ACTIVITY AND MOTOR UNIT SIZE IN PARTIALLY DENERVATED RAT MEDIAL GASTROCNEMIUS

The aim of the study was to determine whether increased motoneuron act ivity induced by treadmill walking would alter the extent of motoneuro n sprouting in the partially denervated rat medial gastrocnemius muscl e. An extensive partial denervation was effected by unilateral section of the L(5) ventral root, and it is very likely that all units remain ing in the medial gastrocnemius were used in treadmill walking. Rats w ere trained for 1.5 h/day and after 14 days were walking at least 1 km /day. Motor unit characteristics were determined 24 days after the par tial denervation and were compared with units from partially denervate d control (PDC) animals and with units from normal (control) animals. In PDC rats, force developed by slow, fast fatigue-resistant, and fast intermediate-fatigable motor units increased substantially compared w ith control animals; that of fast-fatigable units did not increase. In partially denervated exercised animals, force developed by slow and f ast-fatigue-resistant units showed no further increase, but fast-inter mediate- and fast-fatigable units showed significant increases compare d with those in PDC animals. The changes in force were closely paralle led by changes in innervation ratios. We concluded that neuronal activ ity is an important factor in determining the rate of motoneuron sprou ting.