Response properties of raccoon cuneothalamic neurons

Microelectrodes were used to record the extracellular activity of 80 single neurons of the main cuneate nucleus (MCN) of raccoons anesthetized with either methoxyflurane or pentobarbital sodium. All 80 MCN neurons had peripheral receptive fields (RFs) that lay entirely on the glabrous surfaces of the forepaw and were responsive to light mechanical stimulation. Neurons were characterized according to the nature of their response to mechanical stimulation of their RFs, as well as to their response to electrical stimulation of the contralateral thalamic ventrobasal complex (VB). All antidromically activated neurons (64% of sample) were histologically verified as falling within the clusters region of the MCN, while synaptically activated neurons (19% of sample), as well as neurons not responsive to VB stimulation (17% of sample), were located in both the clusters and the polymorphic regions. Antidromically activated neurons typically responded with a single fixed-latency spike, although a few responded with a burst of 3 or more spikes. Others responded with a single antidromic spike followed by a train of synaptically activated spikes. In these latter neurons, it was often possible to block the synaptic spikes selectively. MCN neurons were classed according to their response to controlled mechanical stimuli as rapidly adapting (RA), slowly adapting (SA), or Pacinian (Pc). The proportions of neurons falling into these categories did not vary significantly with the type of response to thalamic stimulation, and the overall percentages were 56% RA, 24% SA, and 20% Pc. These figures are very similar to those previously obtained in a sample of primary afferent fibers of the raccoon cervical cuneate fasciculus (L. M. Pubols and Pubols, 1973). Absolute displacement, displacement velocity, and force thresholds, which ranged between 4 and 326 micron, 0.01 and 16.3 micron/msec, and 120 and 3600 mg, respectively, are comparable to those previously found for primary afferents supplying mechanoreceptors of the glabrous surfaces of the raccoon's forepaw. Neither displacement nor force thresholds differed for RA versus SA neurons; however, displacement velocity thresholds were significantly lower for SA than for RA neurons.