Collateral projections are an important feature of the organization of ascending projections from the spinal cord to the brain. Primate spinothalamic tract (STT) neurons with collaterals to the periaqueductal gray (PAG) were studied by means of a fluorescent double‐labeling method. Granular Blue and rhodamine‐labeled latex microspheres were placed in the ventral posterior lateral (VPL) nucleus of the thalamus and the periaqueductal gray, respectively. Single and double labeled neurons were studied in the upper cervical cord, cervical enlargement, thoracic cord, lumbar enlargement, and sacral segments. The laminar distribution of double labeled neurons was similar to that of spinomesencephalic tract (SMT) neurons. Most double labeled (STT‐SMT) neurons were located in contralateral laminae I, V, VII, and X. Relatively more lamina I STT‐SMT neurons were found in the cervical enlargement and more lamina V STT‐SMT neurons in the lumbar enlargement. The density of STT‐SMT neurons in the upper cervical segments and cervical enlargement was almost equal. The density of STT‐SMT neurons in the lumbar enlargement was 40% of that in the cervical enlargement. The thoracic and sacral segments had the lowest density of STT‐SMT neurons, about 10% of that in the cervical enlargement. STT‐SMT neurons constituted 14.7% of SMT neurons and 6% of STT neurons in the cervical enlargement and 15.3% of SMT neurons and 2.9% of STT neurons in the lumbar enlargement. The branch points of eight STT‐SMT axons were studied electrophysiologically. The average percentage of conduction time spent in the parent axon was more than 85% for an antidromic action potential from the VPL nucleus and 91% from the PAG. Branch points of STT‐SMT axons were calculated to be 9–13 mm caudal to the PAG, in the pons or rostral medulla.
- spinal cord
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