely controlled mechanical stimulatin of vibrissae, to ascertain response characteristics as fully as possible. Cerebellar-projecting neurons and thalamic-projecting neurons had significantly different population distributions for several of the variables: cerebellar-projecting neurons had smaller receptive fields and more directional sensitivity than did thalamic-projecting neurons. Furthermore, thalamic-projecting neurons were more likely to have nonmonotonic responses to increasing deflection amplitudes. Although significant interpopulation differences were found for several single variables, none could reliably distinguish between cerebellar- and thalamic-projecting neurons. A multivariate approach to population characterization was undertaken using a discriminant-function analysis program. This program calculated a discriminant function based on 9 variables that correctly classified all but 7 of the neurons as to projection locus. The cerebellar-projecting neurons sampled had smaller receptive fields than the thalamic-projecting neurons which, is consistent with some recent literature. The possibility of stimulus-intensity coding by neruons with nonmonotonic amplitude response functions is also discussed.