Journal of Neurophysiology, Vol 40, Issue 1 16-25, Copyright © 1977 by APS
Identification of two species of alpha motoneurons in cat's plantaris pool
D. A. Harris and E. Henneman
1. Single units of the plantaris pool were isolated in ventral root
filaments of decerebrate cats and their critical firing levels (CFLs) were
determined. Motoneurons of similar size, as judged by their CFLs and other
criteria, were compared in firing rate (FR) during repetitive stimulation
of the plantaris nerve. 2. Such units either differed very little or quite
widely, suggesting that they were sampled randomly from two populations,
one firing rapidly, the other slowly. The relationship between the two
rates remained approximately constant, regardless of the intensity or rate
of input the units received, as long as both of them discharged
rhythmically. 3. In single experiments 10-15 of the smallest units in the
pool (all with CFLs in the 0-8% range) were isolated and compared.
Statistical analyses and visual inspection of these small samples again
suggested the existence of two species of motoneurons. 4. Statistical
analyses also indicated that the FRs of units in single experiments were
not sampled from any one of a variety of parametric, single-modal
distributions. This suggests that the data were sampled from a distribution
having more than one mode, indicating the existence of separate populations
or species of motoneurons among the small units of the pool (0-8% range of
CFL). 5. Pooling of the normalized data from different experiments revealed
a bimodal histogram, reinforcing the conclusion that there are two species
of small alpha motoneurons in the plantaris pool.
