PROPRIOSPINAL INPUT TO THORACOLUMBAR SYMPATHETIC NUCLEI FROM CERVICALAND LUMBAR LAMINA-I NEURONS IN THE CAT AND THE MONKEY

The possibility that specific thermoreceptive and nociceptive influenc es on sympathetic outflow are conveyed directly to spinal sympathetic regions by lamina I neurons was investigated anatomically with the imm unofluorescent PHA-L technique in the cat and the cynomolgus monkey. I ontophoretic injections made with physiological guidance were restrict ed to lamina I or to laminae I-II in the cervical (C6-8) or lumbar (L6 -7) enlargement. Bilateral (symmetric) terminal arborizations were obs erved (with an ipsilateral predominance) in the intermediolateral, int ermediomedial, and intervening regions of the thoracolumbar intermedia te zone. In serial horizontal sections, patches of terminal labeling a ppeared at regular longitudinal intervals in the intermediolateral reg ion. Longitudinally coursing fibers that had multiple varicosities and gave off small terminal branches were observed in the intermediolater al and the intermediomedial regions. Mediolateral strips of labeling t hat extended from labeling in the intermediolateral region to labeling in the intermediomedial region occurred at fairly regular longitudina l intervals. Because the longitudinal distribution of these terminatio ns corresponds very well with the characteristic (ladder-like) longitu dinal pattern of organization of the neuropil of the thoracolumbar sym pathetic nuclei, i.e., the principal part of the intermediolateral cel l column, the central autonomic n., and the intervening n. intercalatu s, it is inferred that these lamina I terminations occur within these nuclei. After cervical injections, the labeling was most dense in the upper thoracic T2-4 spinal cord segments in both the cat and the monke y; labeling was also present in the T10-12 segments. After lumbar inje ctions, labeling in the cat was located in the L4 segment; labeling in the monkey was present in the T4-6 and T10-12 segments. The labeling obtained was much more dense in the monkey than in the cat. These obse rvations reveal a spinal lamina I projection that could provide a dire ct pathway for the somatosympathetic reflex effects of thermal and nox ious stimuli. Considered together with reports that lamina I and the s ympathetic nuclei both receive descending input from certain key auton omic regions, this result emphasizes the importance of lamina I for ho meostasis, in addition to its probable roles in behavioral arousal, af fect, and sensation. These observations thus support the proposed conc ept that lamina I processes and distributes in a functionally specific manner the sensory input relevant to the physiological status of the tissues and organs of the entire organism.