MORPHOLOGICAL CLASSES OF SPINOTHALAMIC LAMINA-I NEURONS IN THE CAT

We examined the morphology and distribution of retrogradely labeled sp inothalamic tract (STT) neurons in lamina I (the marginal zone) of the spinal dorsal horn after large injections of cholera toxin subunit B (CTb) or Fast Blue (FB) into the contralateral thalamus of cats. Based on the shape and orientation of the somata and proximal dendrites in horizontal sections, three distinct cell types were identified: (1) fu siform cells with small, spindle-shaped somata and bipolar, longitudin al dendritic arbors; (2) pyramidal cells with triangular somata and th ree main dendritic origins with primarily longitudinal arborizations; and (3) multipolar cells with larger, multiangular somata and four or more radiating dendritic arbors directed both longitudinally and medio laterally. These three morphological types differed significantly in t he number of primary dendrites and the size of the somata. Subclasses of multipolar cells were noted. Nearly all cells could be categorized into these three classes consistently in horizontal sections. A small number of cells with transitional shapes or with small, round somata w ere unclassified. The proportional distributions of these cell types w ere found to vary over the length of the spinal cord (from the third c ervical through the coccygeal segments) in three cats. The overall pro portions of cell types were 34% fusiform, 36% pyramidal, 25% multipola r, and 5% unclassified. The proportions of pyramidal and multipolar ce lls were strikingly higher within the C7-8 and L6-7 segments and lowes t in the thoracic segments. In contrast, fusiform cells formed about 2 0% of the labeled lamina I STT population in the C7-8 and L6-7 segment s but more than 60% in thoracic segments. Across all nine cats, the pr oportions were similar within the cervical (C5-8) and lumbosacral (L5- S1) enlargements, although considerable interanimal variability was no ted. These distinct morphological types of lamina I STT cells with dif ferential longitudinal distributions probably have different functiona l roles. They may correspond to the three main physiological classes o f lamina I STT cells. (C) 1996 Wiley-Liss, Inc.