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.

Threshold tracking techniques in the study of human peripheral nerve.

Conventional electrophysiological tests of nerve function focus on the number of conducting fibers and their conduction velocity. These tests are sensitive to the integrity of the myelin sheath, but provide little information about the axonal membrane. Threshold tracking techniques, in contrast, test nerve excitability, which depends on the membrane properties of the axons at the site of stimulation. These methods are sensitive to membrane potential, and to changes in membrane potential caused by activation of ion channels and electrogenic ion pumps, including those under the myelin sheath. This review describes the range of threshold tracking techniques that have been developed for the study of human nerves in vivo: resting threshold is compared with the threshold altered by a change in environment (e.g., ischemia), by a preceding single impulse (e.g., refractoriness, superexcitability) or impulse train, or by a subthreshold current (e.g., threshold electrotonus). Few clinical studies have been reported so far, mainly in diabetic neuropathy and motor neuron disease. Threshold measurements seem well suited for studies of metabolic and toxic neuropathies but insensitive to demyelination. Until suitable equipment becomes more widely available, their full potential is unlikely to be realized.