Mechanosensitive afferent units in the lumbar intervertebral disc and adjacent muscle

The purpose of this study was to characterize the somatosensory units of the lumbar intervertebral disc and adjacent muscle. A laminectomy was performed on the lumbar spine of adult male Japanese White rabbits. Receptive fields of mechanosensitive afferent units were investigated in the intervertebral disc and adjacent muscle, and electrophysiologic recordings were obtained from filaments of the dorsal root. Thirteen mechanosensitive units were identified: three units in the intervertebral disc area, and 10 units in the psoas muscle. Of the three units in the disc area, one was identified at the surface of the L5-6 anulus fibrosus and two at the insertion of the anulus fibrosus to the L5 and L6 vertebral bodies. These three units belonged to Group III and had very high mechanical thresholds, more than 160 g. The 10 units in the psoas muscle belonged to Groups II, III, and IV, and had mechanical thresholds ranging from 0.22 to 12.6 g. The units in the disc area may serve as nociceptors sensitive to strong noxious stimulation that could traumatize the tissue. The units in the psoas muscle may contribute to nociception and proprioception.