The resting membrane potential of large nerve fibers when they are not transmitting nerve signals is about −90 millivolts. That is, the potential inside the fiber is 90 millivolts more negative than the potential in the extra cellular fluid on the outside of the fiber. In the next few paragraphs, the transport properties of the resting nerve membrane for sodium and potassium and the factors that determine the level of this resting potential are explained.

Active Transport of Sodium and Potassium Ions Through the Membrane—The SodiumPotassium (Na⁺K⁺) Pump. Recall from Chapter 4 that all cell mem branes of the body have a powerful Na+-K+ pump that continually transports sodium ions to the outside of the cell and potassium ions to the inside, as illustrated on the left side in Figure 1. Note that this is an electrogenic pump because more positive charges are pumped to the outside than to the inside (three Na+ ions to the outside for each two K+ ions to the inside), leaving a net deficit of positive ions on the inside and causing a negative potential inside the cell membrane.

Fig1. Functional characteristics of the Na+-K+ pump and of the  K+ “leak” channels. ADP, adenosine diphosphate; ATP, adenosine  triphosphate. The K+ leak channels also leak Na+ ions into the cell  slightly but are much more permeable to K+. 

T he Na+-K+ pump also causes large concentration gradients for sodium and potassium across the resting nerve membrane. These gradients are as follows:

T he ratios of these two respective ions from the inside to the outside are:

nerve membrane through which potassium can leak even in a resting cell. The basic structure of potassium channels was described in Chapter 4 . These K+ leak channels may also leak sodium ions slightly but are far more permeable to potassium than to sodium—normally about 100 times as permeable. As discussed later, this differential in permeability is a key factor in determining the level of the normal resting membrane potential.

ORIGIN OF THE NORMAL RESTING MEMBRANE POTENTIAL

 Figure 2 shows the important factors in the establishment of the normal resting membrane potential of −90 millivolts. They are as follows.

Fig2.  Establishment of resting membrane potentials in nerve  fibers under three conditions: A, when the membrane potential is  caused entirely by potassium diffusion alone; B, when the membrane  potential is caused by diffusion of both sodium and potassium ions;  and C, when the membrane potential is caused by diffusion of both  sodium and potassium ions plus pumping of both these ions by the  Na+-K+ pump. 

Contribution of the Potassium Diffusion Potential. In Figure 2A, we assume that the only movement of ions through the membrane is diffusion of potassium ions, as demonstrated by the open channels between the potassium symbols (K+) inside and outside the mem brane. Because of the high ratio of potassium ions inside to outside, 35 : 1, the Nernst potential corresponding to this ratio is −94 millivolts because the logarithm of 35 is 1.54, and this multiplied by −61 millivolts is −94 millivolts. Therefore, if potassium ions were the only factor causing the resting potential, the resting potential inside the fiber would be equal to −94 millivolts, as shown in the figure.

Contribution of Sodium Diffusion Through the Nerve Membrane. Figure 2B shows the addition of slight permeability of the nerve membrane to sodium ions, caused by the minute diffusion of sodium ions through the K+-Na+ leak channels. The ratio of sodium ions from inside to outside the membrane is 0.1, which gives a calculated Nernst potential for the inside of the membrane of +61 millivolts. Also shown in Figure 5-5B is the Nernst potential for potassium diffusion of −94 millivolts. How do these interact with each other, and what will be the summated potential? This question can be answered by using the Goldman equation described previously. Intuitively, one can see that if the membrane is highly permeable to potassium but only slightly permeable to sodium, it is logical that the diffusion of potassium con tributes far more to the membrane potential than does the diffusion of sodium. In the normal nerve fiber, the permeability of the membrane to potassium is about 100 times as great as its permeability to sodium. Using this value in the Goldman equation gives a potential inside the membrane of −86 millivolts, which is near the potassium potential shown in the figure.

Contribution of the Na+K+ Pump. In Figure 2C, the Na+-K+ pump is shown to provide an additional contribution to the resting potential. This figure shows that continuous pumping of three sodium ions to the outside occurs for each two potassium ions pumped to the inside of the membrane. The pumping of more sodium ions to the outside than the potassium ions being pumped to the inside causes continual loss of positive charges from inside the membrane, creating an additional degree of negativity (about −4 millivolts additional) on the inside beyond that which can be accounted for by diffusion alone. Therefore, as shown in Figure 2C, the net mem brane potential when all these factors are operative at the same time is about −90 millivolts.

In summary, the diffusion potentials alone caused by potassium and sodium diffusion would give a membrane potential of about −86 millivolts, with almost all of this being determined by potassium diffusion. An additional −4 millivolts is then contributed to the membrane potential by the continuously acting electrogenic Na+-K+ pump, giving a net membrane potential of −90 millivolts.

مواضيع ذات صلة

MEASURING THE MEMBRANE POTENTIAL

BASIC PHYSICS OF MEMBRANE POTENTIALS

ACTIVE TRANSPORT” OF SUBSTANCES THROUGH MEMBRANES

The Cell Membrane Consists of A Lipid Bilayer With Cell Membrane Transport Proteins

Apoptosis—Programmed Cell Death

Cilia and Ciliary Movements

تركيب الخلية Structure of the cell

LYSOSOMES

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