Journal of Neuroscience, Vol 8, 2142-2152, Copyright © 1988 by Society for Neuroscience
Spatial distribution of motor unit fibers in the cat soleus and tibialis anterior muscles: local interactions
SC Bodine, A Garfinkel, RR Roy and VR Edgerton
Brain Research Institute, University of California, Los Angeles 90024.
The spatial distribution of muscle fibers belonging to a motor unit was
studied in the soleus and tibialis anterior muscles of adult cats. Motor
unit fibers were depleted of their glycogen through repetitive stimulation
of the motoneuron or of the functionally isolated motor axon. Subsequently,
the position of depleted muscle fibers was mapped on serial cross sections
taken along the length of the muscle. A subset of fibers was selected from
the cross section containing the largest number of motor unit fibers and 4
spatial analyses were performed. These analyses were designed to determine
whether the muscle fibers belonging to a single unit were distributed in a
random manner. To test whether the actual distribution was other than
random, Monte Carlo techniques were used to simulate the random innervation
of a muscle. From these simulations, a test statistic was calculated for
comparison with the observed data. Adjacency and nearest-neighbor analyses
revealed no tendency for grouping or dispersion of fibers belonging to a
motor unit. However, measurement of the distances between all motor unit
fibers revealed a greater tendency for grouping than spreading, suggesting
the existence of some mechanism that restricts the absolute distribution
and territory of a motor unit.