CUTANEOUS RESPONSIVENESS OF LUMBAR SPINAL NEURONS IN AWAKE AND HALOTHANE
ANESTHETIZED SHEEP.
Herrero, Juan F. and P. Max Headley.
Department of Physiology, University of Bristol, The School of Medical
Sciences, Bristol BS8 1TD, UK.
APStracts 2:0166N, 1995.
SUMMARY AND CONCLUSIONS
1. To compare the responsiveness of lumbar spinal neurons to peripheral
sensory stimuli under normal physiological conditions and under halothane
anesthesia, a study has been performed in sheep that were prepared
chronically. This permitted recordings to be made in the same animals, either
when they were awake, free from recent surgery, free from drugs and training
and only partially restrained, or when they were anesthetized with halothane.
2. 261 units were recorded in dorsal and ventral horns under conscious
conditions. Of these, 19% had no detectable receptive field (RF) and 44% had
responses dominated by proprioceptive inputs; these units were not
investigated in detail. The remaining 96 neurons (37%) had clearly defined
cutaneous RFs. Of these, most (72%) had wide dynamic range (WDR; convergent,
multireceptive) properties, 19% were low threshold mechanoreceptive (LTMR) and
9% were high threshold mechanoreceptive (HTMR). These units with cutaneous RFs
were investigated in greater detail. 3. The spontaneous activity under these
awake conditions were low (<4 spikes.s-1) for nearly all units in all 3
categories. The mechanical threshold of the most sensitive (central) part of
the cutaneous RF was assessed with von Frey bristles. Thresholds were <5 mN
for all LTMR, <1-30 mN for WDR and over 80 mN for HTMR neurons. The size of
the low threshold cutaneous RFs was significantly larger for WDR (mean 46 cm2)
and HTMR (45 cm2) than for LTMR (24 cm2). The RFs were distributed all over
the ipsilateral hindlimb. Large RFs were mostly proximal whereas small RFs
were distributed relatively evenly over the limb. 4. Recordings were made from
a further 156 units whilst the animals were under halothane anesthesia. With
86 neurons having cutaneous peripheral receptive fields, the proportions
having LTMR, HTMR or WDR characteristics were very similar to those in awake
animals. Under halothane the ongoing activity of WDR units was slightly (but
significant) less. The threshold to von Frey bristle stimulation was
significantly higher only for WDR units, in both dorsal and ventral horns. The
mean size of cutaneous receptive fields was significantly larger in all
classes of unit recorded under halothane anesthesia. For WDR units this was
true for cells in both dorsal and ventral horns. This effect on mean values
was due to a larger proportion of units with very large fields under
anesthesia, particularly in the dorsal horn. 8. Comparison of the data from
conscious animals with published results 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. 9. Halothane does not have major
effects on the resting sensory responsiveness of spinal neurons with cutaneous
receptive fields. The increase in receptive field area, which contrasts with
most results from acute studies, is likely to be due to a dampening of
descending inhibitory control mechanisms.
Received 23 September 1994; accepted in final form 25 May 1995.
APS Manuscript Number J599-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 8 June 1995.