Trigeminothalamic neurons in nucleus caudalis responsive to tactile, thermal, and nociceptive stimulation of monkey's face.

1. A total of 113 trigeminothalamic neurons and over 200 presumed interneurons of nucleus caudalis (0-5 mm below the obex) and subjacent reticular formation were studied in rhesus monkeys anesthetized with chloralose or nitrous oxide. Each cell was characterized in terms of its antidromic responses to stimulation of ventral posterior medial and/or posterior thalamic nuclei and to three types of stimuli applied to its receptive field: a) graded 5-s temperature shifts at a rate of 9 degrees C/s from 35 degrees C to final temperatures of 20-52 degrees C, generated by a contact thermode; b) graded intensities of electrical stimulation to determine the conduction velocities of converging primary afferent fiber populations; and c) mechanical stimulation ranging from light touch to pinch with serrated forceps. 2. This analysis yielded five classes of units distinguished by the range of responses to mechanical stimuli and by the convergence of different primary afferent fiber populations. These five classes were found among both trigeminothalamic neurons and neurons which could not be antidromically activated. Class 1 units exhibited rapidly adapting responses to hair movement or light touch and received only A-beta primary afferent input. Class 2 units responded to light touch and pressure with maintained discharges and received A-beta primary afferent input. Class 3 units responded maximally to pinch with serrated forceps but also were activated by light touch and pressure. They received A-beta, A-delta, and C fiber input. Class 4 units responded to firm pressure and maximally to pinch with serrated forceps. These units had A-delta and sometimes C fiber input. Class 5 units responded only to pinch with serrated forceps and had exclusive A-delta fiber input. Some cells in all five classes responded antidromically to stimulation of the thalamus. Antidromic action-potential latencies of classes 1,2, and 3 units were shorter than those of classes 4 and 5 units (P less than 0.001). Receptive-field sizes were usually small (1-2 cm2) for classes 1, 2, 4, and 5 units, and larger for class 3 units (one to three trigeminal divisions). The marginal layer of nucleus caudalis contained mostly classes 4 and 5 units, some class 3 units, but no classes 1 or 2 units. The superficial portion of the magnocellular layer contained mostly classes 1 and 2 units, while neurons at the base of this layer contained class 3 units and some classes 4 and 5 units. Cells in the sujacent reticular formation included all 5 classes but showed a tendency to have large receptive fields (greater than 1 trigeminal division). 3. Neurons responding to noxious thermal stimuli (44-52 degrees C) were classes 3 or 4 units. The response patterns of classes 3 and 4 units to noxious thermal stimuli were similar. No classes 1 or 2 units and only one class 5 unit responded to increases in skin temperature. Thermal thresholds ranged from 38 to 50 degrees C and most heat-responsive units responded monotonically to temperatures between 45 and 52 degrees C...