LEVEL OF INDEPENDENCE OF MOTOR UNIT PROPERTIES FROM NEUROMUSCULAR ACTIVITY

Neuromuscular activity was eliminated in the tibialis anterior muscle of adult cats for 6 months by spinal isolation (SI), i.e., complete sp inal cord transections at T-12-13 and at L-7-S-1, plus bilateral dorsa l rhizotomy between the two transection sites. One motor unit from eac h muscle was isolated using ventral root teasing procedures and physio logically tested. The fibers belonging to each motor unit were visuali zed in PAS-stained sections by the loss of glycogen following prolonge d repetitive stimulation. Qualitatively, the normal enzymatic interrel ationships among fibers identified by myosin heavy chain composition w ere unchanged by SI, Generally, each motor unit from SI cats were of a single myosin immunohistochemical type. The same physiological motor unit types that typify control muscles were found in SI cats. In SI co mpared to control cats, there was approximately a 10% increase in the number of muscle fibers expressing fast myosin. Mean fiber activity le vels of ATPase and SDH for a given fiber type (based on MHC antibody r eactions) decreased by approximately 10% and 25%, whereas GPD activity increased approximately 35%. It is concluded that differential levels or patterns of activity are not essential to maintain the range of hi stochemical and physiological motor unit types found in the tibialis a nterior of normal adult cats. (C) 1994 John Wiley & Sons, Inc.