Membrane potential of muscles

Membrane potential of muscles

The interstitial fluid and the intracellular fluid contain 147 mEq/litre of positive ions (cations) outside the cell, and about 155 mEq /L, of negative ions (anions) inside the cell. Higher concentration of Na+ ions (142 mEq/L) and lower concentration of K+ ions (5 mEq/L) characterise the interstitial fluid.

The intracellular fluid has more of K ions (140 mEq/L) and less of Na+ ions (14mEq/L). This concentration difference of ions across a selectively permeable cell membrane creates an electrical potential difference.

This electrical potential difference across the cell membrane is called as membrane potential”.

Basic mechanisms of membrane potential

Three major factors cause the membrane potential-

1. Differential permeability of the membrane to diffusion of ions

During resting state the cell membrane K+ ions is 50 to100 times more 0diffusable to out side than to Na+ ions through non-gated leak channels due to their concentration gradient.

This contributes accumulation of positive charges immediately outside the membrane.

The resting membrane is almost completely impermeable /very slightly permeable to Na+ ions.

2. The Na+, K+ electrogenic pump

It generates positive membrane potential out side the cell by actively pumping 3 Na+ ions out of the cell for every 2 K+ ions pumped into the cell against their concentration gradient.

3. Trapping of negatively charged anions inside the cell

Many intracellular anions are large molecules proteins, organic SO4 and POare trapped within the cell and are attracted to the inner surface of the cell membrane generate negative electrical potential inside the cell.

  • The magnitude of this potential produced by diffusion of ions can be predicted by the Nernst equation
    • Ex = (-60mV/z) x log[X inside/X outside] E= equilibration potential for ion X
    • z = valence of the ion – 1 for Na & K
    • [X] = Concentration of ion X

The diffusion potential caused by K and Na ion diffusion would give a membrane potential of about. An additional giving a net resting membrane potential of – 90 mV.

The resting membrane/ diffusion potentials of skeletal muscle fibers

During resting stage, the muscle fibres have an electrical potential of – 90 mVinside the fiber (range from – 75 to – 90 mV) of which – 86 mV is determined by K diffusion and the remaining – 4mV is contributed by the electrogenic Na+– K+ pump.

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