Effects of temperature on the excitability properties of human motor axons

The effects of temperature on parameters of motor nerve excitability were i nvestigated in 10 healthy human subjects. The median nerve was stimulated a t the wrist and compound muscle action potentials were recorded from the ab ductor pollicis brevis, Multiple excitability measures were recorded: stimu lus-response curves, the strength-duration time constant (tau (SD)), thresh old electrotonus, a current-threshold relationship and the recovery of exci tability following supramaximal activation. Recordings were made at wrist t emperatures of 35, 32 and 29 degreesC 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 cu rrents by 4.0% per degree C (P < 0.0001) and increased tau (SD) by 2.6% per degree C (P < 0.01), but most other excitability parameters were not affec ted significantly. The effects of temperature on threshold electrotonus wer e investigated further in separate studies on two subjects over the range 2 8-36<degrees>C and found to be complex. Whereas the rate of accommodation t o depolarizing current was closely related to instantaneous temperature, th e threshold increase induced by hyperpolarizing current was most sensitive to changes in temperature, probably because warming the nerve causes a tran sient hyperpolarization by accelerating the electrogenic sodium pump. Conse quently, it may be preferable to make allowances for differences in skin te mperature when testing patients for abnormal excitability parameters, rathe r 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.