Further investigations on the influence of motoneurones on the speed of muscle contraction

655

195

SUMMARY1. The time course of contraction and relaxation in the isometric twitch of a rabbit soleus muscle becomes more rapid following tenotomy and spinal cord section. This increase in speed could be prevented by long-term electrical stimulation at frequencies of 5 or 10/sec. It was not prevented by stimulation at frequencies of 20 or 40/sec.2. Long-term electrical stimulation of fast rabbit and cat muscles at a frequency of 10/sec had a slowing effect on the time course of contraction and relaxation.3. It is concluded that the almost continuous low frequency discharge of motoneurones innervating postural muscles plays an important part in establishing and maintaining the slow time course of contraction of these muscles. The characteristically different speeds of contraction of fast and slow striated muscles can in this way be related to the different patterns of impulse activity which they normally receive.

Section: Discussionmentioning

confidence: 99%

The influence of activity on some contractile characteristics of mammalian fast and slow muscles

Salmons¹,

Vrbová²

1969

655

195

“…The longer absolute refractory period may reflect an alteration in membrane properties while the after-contraction suggests a latent predisposition to fibrillation at the characteristic frequency of 10 Hz (see Lewis, 1972). It may be noted that slowing occurred in the absence of active fibrillation which was postulated as a causal factor by Eccles et al (1962).…”

Section: Discussionmentioning

confidence: 99%

“…In pursuit of information concerning the role of activity, a number of disuse situations have been employed including tenotomy (Vrbova, 1963 a;Nelson 1969), limb fixation (Fischbach & Robbins, 1969) cord isolation (Eccles, Eccles & Kozak, 1962) and denervation (Lewis, 1972). These methods are open to certain criticisms and have produced variable results.…”

mentioning

confidence: 99%

The effect of prolonged inactivity upon the contraction characteristics of fast and slow mammalian twitch muscle (cat)

Davis¹,

Montgomery²

1977

SUMMARY1. Prolonged inactivity of soleus (slow twitch) and medial head of flexor digitorum longus (fast twitch: previously commonly known as flexor hallucis longus and hereafter referred to as FDL) muscles of the cat was produced by sodium pentobarbitone anaesthesia or spinal isolation. Isometric contraction characteristics were examined after 4-22 days and 8-49 days respectively.2. Sleep of up to 3 weeks' duration was associated with progressive changes in the weight, maximum tetanic tension, speed of contraction and absolute refractory period of fast twitch muscle. The slow muscle showed corresponding changes which were not so pronounced. Cord isolation resulted in similar changes which tended to be reversed after longer periods of disuse.3. Fast muscle frequently exhibited an after-contraction with corresponding electromyographic activity following two weeks or more of disuse.4. The time to peak of FDL was prolonged, becoming approximately one and a half times normal. In contrast to other reports of disuse, that of soleus was normal or slightly prolonged.5. The ratio of twitch to tetanic tension of FDL increased to twice the normal value or greater and remained high. The soleus muscle showed a smaller increase with evidence of a return towards normal after longer periods of inactivity.6. Absolute refractory period of FDL was considerably prolonged (more than 50 %) following barbiturate sleep and this also occurred, but to a lesser degree, with soleus after 4-7 weeks of cord isolation.

“…When the soleus nerve is sectioned and immediately reunited to the muscle, no functional motor re-innervation of the muscle can be seen for at least 30 days after the operation (Eccles, Eccles & Kozak, 1962;Kuno et al 1974b). Therefore, it is unlikely that muscle contraction could be elicited by intracellular stimulation of the motoneurones whose axons had been sectioned 8-22 days previously by partial denervation of the muscle.…”

Section: Methodsmentioning

confidence: 99%

Reaction of intact spinal motoneurones to partial denervation of the muscle.

Huizar¹,

Kuno²,

Kudo³

et al. 1977

SUMMARY1. The properties of soleus motoneurones of the cat were examined with intracellular electrodes about 3 weeks after partial denervation of the soleus muscle. Soleus motoneurones whose axons had been left intact were distinguished from those whose axons had been sectioned by the presence or absence of muscle contraction in response to intracellular stimulation of each motoneurone.2. The average twitch tension of motor units evoked by intracellular stimulation of intact soleus motoneurones after partial denervation of the muscle was not significantly different from that observed in control, unoperated cats. Therefore, it was assumed that the majority of intact motoneurones had not been subject to injuries in their axons upon partial denervation.3. Soleus motoneurones whose axons had been sectioned showed a significant increase in overshoot of action potentials ajid a significant decrease in resting membrane potential, in axonal conduction velocity and in the duration of after-hyperpolarization. 4. Soleus motoneurones whose axons had been left intact also showed a significantly shorter after-hyperpolarization than that seen in control, unoperated cats. Other electrophysiological properties of the intact soleus motoneurones were indistinguishable from those observed in unoperated animals.5. The decrease of the duration of after-hyperpolarization in intact soleus motoneurones was greater in highly denervated preparations than in moderately denervated preparations.6. The decrease of the duration of after-hyperpolarization in intact soleus motoneurones was associated with a decrease in contraction times of the innervated muscle fibres, the former preceding the latter by one to two weeks.