Brain, Vol 121, Issue 5 851-859, Copyright © 1998 by Oxford University Press
I Mogyoros, MC Kiernan, D Burke and H Bostock
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.
ARTICLES
Strength-duration properties of sensory and motor axons in amyotrophic lateral sclerosis
Department of Neurology, Prince of Wales Hospital, Randwick, Sydney, Australia.
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