SPINOHYPOTHALAMIC TRACT NEURONS IN THE CERVICAL ENLARGEMENT OF RATS -DESCENDING AXONS IN THE IPSILATERAL BRAIN

Spinohypothalamic tract (SHT) cells are spinal cord neurons with axons that project directly to or through the contralateral hypothalamus. F requently, SHT axons decussate in the posterior optic chiasm, turn pos teriorly and descend to unknown locations in the ipsilateral brain. We attempted to determine the course and the termination of these descen ding axons. Sixty neurons in the cervical enlargement of rats were ant idromically activated initially from the contralateral hypothalamus an d then from multiple anterior-posterior levels in the ipsilateral brai n. Fifty-three (88%) were backfired with low currents at increased lat encies from the ipsilateral brain. The axons of 35 neurons were surrou nded with electrode penetrations from which high currents could not ac tivate the neuron antidromically, suggesting the examined axons termin ated in the surrounded areas. Seven SHT axons that were surrounded (20 %) appeared to terminate in the contralateral hypothalamus, 5 (14%) in the ipsilateral hypothalamus, and 9 (26%) in the ipsilateral thalamus . Fourteen SHT axons (40%) ended in the ipsilateral midbrain mainly in the superior colliculus, cuneiform nucleus or nucleus brachium inferi or colliculus. An additional 11 axons were followed even farther poste riorly into the ventral pens or rostral medulla. Each of the 26 neuron s that could be physiologically classified responded either preferenti ally or specifically to noxious mechanical stimuli. These results indi cate that SHT axons course through a surprisingly long and complex pat h. After decussating in the hypothalamus, the axons of many SHT neuron s descend into the ipsilateral posterior thalamus, midbrain, pens, or even rostral medulla. These axons may provide nociceptive information to a variety of nuclei throughout the diencephalon and brainstem bilat erally.