CUTANEOUS RESPONSIVENESS OF LUMBAR SPINAL NEURONS IN AWAKE AND HALOTHANE-ANESTHETIZED SHEEP

1. To compare the responsiveness of lumbar spinal neurons to periphera l sensory stimuli under normal physiological conditions and under halo thane anesthesia, we performed a study in sheep that were prepared chr onically. This permitted recordings to be made in the same animals eit her when they were awake and free from recent surgery, drugs, and trai ning and only partially restrained or when they were anesthetized with halothane. 2. We recorded 261 units in dorsal and ventral horns under conscious conditions. Of these, 19% had no detectable receptive held (RF) and 44% had responses dominated by proprioceptive inputs; these u nits were not investigated in detail. The remaining 96 neurons (37%) h ad clearly defined cutaneous RFs. Of these, most (72%) had wide-dynami c-range (WDR; convergent, multireceptive) properties, 19% were low-thr eshold mechanoreceptive (LTMR), and 9% were high-threshold mechanorece ptive (HTMR). These units with cutaneous RFs were investigated in grea ter detail. 3. The spontaneous activity under these awake conditions w as low (<4 spikes/s) for nearly all units in all three categories. The mechanical threshold of the most sensitive (central) part of the cuta neous RF was assessed with von Frey bristles. Thresholds were <5 mN fo r all LTMR neurons, <1-30 mN for WDR neurons, and >80 mN for HTMR neur ons. The size of the low-threshold cutaneous RFs was significantly lar ger for WDR neurons (mean 46 cm(2)) and HTMR neurons (45 cm(2)) than f or LTMR neurons (24 cm(2)). The RFs were distributed all over the ipsi lateral hindlimb. Large RFs were mostly proximal, whereas small RFs we re distributed relatively evenly over the limb. 4. Recordings were mad e from a further 165 units while the animals were under halothane anes thesia. With 86 neurons having cutaneous peripheral RFs, the proportio ns having LTMR, HTMR, or WDR characteristics were very similar to thos e in awake animals. Under halothane the ongoing activity of WDR units was slightly (but significantly) less. The threshold to von Frey brist le stimulation was significantly higher only for WDR units, in both do rsal and ventral horns. The mean size of cutaneous RFs was significant ly larger in all classes of units recorded under halothane anesthesia. For WDR units this was true for cells in both dorsal and ventral horn s. This effect on mean values was due to a larger proportion of units with very large fields under anesthesia, particularly in the dorsal he m. 5. Comparison of the data from conscious animals with published res ults of acute experiments indicates that acute recording conditions do not distort the relative distribution and resting characteristics of these three functional categories of lumbar spinal neurons as much as might have been expected. 6. Halothane does not have major effects on the resting sensory responsiveness of spinal neurons with cutaneous RF s. The increase in RF area, which contrasts with most results from acu te studies, is likely to be due to a dampening of descending inhibitor y control mechanisms.