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The Journal of Neurophysiology Vol. 87 No. 4 April 2002, pp. 1850-1858
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: Possible Contribution
to Motor Unit Excitation. J. Neurophysiol. 87: 1850-1858, 2002. The main purpose of this study was to
estimate the contribution of intrinsic activation of human motoneurons
(e.g., by plateau potentials) during voluntary and reflexive muscle
contractions. Pairs of motor units were recorded from either the
tibialis anterior or soleus muscle during three different conditions:
1) during a brief muscle vibration followed by a slow
relaxation of a steady isometric contraction; 2) during a
triangular isometric torque contraction; and 3) during
passive sinusoidal muscle stretch superimposed on a steady isometric
contraction. In each case, the firing rate of a tonically firing
control motor unit was used as a measure of the effective
synaptic excitation (i.e., synaptic drive) to a slightly
higher-threshold test motor unit that was recruited and
de-recruited during a contraction trial. The firing rate of the control
unit was compared at recruitment and de-recruitment of the test unit.
This was done to determine whether the estimated synaptic drive needed
to recruit a motor unit was less than the amount needed to sustain
firing as a result of an added depolarization produced from intrinsic
sources. After test unit recruitment, the firing rate of the control
unit could be decreased significantly (on average by 3.6 Hz from an
initial recruitment rate of 9.8 Hz) before the test unit was
de-recruited during a descending synaptic drive. Similar decreases in
control unit rate occurred in all three experimental conditions. This
represents a possible 40% reduction in the estimated synaptic drive
needed to maintain firing of a motor unit compared with the estimated
amount needed to recruit the unit initially. The firing rates of both
the control and test units were modulated together in a highly parallel
fashion, suggesting that the unit pairs were driven by common synaptic inputs. This tight correlation further validated the use of the control
unit firing rate as a monitor of synaptic drive to the test motor unit.
The estimates of intrinsically mediated depolarization of human
motoneurons (
40% during moderate contractions) are consistent with
values obtained for plateau potentials obtained from intracellular recordings of motoneurons in reduced animal preparations, although various alternative mechanisms are discussed. This suggests that similar intrinsic conductances provide a substantial activation of
human motoneurons during moderate physiological activity.
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