Activation of Type-Identified Motor Units during Centrally Evoked
Contractions in the Cat Medial Gastrocnemius Muscle. I. Motor-Unit
Recruitment.
Tansey, K. E. and B. R. Botterman.
Institution: Department of Cell Biology and Neuroscience, University of
Texas Southwestern Medical Center, Dallas, Texas 75235.
APStracts 2:0240N, 1995.
SUMMARY AND CONCLUSIONS
1. The recruitment order of 64 pairs of motor units, comprising 121 type-
identified units, was studied during centrally-evoked muscle contractions of
the cat medial gastrocnemius (MG) muscle in an unanesthetized, high
decerebrate preparation. Motor units were functionally isolated within the MG
nerve by intra-axonal (or intra-myelin) penetration with conventional glass
microelectrodes. 2. Graded stimulation of the mesencephalic locomotor region
(MLR) was used to evoke smoothly graded contractions, which under favorable
conditions was estimated to reach 40% of maximum tetanic tension of the MG
muscle. With this method of activation, 100% of slow twitch (type S) units,
95% of fast twitch, fatigue resistant (type FR) units, 86% of fast twitch,
fatigue intermediate (type FI) units and 49% of fast twitch, fatigable (type
FF) units studied were recruited. 3. Motoneuron size as estimated by axonal
conduction velocity (CV) was correlated with muscle-unit size as estimated by
maximum tetanic tension (Po). Although the correlation between these
properties was significant among type S and FR units, no significant
correlation was found for these properties among type FI and FF units. 4.
Motor-unit recruitment was ordered by physiological type (S>F, 100% of pairs;
S>FR>FI>FF, 93% of pairs). Although none of the motor-unit properties studied
predicted recruitment order perfectly, motor-unit recruitment was found to
proceed by increasing Po (89% of pairs), decreasing contraction time (79% of
pairs), decreasing fatigue index (80% of pairs), and increasing CV (76% of
pairs). These percentages were significantly different from random (i.e. 50%).
Statistically, all 4 motor-unit properties were equivalent in predicting
recruitment order. These results are similar to those reported by other
investigators for motor-unit recruitment order evoked from other supraspinal
centers, as well as from peripheral sites. 5. When, however, motor-unit
recruitment within pairs of motor units containing two fast-twitch (type F)
units was examined, Po was a significantly better predictor of recruitment
order than CV (85% vs. 52% of pairs). One explanation for this observation is
that the correlation between Po and CV is high among type S, type FR units and
possibly among the lower-tension type FF units, but not among the remaining
higher-tension type FF units. 6. The reproducibility of recruitment order in
multiple contractions was investigated in 16 motor-unit pairs. Recruitment
order was found to be variable in only 3 motor-unit pairs, all of which
contained units of similar physiological type and recruitment threshold.7.
Analysis of recruitment order by pair-wise testing confirms the general
conclusion reached in human studies that the muscle force level at recruitment
for a motor unit is highly correlated with its strength. As an additional
confirmation, the whole-muscle force level at recruitment for 41 units was
measured in a series of contractions in which the rate of rise of muscle
tension was limited to rates below 1000 g/s. For these contractions, a
significant correlation was found between muscle tension at recruitment and
motor unit Po.
Received 13 September 1994; accepted in final form 9 August 1995.
APS Manuscript Number J576-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 24 August 1995.