The method of retrograde axonal transport of horseradish peroxidase (HRP) was used to identify t h e locations of cells of origin of t h e spinothalamic tract in t h e cat. Injections of from 0.2-3.0 pl of 30% HRP were made unilaterally in various regions of t h e somatosensory thalamus. Massive injections of the caudal thalamus in several cats showed t h e spinothalamic cells of origin to be located mainly in laminae I, VII and VIII in the lumbar enlargement, and in laminae I, V and VII-VIII in t h e cervical enlargement.Small injections of HRP were made into t h e three major spinothalamic terminal zones in t h e thalamus, to determine t h e laminar origink) of the spinal projections to each zone. Neurons in lamina I in both cervical and lumbar enlargements were found to project almost exclusively to t h e rostra1 VB-caudal VL border zone. A small number of neurons in laminae VII and VIII also project there but a larger number project to the intralaminar region. Neurons projecting to t h e PO regions were located mainly in laminae IV and V. This anatomical segregation of thalamic afferents probably reflects a functional segregation of input, since t h e functional properties of spinal neurons vary according t o their laminar location.Comparison of these data with t h e differential projection of spinothalamic neurons in t h e rat and monkey indicate t h a t it is unlikely t h a t t h e proposed "paleo-" and "neospinothalamic" systems would arise from anatomically separ a t e groups of spinal neurons.The spinothalamic tract has long been of interest beginning with t h e early clinical reports t h a t analgesia and thermanesthesia without a significant impairment of other types of somatic sensibility were t h e result of ventrolateral cord lesions (Muller, 1871; Gowers, 1878; Petren, '02; Spiller, '05). These and other findings, including t h e intentional, surgical section of t h e ventrolateral quadrant, led to t h e hypothesis t h a t t h e spinothalamic tract was conveying information necessary for pain and temperature sensation i n humans (Spiller and Martin, '12; Beer, '13; Frazier, '20; Foerster and Gagel, '32; Kuru, '49). More recent electrophysiological and clinical evidence indicates that spinothalamic fibers transmit additional types of somatosensory information which may contribute to sensations of touch, pressure and joint movement in primates (Willis et al., '74; Price and Mayer, '74; Noordenbos and Wall, '76). The functional role of the spinothalamic tract in other species is less well understood, even though t h e tract carries a substantial proportion of t h e total afferent input to t h e thalamus from t h e spinal cord.The course and terminations of t h e spinothalamic tract have been extensively studied in a variety of mammalian species with early and more recent techniques for visualizing degenerating
