Activation patterns of hindlimb motor units in the awake rat and their relation to motoneuron intrinsic properties

The activity of hindlimb motor units from the lateral gastrocnemius and tib ialis anterior muscles in the awake rat was compared during locomotion and during slow, sinusoidal muscle stretch. The majority of units were activate d with high initial frequencies and often commenced firing with an initial doubler or tripler. even when activated by slow muscle stretch. The high fi ring rates at recruitment occurred without jumps in the firing rates of oth er concurrently activated units, the firing rate profiles of which were use d as;I measure of the net synaptic drive onto the motoneuronal pool. This s uggested that the sharp recruitment jumps were not due to an abrupt increas e in synaptic drive but rather due to intrinsic properties of the motoneuro n. In addition, motor units that were activated phasically by the muscle st retch fired more action potentials during muscle shortening than during mus cle lengthening, resulting in rightwardly skewed, asymmetrical firing profi les. In contrast, when thr same units fired tonically during the imposed mu scle stretch, the frequency profiles were modulated symmetrically and no no nlinearities were observed. Tonically firing units were modulated symmetric ally throughout a wide range of firing frequencies, and discrete jumps in r ate (i.e., bistable firing) were not observed. The sharp recruitment jumps during locomotion and muscle stretch are proposed to have resulted from the additional depolarization produced by the activation of plateau potentials at recruitment. Likewise, the sustained activation of plateaus subsequent to recruitment may have produced the prolonged firing of the motor units du ring sinusoidal muscle stretch.