Skeletal muscles consist of motor units which may differ considerably in contractile properties and types of usage. Some units participate mainly in relatively rare, quick movements and contract rapidly and are easily fatigued (type FF); others contribute to the maintenance of posture and hence contract slowly and are fatigue-resistant (type S), while others are both fast and fatigue-resistant (type FR). Our understanding of motor control mechanisms and the dependence of contractile properties on usage would be enhanced if more quantitative information were available concerning the firing patterns of individual motor units during normal motor behaviour. Therefore, we have made continuous recordings for extended periods from single motor units in the fast extensor digitorum longus (edl) and the slow soleus (sol) muscle of freely moving adult rats. By counting the total number of discharges for each unit, and by determining the distributions of interspike intervals and the duration of the individual impulse trains, we have obtained information about firing rate, amount of use, modulation of muscle force and tonic and phasic behaviour for 16 motor units. We now report that these units fall into three classes apparently corresponding to type FF and FR in the edl muscle and type S in the soleus muscle.
The discharge patterns of 16 motor units in extensor digitorum longus (EDL) and soleus (SOL) muscles of freely moving adult rats, described by Hennig & Lømo (1985), were further analysed with respect to their role in grading muscle force output. The units fell into three distinct classes, termed EDL-1, EDL-2 and SOL-1, probably corresponding to type FF, FR and S units. The EDL-1 units generated only single impulses or impulse trains of short duration (phasic firing) which had high frequency and usually started with a short interspike interval (initial doublet). The EDL-2 and SOL-1 units generated single impulses and impulse trains of both short and long durations (phasic and tonic firing) without initial doublets. The frequency was high in EDL-2 and low in SOL-1 units. In EDL-2 and SOL-1 units, the mean durations of the first interspike intervals in a train decreased as the number of impulses per train increased. In EDL-1 units they did not change. Both SOL and EDL muscles were simulated through the nerve at different regular frequencies and tension-frequency (T-F) curves constructed. The EDL-2 units fired naturally most often at frequencies corresponding to the steepest part of the EDL T-F curve. The EDL-1 and SOL-1 units fired naturally most often at frequencies where the T-F curves of their respective muscles began to flatten before maximum tetanic tension was reached. Stimulus trains starting with an initial doublet produced maximum rate of tension development (optimum impulse pattern). At optimum intervals the force increased from about 20 to 85% of maximum tetanic tension when the number of stimuli was increased from 1 to 7. It is concluded that the natural firing pattern of EDL-1 units and the contractile properties of EDL muscle fibres are normally matched so that the force can develop at maximum rate to maximum levels at the start of contractions. Tension output is apparently regulated primarily through varying number of impulses per train in EDL-1 units; in SOL-1 and EDL-2 units both rate and number of impulses are important.
Serum levels of GFAP demonstrated a linear correlation to tumour volume in patients with high-grade gliomas. GFAP seems to be a more reliable biomarker in patients with high-grade gliomas than the commercially available S-100B.
This study seeks to identify the mechanisms which motoneurones use to control the contractile force and speed of skeletal muscles. We have stimulated directly slow soleus (SOL) and fast extensor digitorum longus (EDL) muscles of adult rats intermittently at 100 Hz for 1-9 months. The muscles were either chronically denervated, denervated and reinnervated, or normally innervated. The stimulation started either immediately, or more commonly, after 1-9 months of denervation. Stimulation starting several months after denervation increased the mean maximum tetanic tension 37 times in SOL and eight times in EDL. These values represented 40 and 12% of the increases obtained by reinnervation after comparable periods of time. In denervated SOL and EDL muscles stimulated directly for more than 2 months, the mean isometric twitch contraction times were 13 and 12.7 ms, as in normal EDL muscles (13 ms). In innervated SOL muscles stimulated directly for 1-4 months, the mean twitch contraction times were 23.6 ms (normally innervated) and 19.2 ms (reinnervated), which were considerably shorter than in normal control SOL muscles (39.2 ms). Single motor unit recordings revealed that the natural (background) nerve impulse activity was essentially unaffected by the stimulation. Twitch contraction time and percentage of type II fibres in SOL muscles were related. The fastest muscles (denervated and stimulated) consisted of 100% type II fibres (with one exception), the second fastest (reinnervated and stimulated) of 70-50%, the third fastest (normally innervated and stimulated) of 45-0%, the second slowest (reinnervated) of 15-0%, and the slowest muscles (innervated controls) of 5-0% type II fibres.
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