Parallel distributed network characteristics of the DSCT

1. We examined the functional organization of the dorsal spinocerebellar tract (DSCT) and found that it is similar to that of a parallel distributed network having widespread connectivity among parallel elements. The prevailing view is that the DSCT provides receptor-specific information to the cerebellum regarding muscle and cutaneous inputs from the hindlimbs, but that view does not consider the convergent inputs to DSCT neurons from multimodal polysynaptic pathways. 2. Spontaneously active DSCT neurons respond to peripheral stimulation with changes in their firing probability. We characterized the temporal patterns of poststimulus excitability changes for a large number of neurons using principal component analysis. The response of each neuron was represented by a response vector in three-dimensional principal component space, in which similar vectors represent responses having a similar waveform for their poststimulus activity patterns. 3. We compared the responses of large populations of DSCT units to two types of stimuli: small (3-8 deg) passive rotations of the foot at the ankle of an intact limb (234 cells) and stretch or contraction of an isolated muscle group (gastrocnemius-soleus, 168 cells). Most of the cells tested had significant responses (P < 0.05) to both types of stimuli (40-78% responded to muscle stimulation and 88% to foot rotation), and they exhibited similar patterns of poststimulus activity. Long-lasting inhibitory responses and excitatory responses with a range of peak times (< 10- > 60 ms) were prevalent in all cases. The population response to each stimulus was characterized by the relative incidence of response types among the units in a representative sample of the population. 4. The time course of excitability changes induced in DSCT cells by the stimuli could have been determined primarily by the presynaptic circuitry or by postsynaptic factors intrinsic to the DSCT cells. The evidence presented suggests that the selection of response waveforms and their distribution among the DSCT cells was determined presynaptically. We found that individual cells were capable of diverse responses to different stimuli. 5. Sample groups of 7-30 cells were selected at random and also on the basis of the similarity of their responses to one type of stimulus. The distributions of response types among the cells of the sample groups were compared to the distributions for the entire population recorded for each stimulus.(ABSTRACT TRUNCATED AT 400 WORDS)