The reason that a cell has a resting membrane potential is that electrolytes are unequally distributed between the
Saladin: Anatomy & I 12. Nervous Tissue I Text I I © The McGraw-Hill
Physiology: The Unity of Companies, 2003 Form and Function, Third Edition
ro extracellular fluid (ECF) on the outside of the plasma membrane and the intracellular fluid (ICF) on the inside. The RMP results from the combined effect of three factors: (1) the diffusion of ions down their concentration gradients through the plasma membrane; (2) selective permeability of the plasma membrane, allowing some ions to pass more easily than others; and (3) the electrical attraction of cations and anions to each other.
Potassium ions (K+) have the greatest influence on the RMP because the plasma membrane is more permeable to K+ than to any other ion. Imagine a hypothetical cell in which all the K+ starts out in the ICF, with none in the ECF. Also in the ICF are a number of cytoplasmic anions that cannot escape from the cell because of their size or charge—phosphates, sulfates, small organic acids, proteins, ATP, and RNA. Assuming K+ can diffuse freely through channels in the plasma membrane, it diffuses down its concentration gradient and out of the cell, leaving these cytoplasmic anions behind. As a result, the ICF grows more and more negatively charged. But as the ICF becomes more negative, it exerts a stronger and stronger attraction for the positive potassium ions and attracts some of them back into the cell. Eventually an equilibrium (balance) is reached in which K+ is moving out of the cell (diffusion down its concentration gradient) and into the cell (by electrical attraction) at equal rates. The net diffusion of K+ then stops. At the point of equilibrium, K+ is about 40 times as concentrated in the ICF as in the ECF (fig. 12.9).
If K+ were the only ion affecting the RMP, it would give the membrane a potential of about —90 mV. However, sodium ions (Na+) also enter the picture. Sodium is about 12 times as concentrated in the ECF as in the ICF. The rest-
Figure 12.9 Ionic Basis of the Resting Membrane Potential.
Note that sodium ions are much more concentrated in the extracellular fluid (ECF) than in the intracellular fluid (ICF), while potassium ions are more concentrated in the ICF. Large anions unable to penetrate the plasma membrane give the cytoplasm a negative charge relative to the ECF. If we suddenly increased the concentration of Cl" ions in the ICF,would the membrane potential become higher or lower than the RMP?
ing plasma membrane is much less permeable to Na+ than to K+, but Na+ does diffuse down its concentration gradient into the cell, attracted by the negative charge in the ICF. This sodium leak is only a trickle, but it is enough to neutralize some of the negative charge and reduce the voltage across the membrane.
Sodium leaks into the cell and potassium leaks out, but the sodium-potassium (Na+-K+) pump described in chapter 3 continually compensates for this leakage. It pumps 3 Na+ out of the cell for every 2 K+ it brings in, consuming 1 ATP for each exchange cycle. By removing more cations from the cell than it brings in, it contributes about — 3 mV to the resting membrane potential. The net effect of all this—K+ diffusion out of the cell, Na+ diffusion inward, and the Na+-K+ pump—is the resting membrane potential of —70 mV.
The Na+-K+ pump accounts for about 70% of the energy (ATP) requirement of the nervous system. Every signal generated by a neuron slightly upsets the distribution of Na+ and K+, so the pump must work continually to restore equilibrium. This is why nervous tissue has one of the highest rates of ATP consumption of any tissue in the body, and why it demands so much glucose and oxygen. Although a neuron is said to be resting when it is not producing signals, it is highly active maintaining its RMP and "waiting," as it were, for something to happen.
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