Effects of age at cordotomy and subsequent exercise on contraction times of motor units in the cat.
The contraction times (CTs) of functionally isolated motor units (MUs) in the soleus (SOL) and medial gastrocnemius (MG) muscles were determined in cats that had been spinalized at ages 2 (n = 15) or 12 (n = 9) wk and then either subjected to exercise on a treadmill or simply given manipulative care of the hindlimbs. The MUs were tested approximately 12 wk after the low-thoracic cordotomy, and comparisons were made with data from control animals. The CT of 50.9 ms obtained for SOL units (n = 163) in the spinal cats was 22% shorter than the mean of 65.0 ms for MUs (n = 57) from control cats (n = 4). Contrary to expectation, the CT in animals spinalized at 12 wk was significantly shorter than that in the 2-wk group. The CT for MG units (n = 105) in spinal cats was also significantly shorter (11%) than that in controls cats (n = 66, 6 cats), and those units identified by their high fatigue index as being of slow or fatigue-resistant type had a shorter CT than units with a low index. No distinction in CT of exercised and nonexercised groups was detected for either muscle. These findings are discussed in relation to the bearing influences of supraspinal and segmental origin have on CT duration in SOL and MG muscles during growth of the kitten. A slight, significant decrease (6%) in the fatigue index of SOL MUs (n = 144) was detected, but the values remained high (mean 0.87).
Smith
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LA
Brain Research Institute, University of California, Los Angeles 90024-1761.
Eldred
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E
Edgerton
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VR
D000375:Aging / Q000502:physiology*; D000818:Animals; D002415:Cats; D002818:Cordotomy; D005221:Fatigue / Q000503:physiopathology; D009119:Muscle Contraction / Q000502:physiology; D009132:Muscles / Q000166:cytology / Q000294:innervation / Q000502:physiology*; D010805:Physical Conditioning, Animal