MUSCLE ACTION POTENTIAL AND ACTION POTENTIAL CURVE
The process of generating a muscle action potential shares similarities with the initiation and conduction of action potentials in nerve fibres, with some quantitative differences.
In skeletal muscle fibres, the resting membrane potential is about -80 to -90 millivolts, which is more negative than in neurons. The action potential’s duration ranges from 1 to 5 milliseconds, which is about five times longer than in large myelinated nerves. Additionally, the velocity of conduction is 3 to 5 m/sec, which is approximately 1∕13th of the velocity of conduction in the large myelinated nerve fibres that stimulate skeletal muscle. As shown in Figure 7.11 in muscle tissue, the action potential curve has three major parts: the latent period, depolarisation, and repolarisation.The latent period, which occurs immediately after applying a stimulus, is a brief phase lasting 0.5 to 1 millisecond, during which no change in the electrical potential is observed. Following the stimulus, there is a slight irregular deflection of the baseline for a very short period, known as the stimulus artefact. The current leakage from the stimulating electrode to the recording electrode creates a disturbance in the muscle, which results in the artefact. Subsequently, the stimulus artefact is followed by the latent period, which exists as a brief pause of 1-2 milliseconds after the action potential reaches the muscle before tension becomes noticeable. During the latent period, the action potential is propagated along the sarcolemma, and upon the release of calcium ions from the sarcoplasmic reticulum, the subsequent phases of muscle contraction occur.
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