Abstract
The cortical control of fast and slow muscles of the ankle has been the subject of numerous reports yielding conflicting results. Although it is generally agreed that cortical stimulation yields short latency facilitation of fast muscles, the effects on the slow muscle, soleus, remain controversial. Some studies have shown predominant facilitation of soleus from the cortex while others have provided evidence of differential control in which soleus is predominantly inhibited from the cortex. The objective of this study was to investigate the cortical control of fast and slow muscles of the ankle using stimulus triggered averaging (StTA) of EMG activity, which is a sensitive method of detecting output effects on muscle activity. This method also has relatively high spatial resolution and can be applied in awake, behaving subjects. Two rhesus macaques were trained to perform a hindlimb push-pull task. Stimulus triggered averages (StTAs) of EMG activity (15, 30, and 60 μA at 15 Hz) were computed for four muscles of the ankle [tibialis anterior (TA), medial gastrocnemius (MG), lateral gastrocnemius (LG), and soleus] as the monkeys performed the task. Poststimulus facilitation (PStF) was observed in both the fast muscles (TA, MG, and LG) as well as the slow muscle (soleus) and was as common and as strong in soleus as in the fast muscles. However, while poststimulus suppression (PStS) was observed in all muscles, it was more common in the slow muscle compared to the fast muscles and was as common as facilitation at low stimulus intensities. Overall, our results demonstrate that cortical facilitation of soleus has an organization that is very similar to that of the fast ankle muscles. However, cortical inhibition is organized differently allowing for more prominent suppression of soleus motoneurons.
Highlights
The existence of fast and slow motor units is well known (Eccles et al, 1958; Andersen and Sears, 1964; Kugelberg and Edstrom, 1968; Ranvier, 1874; Kronecker and Stirling, 1878)
While 19 hindlimb muscles were implanted in each monkey, this paper focuses on the results of EMGs recorded from four ankle muscles: tibialis anterior (TA), medial gastrocnemius (MG), lateral gastrocnemius (LG) and soleus (SOL) (Figure 1B)
It was proposed that cortically initiated movements could be enhanced through cortical inhibition of soleus as a slow, tonically contracting postural muscle, coupled with excitation of its agonists—MG and LG. We examined this issue by determining the relative prevalence of poststimulus suppression (PStS) and Poststimulus facilitation (PStF) in the gastrocnemius muscles and TA when (1) PStS was present in soleus, and (2) PStF was present in soleus
Summary
The existence of fast and slow motor units is well known (Eccles et al, 1958; Andersen and Sears, 1964; Kugelberg and Edstrom, 1968; Ranvier, 1874; Kronecker and Stirling, 1878). Preston and Whitlock (1963) and Uemura and Preston (1965), studying monosynaptic reflex conditioning in the “pyramidal” monkey preparation, in which the brainstem is destroyed leaving only the pyramidal tract intact, reported corticospinal output to soleus motoneurons was predominantly inhibitory while output to motoneurons of fast muscles (gastrocnemius and TA) was excitatory. Binder et al (1998) measured effective synaptic currents in fast and slow motoneurons of triceps surae associated with stimulating the contralateral pyramidal tract. They reported that more than 60% of putative slow motoneurons received a net hyperpolarizing effective synaptic current from pyramidal tract stimulation compared to only 33% of fast motoneurons. Consistent with this result, they found that pyramidal tract stimulation increased the discharge rate of motoneurons receiving depolarizing effective currents while decreasing the rate of those receiving hyperpolarizing currents
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