Mechanosensitivity of dorsal root ganglia and chronically injured axons: A physiological basis for the radicular pain of nerve root compression

Abstract

The radicular pain of sciatica [in man] was ascribed by Mixter and Barr to compression of the spinal root by a herniated intervertebral disc. It was assumed that root compression produced prolonged firing in the injured sensory fibers and led to pain perceived in the peripheral distribution of those fibers. This concept was challenged on the basis that acute peripheral nerve compression neuropathies are usually painless. Furthermore, animal experiments have rarely shown more than several seconds of repetitive firing in acutely compressed nerves or nerve roots. Radicular pain may actually be pain referred to the extremity through activation of deep spinal and paraspinal nociceptors. Experiments on cat lumbar dorsal roots and rabbit sural nerves confirmed that acute compression of the root or nerve did not produce more than several seconds of repetitive firing. However, long periods of repetitive firing (5-25 min) followed minimal acute compression of the normal dorsal root ganglion. Chronic injury of dorsal roots or sural nerve produced a marked increase in mechanical sensitivity; several minutes of reptitive firing may follow acute compression of such chronically injured sites. Such prolonged responses could be evoked repeatedly in a population of both rapidly and slowly conducting fibers. Since mechanical compression of either the dorsal root ganglion or of chronically injured roots can induce prolonged repetitive firing in sensory axons, radicular pain is apparently due to activity in the fibers appropriate to the area of perceived pain.