In muscle contractions the muscle fiber cells receive action potentials from a n

Question

In muscle contractions the muscle fiber cells receive action potentials from a nerve fiber. The resting concentrations of Na+ and K+ (respectively) are 15 mM and 150mM inside the cytosol, and 150mM and 5mM outside the cell. a. Calculate the peak depolarization voltage of the muscle fiber cell membrane. b. Suppose that an individual suffers from a diseased membrane that has reduced transport permeability of Na+ by a factor of 10. What would be this individual

Explanation / Answer

Short muscle fibers (1 .5 mm) were dissected from hindlimb muscles of frogs and voltage clamped with two microelectrodes to study phenomena related to depolarization-contraction coupling . Isometric myograms obtained in response to depolarizing pulses of durations between 10 and 500 ms and amplitudes up to 140 mV had the following properties . For suprathreshold pulses of fixed duration (in the range of 20-100 ms), the peak tension achieved, the time to peak tension, and contraction duration increased as the internal potential was made progressively more positive. Peak tension eventually saturates with increasing internal potentials. For pulse durations of >_50 ms, the rate of tension development becomes constant for increasing internal potentials when peak tensions become greater than one-third of the maximum tension possible . Both threshold and maximum steepness of the relation between internal potential and peak tension depend on pulse duration . The relation between the tension-time integral and the stimulus amplitude-duration product was examined. The utility of this relation for excitation-contraction studies is based on the observation that once a depolarizing pulse configuration has elicited maximum tension, further increases in either stimulus duration or amplitude only prolong the contractile response, while the major portion ofthe relaxation phase after the end of a pulse is exponential, with a time constant that is not significantly affected by either the amplitude or the duration of the pulse. Hence, the area under the tension-response curve provides a measure of the availability to troponin of the calcium released from the sarcoplasmic reticulum in response to membrane depolarization . The results from this work complement those obtained in experiments in which intramembrane charge movements related to contractile activation were studied and those in which intracellular Ca" transients were measured.

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