Strength-duration properties and their voltage dependence at different sites along the median nerve.
OBJECTIVE
There is some evidence that the ease with which ectopic activity can be induced varies systematically along the course of a nerve and is greater at more proximal sites. Recent studies have implicated a non-inactivating threshold conductance, possibly due to persistent Na+ channels, in ectopic activity associated with ischaemia and hyperventilation. This conductance is largely responsible for the voltage dependence of strength-duration time constant (tauSD), and changes in it can explain the time constant changes that occur during hyperventilation and ischaemia.


METHODS
To determine whether the strength-duration properties of motor axons of the median nerve vary along the course of the nerve, tauSD and rheobase were calculated at wrist, elbow and axilla in 15 healthy subjects, and the relationship of these properties to threshold was assessed using DC polarizing current to change axonal excitability.


RESULTS
tauSD was similar at the 3 stimulating sites but increased less at the axilla with depolarizing current.


CONCLUSIONS
These data indicate that the greater tendency for ectopic activity to arise from proximal segments of motor axons cannot be explained by differences in the conductances that contribute to tauSD and underlie its dependence on axonal excitability. The findings provide further support for the view that the precise relationship of the stimulating electrodes to the nerve has little effect on tauSD, at least when it is measured in the forearm.
Mogyoros
I
I
Department of Neurology, Prince of Wales Hospital, Prince of Wales Medical Research Institute, Randwick, N.S.W., Australia. ilona.mogyoros@unsw.edu.au
Lin
C
C
Dowla
S
S
Grosskreutz
J
J
Burke
D
D
D000200:Action Potentials / Q000502:physiology; D000293:Adolescent; D000328:Adult; D000704:Analysis of Variance; D001369:Axons / Q000502:physiology; D005260:Female; D006801:Humans; D008297:Male; D008475:Median Nerve / Q000502:physiology*; D008875:Middle Aged; D009046:Motor Neurons / Q000502:physiology*; D009132:Muscles / Q000502:physiology; D013997:Time Factors