Effects of temperature on the excitability properties of human motor axons

The effects of temperature on parameters of motor nerve excitability were investigated in 10 healthy human subjects. The median nerve was stimulated at the wrist and compound muscle action potentials were recorded from the abductor pollicis brevis. Multiple excitability measures were recorded: stimulus-response curves, the strength-duration time constant (tauSD), threshold electrotonus, a current-threshold relationship and the recovery of excitability following supramaximal activation. Recordings were made at wrist temperatures of 35, 32 and 29 degrees C by immersing the arm proximal to the wrist in a water-bath. Cooling increased the relative refractory period by 7.8% per degree C (P < 0.0001), slowed the accommodation to depolarizing currents by 4.0% per degree C (P < 0.0001) and increased tauSD by 2.6% per degree C (P < 0.01), but most other excitability parameters were not affected significantly. The effects of temperature on threshold electrotonus were investigated further in separate studies on two subjects over the range 28-36 degrees C and found to be complex. Whereas the rate of accommodation to depolarizing current was closely related to instantaneous temperature, the threshold increase induced by hyperpolarizing current was most sensitive to changes in temperature, probably because warming the nerve causes a transient hyperpolarization by accelerating the electrogenic sodium pump. Consequently, it may be preferable to make allowances for differences in skin temperature when testing patients for abnormal excitability parameters, rather than to change the temperature to a standard value. For most excitability parameters, however, temperature control is not as important as it is for conduction velocity measurements.