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The Journal of Neurophysiology Vol. 87 No. 4 April 2002, pp. 1859-1866
Copyright ©2002 by the American Physiological Society
1Division of Neuroscience and 2Department of Physical Therapy, Faculty of Rehabilitation Medicine, University of Alberta, Edmonton, Alberta T6G 2S2, Canada
Gorassini, Monica,
Jaynie F. Yang,
Merek Siu, and
David J. Bennett.
Intrinsic Activation of Human Motoneurons: Reduction of Motor
Unit Recruitment Thresholds by Repeated Contractions. J. Neurophysiol. 87: 1859-1866, 2002. The main
purpose of this study was to examine whether facilitation of human
motor unit recruitment by repeated voluntary contractions is mediated,
in part, by time and activity-dependent increases in the intrinsic
excitability of the parent motoneuron. To do this, pairs of tibialis
anterior or soleus motor units were recorded during slowly increasing
and then decreasing voluntary contractions. The firing rate of the
lower-threshold motor unit of the pair (control unit) was used as a
measure of effective synaptic excitation (i.e., drive) to the
motoneurons. This rate was used to estimate the recruitment threshold
of the higher-threshold unit of the pair (test unit). The test unit was
repeatedly recruited and de-recruited in a series of contractions, and
the interval between the de-recruitment and re-recruitment of the test
unit (interactivation interval) was systematically varied between 0.6 and 60 s. An increase in intrinsic excitability of a unit was
considered to have occurred if the level of estimated synaptic input
(as measured by the firing rate of the control motor unit) needed to
recruit a unit was reduced. At short interactivation intervals (1-2
s), the control unit firing frequency was significantly lower when the
test unit was recruited on the second contraction, compared with the
first (by 3.9 Hz or a 64% reduction). This suggested that the
intrinsic excitability of the test motor unit had increased during the
second contraction because it could be recruited at a much lower level
of estimated synaptic drive. Longer interaction intervals (2-6 s)
produced less recruitment facilitation. At even longer interactivation intervals (>6 s) there was no significant facilitation (time constant of effect was 4.8 s). In some motor units, the effect of this short-term facilitation appeared to be so pronounced that it resulted in reversing the order of de-recruitment with the other initially lower-threshold motor units. Such reversals were occasionally observed
for orderly re-recruitment. The time course and behavior of the
observed short-term facilitation of motor unit discharge was
qualitatively similar to the warm-up phenomenon of plateau potentials
seen in motoneurons of reduced preparations (e.g., 4-6 s). The
possibility of warm-up contributing to the time and activity-dependent
facilitation of human motor unit recruitment is discussed.
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