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Effects of membrane potential on mechanical activation in skeletal muscle
Abstract:
The effect of subthreshold depolarization on mechanical threshold was investigated in tetrodotoxin-poisoned mammalian and amphibian skeletal muscle fibers using a two-microelectrode voltage-clamp technique. Mechanical threshold was determined with a 2-ms test pulse. The immediate effect of depolarization was inhibition of the mechanical system. The consequent increase in the test pulse threshold was linearly related to the size of the depolarization and there was, on the average, a 10% increase in threshold for a 10-mV depolarization in mammalian fibers. The duration of the inhibitory period was also related to the size of the depolarization. Inhibition was interrupted by the onset of activation (seen as a reduction in the test pulse threshold), and in rat soleus fibers this occurred within 100 ms with a 20-mV depolarization, inhibition decayed within 10 ms. The decay of activation after brief conditioning pulses was initially rapid (on the average, the test pulse threshold recovered to 80% of its control value within 1 ms) and then slow (full recovery took 100-500 ms). After long conditioning pulses, activation often decayed into a period of inhibition. When depolarization (of 20 mV or more) was maintained for several seconds, the fibers became inactivated. Rat extensor digitorum longus and sternomastoid fibers were strongly inactivated by depolarization to -40 mV and the test pulse to +40 mV did not cause contraction.
Insights
Subthreshold depolarization inhibits skeletal muscle mechanical responses. This inhibition increases with depolarization magnitude and duration, affecting muscle activation and leading to inactivation with prolonged depolarization.
Area of Science:
- Muscle Physiology
- Cellular Electrophysiology
Background:
- Understanding the electro-mechanical coupling in skeletal muscle is crucial.
- Subthreshold membrane potential changes can significantly influence muscle fiber excitability and contractility.
Purpose of the Study:
- To investigate the impact of subthreshold depolarization on the mechanical threshold of mammalian and amphibian skeletal muscle fibers.
- To characterize the relationship between depolarization magnitude/duration and subsequent muscle inhibition and activation.
Main Methods:
- Utilized a two-microelectrode voltage-clamp technique in tetrodotoxin-poisoned muscle fibers.
- Determined mechanical threshold using short (2-ms) test pulses.
- Applied varying levels of subthreshold depolarization to assess effects.
Main Results:
- Subthreshold depolarization immediately inhibited the mechanical system, increasing the test pulse threshold.
- The increase in threshold was linearly related to depolarization size (e.g., 10% per 10 mV in mammalian fibers).
- Prolonged depolarization led to inactivation, preventing contraction even with strong stimuli.
Conclusions:
- Subthreshold depolarization reversibly alters skeletal muscle mechanical excitability.
- The duration and magnitude of depolarization dictate the balance between inhibition and activation, with sustained depolarization causing inactivation.
- Findings provide insights into the complex regulation of muscle force generation.