Changes in contractile properties of motor units of the rat medial gastrocnemius muscle after spinal cord transection.
The effects of complete transection of the spinal cord at the level of Th9/10 on contractile properties of the motor units (MUs) in the rat medial gastrocnemius (MG) muscle were investigated. Our results indicate that 1 month after injury the contraction time (time-to-peak) and half-relaxation time were prolonged and the maximal tetanic force in most of the MUs in the MG muscle of spinal rats was reduced. The resistance to fatigue also decreased in most of the MUs in the MG of spinal animals. Moreover, the post-tetanic potentiation of twitches in MUs diminished after spinal cord transection. Criteria for the division of MUs into three types, namely slow (S), fast fatigue resistant (FR) and fast fatigable (FF), applied in intact animals, could not be directly used in spinal animals owing to changes in contractile properties of MUs. The 'sag' phenomenon observed in unfused tetani of fast units in intact animals essentially disappeared in spinal rats and it was only detected in few units, at low frequencies of stimulation only. Therefore, the MUs in spinal rats were classified as fast or slow on the basis of an adjusted borderline of 20 ms, instead of 18 ms as in intact animals, owing to a slightly longer contraction time of those fast motor units with the 'sag'. We conclude that all basic contractile properties of rat motor units in the medial gastrocnemius muscle are significantly changed 1 month after complete spinal cord transection, with the majority of motor units being more fatigable and slower than those of intact rats.
Celichowski
Jan
J
Department of Neurobiology, University School of Physical Education, 60-352 Pozñan, Poland. celichowski@awf.poznan.pl
Mrówczyński
Włodzimierz
W
Krutki
Piotr
P
Górska
Teresa
T
Majczyński
Henryk
H
Sławińska
Urszula
U
D000818:Animals; D005260:Female; D009046:Motor Neurons / Q000502:physiology*; D009119:Muscle Contraction / Q000502:physiology*; D018763:Muscle Fatigue / Q000502:physiology; D018656:Muscle Fibers, Fast-Twitch / Q000502:physiology; D018657:Muscle Fibers, Slow-Twitch / Q000502:physiology; D018482:Muscle, Skeletal / Q000294:innervation* / Q000503:physiopathology*; D051381:Rats; D017208:Rats, Wistar; D013119:Spinal Cord Injuries / Q000503:physiopathology*; D013904:Thoracic Vertebrae; D013997:Time Factors