The freezing threshold of the peripheral motor nerve: an electrophysiological and light-microscopical study on the sciatic nerve of the rabbit.

Fifty sciatic nerves of 39 rabbits are treated at different temperatures (+5, +1, 0, ?3, ?5, ?10, ?15 and ?20 °C), for different freezing times (10, 20, 30, 60 and 120 sec), and for different numbers of freeze-thaw cycles (1, 2 and 4). After electric supramaximal stimulation (3.8 V) action potentials of the sciatic nerve are measured before, immediately after, and 1, 3, 5, 10, 20, 30, 60, 90 min, 2, 5 and 10 days after freezing. Two or ten days after freezing, the nerves are examined in a light microscope. The cold threshold of the sciatic nerve was determined, i.e., the temperature at which after supramaximal stimulation it is still possible to measure an action potential within 1.5 hr after freezing. On application of one freeze-thaw cycle, the cold threshold is ?15 °C after a freezing time of 10 sec, ?10 °C after 20 sec and 30 sec, and ?5 °C after 60 and 120 sec. After application of two and four freeze-thaw cycles, the cold threshold is elevated, and after a super-cooling time of 10 sec it is ?10 °C, after 30 sec ?5 °C. The longer the freezing time and the more freeze-thaw cycles, the higher is the cold threshold. At ?20 °C (superthreshold temperature) an action potential can no longer be measured and all myelinated nerve fibres have decayed, except some small-caliber ones.Electrophysiologically, it is evident that some of the myelinated nerve fibres become functionally damaged for 1.5 hr, while other parts of the nerve fibres will degenerate and later regenerate. The amplitudes of the measured action potentials correlate with the decay of myelin sheaths and axons of large- and medium-caliber nerve fibres. Action potentials between 0 and 40% show a gradual paresis, above 40% a physiological motor function. The pathophysiological mechanism of this reversible functional loss after super-cooling and freezing may be a consequence of a disturbed membrane permeability.It is of clinical importance that, if the cold threshold of a peripheral motor nerve is known, the nerve can be frozen concomitantly for a short time at application of low temperatures without suffering any functional loss. This is achieved by controlling during freezing the motor function of the corresponding nerve situated on the periphery of cryolesion, and, if there is a loss of motor function, the freezing process has to be interrupted immediately.