Spread of saphenous somatotopic projection map in spinal cord and hypersensitivity of the foot after chronic sciatic denervation in adult rat.

The left sciatic nerve was cut and ligated in adult rats (chronic denervation). Twenty-one days later the right sciatic nerve was cut and ligated (acute denervation). The somatotopic maps of the surviving intact saphenous nerves (left and right) were compared on day 21 by recording from single interneurons in the dorsal horn of the spinal cord. On the acute side saphenous mediated natural responses were observed only as far caudally as L3, while no natural responses were found in L4 and 5 (this silent zone in L4 and 5 had previously been sciatic territory). In contrast, after chronic sciatic denervation, L4 and L5 were not silent to natural stimulation as the saphenous natural responses had spread into the sciatic territory. Saphenous inputs always won the sciatic territory in L4 and L5 over competing thigh afferents after chronic sciatic denervation. Electrical stimulation of the saphenous nerve on the acute side produced unit responses all the way down to S1 (with no silent areas in L4 and 5). These electrically evoked unit responses in L4 to S1 of the acute side were called 'long range' pathways. There were no differences in 'long range' electrical responses in the acutely and chronically denervated cord. The caudal boundary for electrically evoked saphenous responses was S1 on both sides of the cord, and post stimulus histograms of unit responses were not statistically different on the two sides. Thus after chronic sciatic denervation natural responses mediated by saphenous spread caudally into sciatic territory, but electrically evoked responses did not change. Behaviorally, there was the expected spread of saphenous mediated responses from the medial toe and foot to more lateral regions after chronic sciatic denervation. Unexpectedly, there was an increase in sensitivity (hyperalgesia) of the medial toe and foot. We postulate that this increased sensitivity might be mediated by the spread of saphenous projections into L4 and L5 of the cord after chronic sciatic denervation. Perhaps post traumatic neuralgia in humans could be due to increased number of spinal cord cells responding to stimulation of the receptive field of the surviving nerve.