An Electrophysiological Study of Neurones in the Substantia Gelatinosa Rolandi of the Cat's Spinal Cord

¹ Department of Physiology, Faculty of Veterinary Medicine, University of Edinburgh, Edinburgh EH9 1QH

Single unit neuronal activity has been recorded in the Substantia Gelatinosa Rolandi (SG) of the lumbar spinal cord of chloralose anaesthetized and gallamine paralysed cats. High impedance, fine tipped glass microelectrodes were used to record from the neurones of the SG. A set of five criteria was laid down to differentiate activity generated by SG neurones from activity produced by other neuronal elements. One hundred and ten neurones satisfied all the criteria and the sample consisted of 78 extracellularly and 32 intracellularly recorded SG neurones. The responses of these neurones to natural stimulation of the skin have been analysed. The majority (86%) showed a conspicuous and persistent background activity in the absence of intentional stimulation and a cutaneous receptive field from which they could be inhibited. These SG neurones have been called ‘inverse’ neurones because their response characteristics were the mirror image of larger neurones in Laminae I, IV and V of the dorsal horn. The background activity of the ‘inverse’ SG neurones was post-synaptically generated; different in pattern from that of larger and deeper dorsal horn neurones and dependent on the afferent drive to the SG neurones. Three main groups of ‘inverse’ SG neurones could be distinguished according to the kind of natural stimulation that produced inhibition: (i) Class ‘inverse’ 1 ([unknown]), inhibited only by non-noxious stimulation of the skin. (ii) Class ‘inverse" 2, ([unknown]) inhibited by both noxious and non-noxious stimulation and (iii) Class ‘inverse’ 3 ([unknown]), inhibited only by noxious stimulation. Class [unknown] SG neurones were excited by noxious stimulation of their receptive fields and Class [unknown] SG neurones were activated by non-noxious stimulation of theirs.

It is proposed that SG neurones control the transmission of all sensory messages through the spinal cord by tonically inhibiting the larger dorsal horn neurones in the absence of stimulation and by selectively releasing this inhibition during periods of stimulation.

Note:

This work was supported by a grant from the Science Research Council (GR/A/0106). Our thanks are due to Dr D. R. Ensor for his participation in some of the experiments, to Miss A. Thorburn, Mr C. M. Warwick and Mr S. Robertson for their skilled technical assistance and to the Wellcome Animal Research Unit of the Faculty of Veterinary Medicine for the provision of facilities.

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