Primate spinothalamic pathways: I. A quantitative study of the cells of origin of the spinothalamic pathway

In six monkeys spinothalamic (STT) cells were retrogradely labeled by injecting 2% wheat germ agglutinin–conjugated horseradish peroxidase into the somatosensory thalamus. Following a 5‐day survival period, the animals were perfused and the tissue was removed and processed with the tetramethyl benzidine technique. In all animals there were HRP‐labeled STT cells in all segments of the spinal cord. In one old world monkey, the injection included most of the thalamus and resulted in 18,235 estimated total number of STT cells. Of this total, 35% were located in the upper cervical segments (C1–C3), 18% were located in C4–C8, 19% were in the thoracic spinal cord with most found in T1–T3; 6% were in L1–L3, 13% were in L4–L7, and 7% were in the coccygeal segments. Of the total labeled STT cells, 17% were found in the spinal cord ipsilateral to the thalamic injection; 53% of these cells were located in C1–C3 primarily in lamina VIII. The percentage of label found in the contralateral lower cervical region laminae I–III (43–50%), IV–VI (33–48%), and VII–X (8–17%) was similar among three animals with similar thalamic injections. The distributions of the shapes of the labeled STT cells were similar for each lamina between the lower cervical and lower lumbar regions. The mean diameter of the labeled STT cells varied with spinal cord segment and lamina. The lamina I STT cells were the smallest. In the cervical spinal cord, lamina VIII STT cells had the largest diameters, while in the lumbar region laminae IV–VI had the largest STT cells.