VPB 121: Veterinary Physiology - II (2+1)

MEMBRANE POTENTIAL

• 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.

• 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.
• 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.
• Trapping of negatively charged anions inside the cell
• Many intracellular anions are large molecules proteins, organic SO₄ and PO₄ are trapped within the cell and are attracted to the inner surface of the cell membrane generate negative electrical potential inside the cell.
• These three factors are the primary cause of membrane potential. The magnitude of this potential produced by diffusion of ions can be predicted by the Nernst equation.
  • E_x = (-60mV/z) x log[X _inside/X _outside] E_x = 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 fibres

• During resting stage, the muscle fibres have an electrical potential of - 90 mV inside 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.