Saphenous has weak ineffective synapses in sciatic territory of rat spinal cord: electrical stimulation of the saphenous or application of drugs reveal these somatotopically inappropriate synapses.

In a previous paper, we showed that chronic denervation of the sciatic nerve for more than 21 days in adult rats caused expansion of the saphenous nerve into sciatic territory in the spinal cord (medial L4, L5 and L6). To try to explain this expansion in the present paper, we tested the hypothesis that weak ineffective synapses of saphenous terminals are always present in sciatic territory. For this purpose the sciatic nerve was acutely denervated, the cord mapped with microelectrodes and responses evoked in single cells with natural (mechanical cutaneous) or electrical (pulses to saphenous nerve) stimulation. In the sciatic territory, no natural responses occurred but electrically evoked responses from the saphenous were everywhere. When drugs were applied to potentiate synaptic activity, many of the silent neurons in the sciatic territory in L4, L5 and L6 responded to natural inputs mediated by the saphenous. Picrotoxin was more effective than 4-aminopyridine which was more effective than strychnine in expressing these weak somatotopically inappropriate saphenous inputs. All together, these results support the hypothesis that weak ineffective saphenous inputs exist in sciatic territory of the spinal cord. They can be artificially expressed with electrical volleys or chemical potentiation and may be naturally expressed several weeks after chronic lesions of the sciatic nerve.