1. Voluntary contraction of a muscle causes substantial hyperpolarization o
f the active motor axons due to activation of the electrogenic Na+-K+ pump.
The present study was undertaken to determine whether voluntary effort pro
duces a significant impairment in impulse transmission in normal axone and
whether mechanisms other than membrane hyperpolarization contribute to the
changes in axonal excitability.
2. The compound muscle action potential (CMAP) was recorded after median ne
rve stimulation at the wrist using sub- and supramaximal stimuli, delivered
singly and in pairs at conditioning-test intervals of 2-15 ms. Axonal exci
tability parameters (threshold, refractoriness, supernormality, and strengt
h-duration time constant (tau (SD))) were measured using threshold tracking
. Impulse transmission was assessed using supramaximal stimuli.
3. Maximal voluntary contractions of the abductor pollicis brevis for 1 min
produced a substantial increase in threshold, an increase in supernormalit
y and a decrease in tau (SD), all of which lasted similar to 10 min and ind
icate axonal hyperpolarization. However, immediately after the contraction
there was an unexpected increase in refractoriness. The post-contraction in
crease in refractoriness could mot be mimicked by an imposed ramp of hyperp
olarization that produced changes in the other indices to an extent that wa
s similar to voluntary contraction.
4. The contraction had relatively little effect on the size of the uncondit
ioned maximal CMAP. However, there was failure of transmission of supramaxi
mal conditioned volleys when the conditioning-test interval was short.
5. The relationships between axonal excitability and supernormality and tau
(SD) following voluntary contraction differed significantly from those rec
orded during the hyperpolarization produced by DC current. It is argued tha
t these differences probably result from extra-axonal K+ accumulation with
the voluntary contraction but not with the DC polarization.
6. It is concluded that, following maximal voluntary contraction of a norma
l muscle, the refractory period of transmission is impaired distal to the s
timulus site sufficient to cause transmission failure of the second of a pa
ir of closely spaced impulses. The site of transmission failure is: likely
to be the terminal axon, presumably at branch points, possibly in the unmye
linated pre-terminal segment.