Appearance of "transitional" motor units in overloaded rat skeletal muscle.
The contractile characteristics of single motor units, isolated from rat plantaris muscles subjected to short-term (30 days) compensatory overload, were assessed to determine whether motor units in transition could be detected. In the control plantaris 88% of the motor units were classified as fast. After overload, a large decline (26.5%) in the proportion of typical fast motor units was noted. The estimated contribution of fast fatigable units to whole muscle tetanic tension (Po 200) also declined (from 55 to 25%), whereas that of fast intermediate motor units increased (from 33 to 55%). In the overloaded plantaris, motor units that exhibited unusual "sag" and contraction time characteristics were detected. These motor units, which could be further subdivided into two distinct types by a variety of indexes, exhibited characteristics intermediate to fast and slow units and therefore were termed "transitional." Transitional units accounted for 12% of the estimated whole muscle Po200 after overload. These experiments characterize novel classifications of motor units undergoing transformation and further detail the motor unit shift that accompanies compensatory overload.
Noble
E G
EG
Faculty of Physical Education, University of Western Ontario, London, Canada.
Pettigrew
F P
FP
D000704:Analysis of Variance; D000818:Animals; D006984:Hypertrophy; D008297:Male; D009119:Muscle Contraction; D009132:Muscles / Q000473:pathology / Q000503:physiopathology*; D051381:Rats; D011919:Rats, Inbred Strains