Morphology, response properties, and collateral projections of trigeminothalamic neurons in brainstem subnucleus interpolaris of rat

Intracellular recording, electrical stimulation and horseradish peroxidase (HRP) injection techniques were used to delineate the structural and functional characteristics of trigeminothalamic projection neurons in subnucleus interpolaris of the trigeminal brainstem nuclear complex in rat. Eleven such neurons were successfully characterized and recovered. All were medium to large multipolar neurons in the ventral part of interpolaris and all except one also projected to the superior colliculus. Six of these cells also sent axon collaterals to subnucleus principalis and the medullary parvicellular reticular formation and had local collaterals within interpolaris. None of these trigeminothalamic cells were antidromically activated from the cerebellum. All but one of the recovered cells were responsive to deflection of any one of a number (4-19) of vibrissae. The remaining cell was discharged by displacement of mystical guard hairs. Analysis of electrophysiological and anatomical data revealed significant correlations between receptive field size and dendritic area, thalamic conduction latency and axon diameter, and number of targets innervated and axon diameter.