Cat Hindlirnb Motoneurons During Locomotion . 111 . Functional Segregation in Sartorius

1. Cat sartorius has two distinct anatomical portions, anterior (SA-a) and medial (SA-m). SA-a acts to extend the knee and also to flex the hip. SA-m acts to flex both the knee and the hip. The objective of this study was to investigate how a "single motoneuron pool" is used to control at least three separate functions mediated by the two anatomical portions of one muscle. 2. Discharge patterns of single motoneurons projecting to the sartorius muscle were recorded using floating microelectrodes implanted in the L5 ventral root of cats. The electromyographic activity generated by the anterior and medial portions of sartorius was recorded with chronically implanted electrodes. The muscle portion innervated by each motoneuron was determined by spike-triggered averaging of the EMGs during walking on a motorized treadmill. 3. During normal locomotion, SA-a exhibited two bursts of EMG activity per step cycle, one during the stance phase and one during the late swing phase. In contrast, every recorded motoneuron projecting to SA-a discharged a single burst of action potentials per step cycle. Some SA-a motoneurons discharged only during the stance phase, whereas other motoneurons discharged only during the late swing phase. In all cases, the instantaneous frequencygram of the motoneuron was well fit by the rectified smoothed EMG envelope generated by SA-a during the appropriate phase of the step cycle. 4. During normal locomotion, SA-m exhibited a single burst of EMG activity per step cycle, during the swing phase. The temporal characteristics of the EMG bursts recorded from SA-m differed from the swing-phase EMG bursts generated by SA-a. The SA-m EMG bursts typically began earlier and peaked earlier than the swing-phase SA-a EMG bursts. Activity in SA-m declined progressively during the El phase of swing, whereas activity during swing in SA-a peaked late in the El phase and shut off abruptly before footfall. 5. Each sartorius motoneuron active during swing had an instantaneous frequencygram that fell into one of two classes: a group whose discharge profiles were best fit by the SA-a EMG profile and a group whose frequencygrams were best fit by the SA-m EMG profile. 6. We conclude that the motoneuron pool that innervates the cat sartorius muscle consists of three functionally separate motoneuron groups, each of which is independently recruited to perform one of three tasks: knee extension (stance phase), knee and hip flexion (early swing phase), or knee extension and hip flexion (late swing phase). 7. These distinctions are correlated with heterogeneities in both skeletal action (SA-a vs. SA-m) and kinematic function (SA-a actively shortens during swing and actively lengthens during stance). The existence of such functional distinctions suggests circumstances for which the traditional concepts of "one muscle-one motoneuron pool" and orderly recruitment by size of the motoneurons comprising a motor nucleus may require modification.