Structure-function relationships in rat brainstem subnucleus interpolaris: IV. Projection neurons.

In a companion paper (Jacquin et al., '89), the structure and function of local circuit (LC) neurons in spinal trigeminal (V) subnucleus interpolaris (Sp Vi) were described. The present report provides similar data for 44 projection neurons in Sp Vi. Of these, 25 thalamic, 16 cerebellar, 2 superior collicular, and 1 inferior olivary projecting neurons were studied. The majority responded to vibrissa(e) deflection, and all except 4 of these had multivibrissae receptive fields. The remainder were responsive to either guard hair deflection or indentation of glabrous skin. Latencies to V ganglion shocks were suggestive of monosynaptic activation from the periphery. Sp Vi projection neurons were topographically organized in a manner consistent with that of their primary afferent inputs. Nonvibrissa sensitive cells had diverse morphologies. Morphometric analyses of the more heavily sampled thalamic and cerebellar projecting, vibrissa(e)-sensitive cells indicated the following. (1) As compared to LC neurons, projection neurons had bigger receptive fields, cell bodies, dendritic trees, and axons; less circular dendritic trees; a greater preponderance of spiny dendrites and fewer axon collaterals in Sp Vi. (2) Dendritic tree extent correlated significantly with receptive field size, thus suggesting that dendritic tree size is one mechanism contributing to receptive field size in vibrissae-sensitive projection neurons. (3) V thalamic cells had significantly bigger receptive fields and dendritic trees, and also give off more local axon collaterals, than V cerebellar neurons. Collicular and inferior olivary projecting neurons shared structural and functional attributes with other Sp Vi long-range projecting cells. Structure-function relationships exist for vibrissa-sensitive projection neurons in Sp Vi. The relevant parameters correlating with projection neuron morphology are receptive field size and projection status, whereas for Sp Vi LC neurons the relevant correlative parameter is peripheral receptor association.