We have undertaken a series of electrophysiological experiments using animals in order to identify and characterize the somatosensory units of the lumbar spine and adjacent tissues. A laminectomy was performed on the lumbar spine of anesthetized rabbits. Receptive fields of mechanosensitive afferent units were investigated in the lumbar spine and adjacent tissues, and electrophysiological recordings were obtained from filaments of the dorsal root. Thirty mechanosensitive units were identified at the facet joint, and 27 others were found in the muscles and tendons near their insertion into the facet joint. Three units were identified in the intervertebral disc area, and ten units in the psoas muscle. Some of these units had high mechanical thresholds and belonged to group-III. These units may serve as nociceptors. Other units with low thresholds may serve as proprioceptors. The units in the intervertebral disc had very high mechanical thresholds, so that these units may serve as nociceptors sensitive to strong noxious stimulation which could traumatize the tissue. Substance P excited 20 of 25 mechanosensitive units in the facet joint area. Both high and low- threshold units were excited. This result suggests that substance P has an excitatory effect on both nociceptive and proprioceptive units.
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