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Block AP: Page 3

Effect of localized Nerve Cooling on Compound Action Potentials.

In clinical studies, nerve conduction velocity is considered to decrease by roughly 2 m/s per °C of cooling. Decrease in nerve conduction velocity have been observed following cold water immersion [1]. A positive correlation has been observed between decrease in surface temperature and decrease in nerve conduction velocity of 1.7 m/sec for every 1°C decrease in surface temperature [2].
There have been conflicting results reported on the blocking effect of cooling. Taken together, some of the data suggested that smaller diameter axons were blocked at lower temperatures than larger axons subjected to the same temperature. During cooling, the latencies increase slowly and then rapidly before conduction was blocked. The differential effect of axon diameter that was observed was due to the effect of the latencies on the Compound Action Potential. Lowering of conduction velocities destroyed components of the CAP leading to the impression that block was occurring. Single fiber recordings revealed that conduction velocities slowed before block occurred and the blocking temperature was between 3∞C and 9∞C for all healthy fibers, higher if the nerves were stretched or injured [3].
Lowering the temperature of a nerve might block conductions because gating ion channels involves conformational changes in the protein and the rates at which these conformational changes take place are voltage and temperature sensitive. The Q10 for the rate constants are reported to be 2 to 4, meaning that the rate changes by a factor of 2 to 4 for each 10¾C change in temperature [4]. So if Delta T is -25¾C then Q.Delta T = (Q10)Delta T/10 = 0.06 or about 16 times slower (assuming Q10=3).


[1.] Abramson DI, Chu LS, Tuck S Jr, Lee SW, Richardson G, Levin M. (1966) Effect of tissue temperatures and blood flow on motor nerve conduction velocity. JAMA. 198:1082-1088.
[2.] Halar EM, DeLisa JA, Brozovich FV. (1980) Nerve conduction velocity: relationship of skin, subcutaneous and intramuscular temperatures. Arch Phys Med Rehabil. 61(5):199-203.
[3] Paintal, A.S. (1965) Block of conduction in mammalian nerve fibers by low temperatures. J. Physiol. 180 (1) 1-19.
[4] Hille, B. (2001) Ion Channels of Excitable Membrane (3rd Edition). Sunderland, Mass. Sinauer Associates, Inc.

 

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