Strength-duration properties of sensory and motor axons in amyotrophic lateral sclerosis

In normal subjects, the strength-duration time constant is longer for cutaneous afferents than for motor axons, probably because the former express a greater non-inactivating (persistent) Na+ conductance that is active at threshold. Using a threshold-tracking system the strength-duration properties of cutaneous afferents and motor axons were recorded from 23 patients with amyotrophic lateral sclerosis, and compared with those of 32 healthy subjects. In control subjects and patients, the strength-duration time constant of sensory fibres declined with age, and there was no difference between the two groups when age was taken into account. The motor time constant did not change with age when expressed as a percentage of the time constant for sensory fibres in the same nerve, but was significantly longer for the patients than control subjects. In addition, motor rheobase was significantly lower for the patients, when expressed as a percentage of sensory rheobase. There was an inverse relationship between the time constant and rheobase for sensory and motor axons, and this was the same for the patients and the control subjects, suggesting that the variations in time constant within and between the groups were related to the expression of a common factor. Measurements of refractoriness and supernormality provided no evidence for a difference in resting membrane potential between the patients and control subjects. These findings are consistent with the interpretation that motor axons of the patients with amyotrophic lateral sclerosis have a greater persistent Na+ conductance than normal motor axons. This could contribute to the ectopic activity responsible for fasciculation.