Distribution of activity within the cat's peroneus longus muscle when activated in different ways via the central...

We have studied whether a mechanically homogenous muscle (all units giving joint-torques in the same direction) would display different distributions of activity in response to different types of activation via the central nervous system. In adult cats anaesthetized with pentobarbitone, long-lasting contractions were evoked in m. peroneus longus (PerL), a mixed ankle dorsiflexor muscle. The contractions were elicited by continuous repetitive stimulation of: (1) the superficial peroneal nerve (NP, flexion reflex); and (2) the contralateral motor cortex (MC). During these contractions, isometric force was monitored and fine-wire electromyographic recordings (EMG) were simultaneously obtained from anterior and posterior portions of the PerL. The relative degree of activation in anterior vs posterior muscle portions was quantified by measuring, at various force levels: (1) the average spike amplitude (Spike); and (2) the total number of spikes per unit time (Count; 0.5-s measurement periods). For both types of stimulation (MC and NP), the results indicated that activation was more effective for posterior than for anterior muscle portions: the Spike-measurements were typically higher in posterior than in anterior PerL and, for MC-elicited contractions, this was true for the Count-measurements as well. With respect to both types of EMG-quantification (Spike and Count), the 'posterior bias' of activity was significantly more pronounced for the cortically evoked contractions than for those elicited via stimulation of the peroneal nerve. As it is known that anterior and posterior PerL portions tend to be innervated by rostral and caudal motoneurones, respectively, these findings indicate that different inputs to the PerL motoneurone pool may differ significantly with respect to the intraspinal spatial distribution of synaptic effects among the motoneurones. The results were discussed in relation to the organization of task-related recruitment schemes.