Stability of Motor Unit Force Thresholds in the Decerebrate Cat.
Timothy C.Cope, Alan J. Sokoloff, Stan M. Dacko, Rebecca Huot, and Eleanor
Feingold.
Departments of Physiology and Biostatistics, and Neuroscience Program,
Emory University, Atlanta, GA 30322, Department of Physical Therapy, Thomas
Jefferson University, Philadelphia, PA 19107.
APStracts 4:179N, 1997.
ABSTRACT
To further test the hypothesis that some fixed property of motoneurons
determines their recruitment order, we quantified the variation in force
threshold for motoneurons recruited in muscle stretch reflexes in the
decerebrate cat. Motor axons supplying the medial gastrocnemius (MG) muscle
were penetrated with micropipettes and physiological properties of the
motoneuron and its muscle fibers, i.e. the motor unit, were measured. Force
threshold (FT), defined as the amount of MG force produced when the isolated
motor unit was recruited, was measured from 20-93 consecutive stretch trials
for 29 motor units. Trials were selected for limited variation in base force
and rate of rise of force, which have been shown to co-vary with FT, and in
peak stretch force, which gives some index of motor pool excitability. Under
these restricted conditions, large variation in FT would have been
inconsistent with the hypothesis. Analysis of the variation in FT employed the
coefficient of variation (CV), because of the tendency for FT variance and
mean to increase together. We found that CV was distributed with a median
value of 10% and with only 2/29 units exceeding 36%. Some of this variation
was associated with measurement error and with inter-trial fluctuations in
base, peak, and the rate of change of muscle force. CV was not significantly
correlated with motor unit axonal conduction velocity, contraction time, or
force. In three cases FT was measured simultaneously from two motor units in
the same stretch trials. Changes in recruitment order were rarely observed
(5/121 stretch trials), even when FT ranges for units in a pair overlapped. We
suggest that the large variation in recruitment threshold observed in some
earlier studies resulted, not from wide variation in the recruitment ranking
of motoneurons within one muscle, but rather from variation in the relative
activity of different pools of motoneurons. Our findings are consistent with
the hypothesis that recruitment order is determined by some fixed property of
alpha motoneurons and/or by some unvarying combination of presynaptic inputs
that fluctuate in parallel.
Received 5 February 1996; accepted in final form 1 August 1997.
APS Manuscript Number J109-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 28 August 1997