Spinohypothalamic Tract Neurons in the Cervical Enlargement of Rats: Locations of Antidromically Identified Ascending Axons and Their Collateral Branches in the Contralateral Brain. Kostarczyk, Ewa, Xijing Zhang and Glenn J. Giesler. Department of Cell Biology and Neuroanatomy, University of Minnesota, Minneapolis, MN 55455.
APStracts 3:0193N, 1996.
Antidromic activation was used to determine the locations of ascending spinohypothalamic tract (SHT) axons and their collateral projections within C1, medulla, pons, midbrain and caudal thalamus. Sixty-four neurons in the cervical enlargement were antidromically activated initially by stimulating within the contralateral hypothalamus. All but one of the examined SHT neurons responded either preferentially or specifically to noxious mechanical stimuli. A total of 239 low threshold points were classified as originating from 64 ascending (or parent) SHT axons. Within C1, 38 ascending SHT axons were antidromically activated. These were located primarily in the dorsal half of the lateral funiculus. Within the medulla, the 29 examined ascending SHT axons were located ventrolaterally, within or adjacent to the lateral reticular nucleus or nucleus ambiguus. Within the pons, the 25 examined ascending SHT axons were located primarily surrounding the facial nucleus and the superior olivary complex. Within the caudal midbrain, the 23 examined SHT ascending axons coursed dorsally in a position adjacent to the lateral lemniscus. Within the anterior midbrain, SHT axons traveled rostrally near the brachium of the inferior colliculus. Within the posterior thalamus, all 17 examined SHT axons coursed rostrally through the posterior nucleus of thalamus. A total of 114 low threshold points were classified as collateral branch points. Sixteen collateral branches were seen in C1; these were located primarily in the deep dorsal horn. Forty-five collateral branches were located in the medulla. These were primarily within or near the medullary reticular nucleus, nucleus ambiguus, lateral reticular nucleus, parvocellular reticular nucleus, gigantocellular reticular nucleus, cuneate nucleus, and the nucleus of the solitary tract. Twenty-six collateral branches from SHT axons were located in the pons. These were in the pontine reticular nucleus caudalis, gigantocellular reticular nucleus, parvocellular reticular nucleus, and superior olivary complex. Twenty-three collateral branches were located in the midbrain. These were in or near the mesencephalic reticular nucleus, brachium of the inferior colliculus, cuneiform nucleus, superior colliculus, central gray and substantia nigra. In the caudal thalamus, two branches were in the posterior thalamic nucleus and two were in the medial geniculate. These results indicate that SHT axons ascend toward the hypothalamus in a clearly circumscribed projection in the lateral brainstem and posterior thalamus. In addition, large numbers of collaterals from SHT axons appear to project to a variety targets in C1, the medulla, pons, midbrain and caudal thalamus. Through its widespread collateral projections, the SHT appears to be capable of providing nociceptive input to many areas that are involved in the production of multifaceted responses to noxious stimuli.

Received 14 June 1996; accepted in final form 28 August 1996.
APS Manuscript Number J473-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 7 October 1996