A quantitative comparison of stimulus-response relationships of vibrissa-activated neurons in subnuclei oralis and interpolaris of the rat's trigeminal sensory complex: receptive field properties and threshold distributions.

Electrical activity of single vibrissa-activated neurons was recorded in pars interpolaris and pars oralis of the nucleus of the trigeminal spinal tract of rats. Stimuli consisted of quantitatively controlled deflections of individual mystacial vibrissae. The evoked spike trains were analyzed as point-process time series with a variety of quantitative procedures. Most second-order trigeminal neurons were directionally sensitive. About one-third of interpolaris neurons and over half of oralis neurons responded to axial push of the hair shaft. About half of the neurons of both oralis and interpolaris had receptive fields that included more than one vibrissa. The upper-quartile receptive field size of neurons of interpolaris was about half that of oralis, although the distributions overlapped. In interpolaris, almost one-fifth of the sample discharged in the absence of an intentionally applied stimulus; in oralis, over one-third of the sample displayed background activity. The ranges of angular displacement thresholds of both samples exceeded three orders of magnitude. The distributions differed quantitatively. They were, nevertheless, similar in shape, and exhibited considerable overlap. The median threshold of oralis neurons was about half that of interpolaris neurons, and about one-fifth that of first-order neurons. About one-fourth of the second-order neurons exhibited a nonmonotonic relationship between pulse displacement and the number of evoked spikes. Neurons of interpolaris tended to be more severely nonmonotonic than those of oralis, and some "turned off" at stimulus magnitudes well above threshold. Nevertheless, the number of spikes evoked in either entire sample was a monotonically increasing function of pulse displacement. The range of angular velocity thresholds observed in both second-order samples exceeded three orders of magnitude. As with the angular displacement thresholds, the distributions of angular velocity thresholds were quantitatively different, although their shapes and extremes were similar, and they overlapped extensively. The observed differences of stimulus-response relationships of the neurons in interpolaris and oralis reflect differences in the ways different trigeminal nuclei receive, process, and distribute information to the circuits in which they participate.