Nociceptor-driven dorsal horn neurones in the lumbar spinal cord of the cat

Abstract

Single dorsal horn neurons were recorded extracellulary in the lumbar spinal cord of cats anesthetized with chloralose. Cold block at L1 was used to provide reversible spinalization. The location of the units in the dorsal horn was marked by the electrophoretic deposition of pontamine sky blue from the recording microelectrode. There was a clear somatotopic representation of the ventrolateral surface of the foot in the L6 segment. Of the 46 units recorded in the marginal zone of the L6 dorsal horn (lamina I), 35 could only be excited by volleys in small afferent fibers and by noxious stimulation of the skin in the foot regions, and were termed class 3 cells. The remaining 11 units could also be excited by sensitive cutaneous mechanosensitive afferent units; they were class 2 units. The specific nociceptor-driven neurons could be divided into 2 subclasses on the basis of their excitability by afferent fibers. Class 3 (a) were excited only by A.delta. cutaneous afferents and class 3 (b) by both A.delta. and C cutaneous afferents. Some of the latter were also excited by A.delta. and C afferent fibers in the lateral gastrocnemius nerve. When tested by natural stimulation all class 3 cells were excited by noxious mechanical stimuli, but only the 3 (b) units were effectively excited by heating the skin. This discharge in 3 (b) units could be suppressed by electrical stimulation of large (group II) cutaneous myelinated afferent fibers, and a similar effect could be produced in responses evoked by A.delta. and C afferent volleys. Additional inhibition was accomplished by electrical stimulation of the slower mylinated cutaneous (A.DELTA. or group III) afferent fibers. The excitability of the class 3 cells was greater in spinal preparations, but the tonic descending inhibition was weaker than the apparently similar descending tonic inhibition of class 2 cells. The results were discussed with reference to pain mechanisms.