Somatic stimuli, spinal pathways, and size of cutaneous fibers influencing unit activity in the medial medulary reticular formation

Abstract

Analysis of unit activity in the medial medullary reticular formation of unanesthetized, decerebrate and decereballate cats showed that: (i) 66% of the units responding to natural somatic stimuli were influenced only by firm pinching of skin folds or by heavy pressure over some part of their receptive fields; (ii) 86% of the units responding to electrical stimulation of cutaneous nerves required volleys containing both A and A-delta fibers for maximum response; (iii) unit responses to nerve volleys including A-delta fibers could be maintained above 70% of maximum levels even though anodal polarization of the nerve had produced differential block of the larger A fibers; (iv) somatically evoked potentials and unit discharge could be markedly reduced or eliminated by section of the ventolateral spinal cord quadrants, but high cervical section of dorsal and dorsolateral fascicles had no effect; and (v) evoked potentials and unit discharge could be elicited by stimulation in the dorsomedial mid-brain tegmentum and by stimulation of the caudal, but not rostral, cut end of the dorsal columns sectioned between C1 and C2. These findings suggest that a neural population in the medial medullary reticular formation is part of a system subserving responses to noxious stimuli.