MORPHOLOGY AND DISTRIBUTION OF SPINOTHALAMIC LAMINA-I NEURONS IN THE MONKEY

Lamina I spinothalamic tract (STT) neurons were identified by retrogra de labeling with cholera toxin subunit b (CTb) in monkeys. On the basi s of the criteria of somatal shape and dendritic orientation in horizo ntal sections used in prior work in the cat, three distinct morphologi cal types were recognized: fusiform (F) cells with spindle-shaped soma ta and two main longitudinal dendritic arbors; pyramidal (P) cells wit h triangular somata and three main dendrites oriented primarily longit udinally; and multipolar (M) cells with polygonal somata and four or m ore dendrites directed longitudinally and mediolaterally. Some cells h ad transitional shapes, but cells with indeterminate shapes and a few with small round, unipolar, or eccentric somata were grouped as unclas sified (U). Greater variation appeared in the monkey than had been see n in the cat, and more subtypes were noted. The overall proportions of these cell types were: 47% F, 27% P, 22% M, and 5% U. Differential lo ngitudinal distributions were found over the length of the spinal cord (from the second cervical through the first coccygeal segments). Pyra midal and multipolar cells together predominated in the enlargements, whereas fusiform cells predominated in thoracic segments. We conclude that three distinct morphological types of lamina I STT cells are pres ent in the monkey as in the cat. Considered with other recent findings , the present results support the possibility that these three cell ty pes may correspond to distinct physiological classes of nociceptive an d thermoreceptive lamina I STT cells.