Origin of the resting membrane potential

2021-06-04 03:23 PM

The relatively static membrane potential of quiescent cells is called the resting membrane potential (or resting voltage), as opposed to the specific dynamic electrochemical phenomena.

Relatively static membrane potential is usually referred to as the ground value for trans-membrane voltage.

The relatively static membrane potential of quiescent cells is called the resting membrane potential (or resting voltage), as opposed to the specific dynamic electrochemical phenomena called an action potential and graded membrane potential.

The figure shows the important factors that establish the membrane potential, which are:

resting membrane potential

Figure. Establish resting membrane potentials in nerve fibres under three conditions: A, when the membrane potential is induced solely by potassium diffusion; B, when the membrane potential is induced by the diffusion of both sodium and potassium ions; and C, when the membrane potential is induced by the diffusion of both sodium and potassium ions plus the pumping of both by the Na + -K + pump.

The contribution of potassium diffusion

In Figure A, we assume that only its transport of potassium ions is diffused across the membrane, as evidenced by the opening of potassium channels inside and outside the cell membrane. Because the ratio of potassium concentration in and out of the membrane is very high, up to 35:1, according to the Nerst equation, the potential potassium diffusion is -61 mV x log35 = -94mV. So if potassium was the only ion generating the resting potential, the resting potential inside the nerve fiber would be -94mV as shown in the figure.

Contribution of sodium ion diffusion

Figure B shows the additional contribution to the membrane potential by sodium diffusion, caused by sodium diffusion through Na+-K+ channel leakage. The concentration ratio to sodium inside and outside the membrane is 0.1, so calculate the potential inside the membrane according to the N equation to be +61mV. Furthermore, shown in Figure 5-5b, the potassium diffusion potential is -94mV. So how will they affect each other? And what is the synthesis potential? This question will be answered by the Goldman equation described earlier. Intuitively, it can be seen that if the membrane is highly permeable to potassium but only low permeable to sodium, it makes sense that the contribution of potassium to the formation of the membrane potential is greater than that of sodium. In normal neurons, the diffusion of potassium is 100 times greater than that of sodium. Using this ratio into the Goldman equation, the potential inside the membrane is -86mV,

Contribution of Na+-K+. pump

In Figure C, pump Na+-K+. shown to provide additional contribution to the resting potential. This figure demonstrates that the continuous pumping of 3 sodium ions to the outside of the membrane occurs when 2 potassium ions are pumped inside. Pumping more sodium ions than potassium causes a loss of positive charge inside the cell membrane, creating an additional negative potential (about -4mV) inside the membrane compared to diffusion alone. Thus, as shown in Figure C, the membrane potential generated by all these factors acting at the same time is -90mV.

In summary, the mere diffusion of potassium and sodium will produce a membrane potential of about -86 mV, which is made up mostly of potassium diffusion. In addition, -4mV is generated by the continuous pumping action of the Na+-K+ pump, creating a membrane potential of -90mV.

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Resting potential