Activity‐dependent hyperpolarization of human motor axons produced by natural activity

Abstract

The changes in excitability of motor axons produced by natural activity were measured in six healthy subjects using voluntary contractions lasting 15 s, 30 s and 1 min, by recording the changes in stimulus current required to produce a compound muscle action potential of ≈60 % of maximum. On cessation of the contractions there was a prominent increase in the current required to produce the target potential, accompanied by an increase in rheobase, a decrease in strength‐duration time constant, and an increase in axonal supernormality. These changes indicate that the hypoexcitability was due to axonal hyperpolarization. The activity‐dependent hypoexcitability increased in depth and duration the longer the contraction. Following a 1 min contraction, it produced a 24 % increase in threshold, waning over 15 min. The hypoexcitability was greater than in cutaneous afferents tetanized to produce an equivalent rate‐dependent stress. It is concluded that natural activity results in substantial hyperpolarization of active axons and that, for similar discharge rates, the degree of hyperpolarization is greater in motor axons than cutaneous afferents. The greater effect of activity on the excitability of motor axons could be due to less inward rectification and less persistent Na+ conductance than in sensory axons. It is suggested that motor axons may therefore be more susceptible than cutaneous afferents to conduction block at sites of impaired safety margin for impulse conduction.

Effect of exercise-induced arterial hypoxemia on quadriceps muscle fatigue in healthy humans

“…Although we cannot absolutely exclude the possibility of submaximal stimulation, we doubt whether this would have influenced our results, particularly because submaximal stimulation tends, if anything, to underestimate the magnitude of fatigue (7). An additional consideration is that repeated voluntary contractions increase the threshold of motor axons due to activity-dependent hyperpolarization (49), which reduces the population of axons excited by the same stimulus intensity after vs. before exercise, even in the face of a slight increase in M-wave amplitude (49). These data point to a reduced motor output to the muscle during magnetic stimulation following exercise, although comparisons between normoxic and hypoxemic conditions should not be influenced, because these conditions required exercise of identical force and duration.…”

Excitation block in a nerve fibre model owing to potassium-dependent changes in myelin resistance

“…The effect of the Na þ ,K þ -pump was studied in human nerves e.g. by Kiernan et al [3] and Vagg et al [4].…”

Chronic Refractory Myofascial Pain: Characteristics of Patients who Self-select Long-term Management with Electrical Twitch-Obtaining Intramuscular Stimulation

“…This may result from transient conduction block. Even natural activity results in substantial hyperpolarization of active axons and, for similar discharge rates; the degree of hyperpolarization is greater in motor axons than cutaneous afferents [29]. There is potential for increased susceptibility of MTrPs in chronic pain patients for further trauma, induced by violent muscle contractions, as well as by new injuries that include falls, lifting injuries, auto accidents, exercise, or even repetitive contractions associated with activities of daily living.…”