Activity of hindlimb motor units during locomotion in the conscious rat

This paper compares the activity of hindlimb motor units from muscles mainl y composed of fast-twitch muscle fibers (medial and lateral gastroenemius: MG/LG, tibialis anterior: TA) to motor units from a muscle mainly composed of slow-twitch muscle fibers (soleus: SOL) during unrestrained walking in t he conscious rat. Several differences in the activation profiles of motor u nits from these two groups of muscles were observed. For example, motor uni ts from fast muscles (e.g., MG/LG and TA) fired at very high mean frequenci es of discharge, ranging from 60 to 100 Hz, and almost always were recruite d with initial doublets or triplets, i.e., initial frequencies greater than or equal to 100 Hz. In contrast, the majority of SOL units fired at much l ower mean rates of discharge, approximate to 30 Hz, and had initial frequen cies of only 30-60 Hz (i.e., there were no initial doublets/triplets greate r than or equal to 100 Hz). Thus the presence of initial doublet or triplet s was dependent on the intrinsic properties of the motor unit, i.e., faster units were recruited with a doublet/triplet more often than slower units. Moreover, in contrast to units from the slow SOL muscle, the activity of si ngle motor units from the fast MG/LG muscle, especially units recruited mid way or near the end of a locomotor burst, was unrelated to the activity of the remainder of the motoneuron pool, as measured by the corresponding gros s-electromyographic (EMG) signal. This dissociation of activity was suggest ed to arise from a compartmentalized recruitment of the MG/LG motoneuron po ol by the rhythm-generating networks of the spinal cord. In contrast, when comparing the rate modulation of simultaneously recorded motor units within a single LG muscle compartment, the frequency profiles of unit pairs were modulated in a parallel fashion. This suggested that the parent motoneurons were responsive to changes in synaptic inputs during unrestrained walking, unlike the poor rate modulation that occurs during locomotion induced from brain stem stimulation. In summary, data from this study provide evidence that the firing behavior of motor units during unrestrained walking is infl uenced by both the intrinsic properties of the parent motoneuron and by syn aptic inputs from the locomotor networks of the spinal cord. in addition, i t also provides the first extensive description of motor-unit activity from different muscles during unrestrained walking in the conscious rat.