Tooth pulp-driven neurons in somatosensory cortex of primates: Role in pain mechanisms including a review of the literature

Abstract

The tooth pulp of primates [6 rhesus monkeys, 7 baboons] was stimulated electrically while searching for evoked unit potentials in the cerebral cortex. Control procedures were employed to assure that the electrical stimuli reached only tooth pulp fibers but no extrapulpal sensory fibers. An electrode was inserted in soft tissue surrounding the tooth for separate excitation of extrapulpal axons. A tooth pulp projection area was identified in the face area of primary somatosensory cortex. Major neuron groups (2) were encountered, 1 excited only by the extrapulpal soft tissue stimulus, the other by tooth pulp stimuli. Within the pulp-projection area, soft tissue-driven neurons were most numerous in superficial cortex of the postcentral gyrus, pulpdriven neurons dominated in deep cortex in the base of the central sulcus. The pulp-driven population divided into several functional subsets: those excited from 1 pulp only (conceivably capable of localizing pulpal stimuli), those excited from > 1 pulp and those excited from both pulp and extrapulpal soft tissue. Within each of these 3 pulp-driven subsets, some units responded to single shock, others only to a train of shocks. Mean discharge latency was shortest for the population excited only for soft tissue, intermediate for pulp-driven units excited by single shock and longest for pulp-driven units excited only by trains of shocks. Both soft tissue and pulp stimuli evoked extensive inhibitory effects. The possible role of pulp-driven neurons in pain is considered. The functional properties of some neurons are consistent with a role in stimulus localization but those of the remaining neurons suggest other roles in pain. An examination of the literature on cortex and pain suggests that normally somatosensory cortex is important for localizing painful stimuli and that it contributes to other pain mechanisms as well. After certain lesions, somatosensory cortex has the capacity for generating central pain just like other structures in the nociceptive pathway.