Observations on the development of muscle hypersensitivity following chronic nerve conduction blockage and recovery

Agar-sleeves containing 0.01%, 0.015% and 0.02% Tetrodotoxin were placed onto the sciatic nerve of the rat. The time-course of the conduction block and the full recovery of the nerve were studied; correlations were drawn with the hypersensitivity developed on the innervated muscles. The earliest sign of a TTX-produced conduction block was a decrease in the amplitude of the faster conducting fibres appearing 3 min later. Complete block was fully established 35 min later. The duration of a complete conduction block was a dose-dependent phenomenon and lasted from 1--4 days. The recovery process was gradual, simulating the reverse pattern of the acute TTX-block but spread over a much longer period with complete conduction recovery occuring 12 to 13 days later. Innervated muscles behaved as paralytic even before the complete establishment of a conduction block and remained so for 2--6 days after which clinical recovery was prompt. Muscles innervated by the TTX-treated nerves developed hypersensitivity to acetylcholine which could be seen within two days. This hypersensitivity continued to increase over the following days, despite some recovery of conduction. Its maximum appeared six to seven days later and then declined to return to normal at the time when nerve conduction properties had fully recovered. A similar degree of partial conduction block when acutely established always resulted in paralysis but when chronically present, the clinical picture of paralysis was fully compensated, due to the hypersensitivity of the muscle and possibly to collateral nerve sprouting.