Neurophysiological study of nociceptive sensory mechanisms

Abstract

A survey of the literature relating to nociception that forms the first part of this thesis reveals the continuing difficulties in defining pain and explaining the mechanisms by which it may be produced. Consideration of the circum¬ stances in which pain may be experienced in skeletal muscles suggests that highthreshold receptors in these muscles are likely to resemble those in the skin and the viscera and respond to severe pressure, gross thermal changes and/or the administration of some putative algetic substances. Neurophysiological recordings of discharges in single sensory nerve fibres show that most individual sensory receptors respond only to specific stimuli, and that skeletal muscle nociceptors, of which very few studies have been made, form a distinct subgroup. Recordings from the spinocervical tract and other sensory pathways in the spinal cord and brain show that significant inhibition of the transmission of discharges from peripheral nociceptors occurs within the lumbar segments of the spinal cord.The experiments summarised in Section II were performed in order to isolate and characterize nociceptors associated with the gastrocnemius and soleus muscles in the cat and the rabbit. Host of the nociceptors isolated were located in the region of the musculotendinous junctions, and responded to severe pressure by producing a relatively low-frequency, low amplitude discharge that disappeared within 30 seconds of the removal of the stimulus. Host of these discharges were transmitted through small-diameter nerve fibres. Some receptors also responded to temperature changes or to infusions of acetylcholine, histamine, 5-hydroxytryptamine or prosta¬ glandin solutions.The chronic experiments in cats and rabbits described in Section III confirmed previous observations, in the macaque mon: ey, that the size of a dermatome isolated by the 'method of remaining sensibility' is greater when the appropriate spinal nerves are sectioned distal to the dorsal root ganglia than it is when the dorsal root3 of the same spinal nerves are sectioned within the dura mater. Recordings of activity in dorsal spinal nerve rootlets made in the acute experiments described in the same Section have revealed for the first time that spontaneous discharges can arise within dorsal root ganglia which have been surgically isolated from the periphery. These discharges are again of low frequency afid amplitude, and appear to be conducted in small-diameter fibres.Experiments undertaken conjointly with Drs A.G. Brora and H.P, Plartin III and summarised in Section IV revealed marked similarities between segmentally-evoked inhibition of transmission through the spinocervical tract and the inhibition evoked by stimulation of the cervical segments of the spinal cord, and it suggested that the same segmental nociceptive mechanisms may operate in each case. The inhibition has its greatest effect on Impulses in polysynaptic pathways and in the smaller-diameter peripheral nerve fibresThe concluding discussion, in Section V, considers the likely significance of the discharges recorded at each of the three levels of the nervous system —- the periphery, the dorsal root ganglia and the spinal cord in nociceptive sensory mechanisms, and suggests that the present study has provided a useful basis for further studies at each of these levels.