A membrane potential of +60 mV would prevent the diffusion of Na+ into the cell, while a membrane potential of -90 mV would prevent the diffusion of K+ out of the cell. It is clear that the membrane potential cannot be both values at the same time; indeed, it is seldom either value but instead is somewhere between these two extremes. We will call this the resting membrane potential to distinguish it from the theoretical equilibrium potentials.
The actual value of the resting membrane potential depends on two factors:
1. The ratio of the concentrations (Xo X) of each ion on the two sides of the plasma membrane.
2. The specific permeability of the membrane to each different ion.
Many ions—including K+, Na+, Ca2+, and Cl-—contribute to the resting membrane potential. Their individual contributions are determined by (a) the differences in their concentrations across the membrane (fig. 6.23), and (b) by their membrane permeabilities. This has two important implications:
1. For any given ion, a change in its concentration in the extracellular fluid will change the resting membrane potential—but only to the extent that the membrane is permeable to that ion. Because the resting membrane is most permeable to K+, a change in the extracellular concentration of K+ has the greatest effect on the resting membrane potential. This is the mechanism behind the fact that "lethal injections" are of KCl (raising the extracellular K+ concentrations and depolarizing cardiac cells.).
2. A change in the membrane permeability to any given ion will change the membrane potential. This fact is central to the production of nerve and muscle impulses, as will be described in chapter 7. Most often, it is the opening and closing of Na+ and K+ channels that are involved, but gated channels for Ca2+ and Cl- are also very important in physiology.
The resting membrane potential of most cells in the body ranges from -65 mV to -85 mV (in neurons it averages -70 mV). This value is close to the EK, because the resting plasma membrane is more permeable to K+ than to other ions. During nerve and muscle impulses, however, the permeability properties change, as will be described in chapter 7. An in-
■ Figure 6.23 Concentrations of ions in the intracellular and extracellular fluids. This distribution of ions, and the different permeabilities of the plasma membrane to these ions, affects the membrane potential and other physiological processes.
creased membrane permeability to Na+ drives the membrane potential toward ENa (+60 mV) for a short time. This is the reason that the term resting is used to describe the membrane potential when it is not producing impulses.
The resting membrane potential is particularly sensitive to changes in plasma potassium concentration. jj Since the maintenance of a particular membrane potential is critical for the generation of electrical events in the heart, mechanisms that act primarily through the kidneys maintain plasma K+ concentrations within very narrow limits. An abnormal increase in the blood concentration of K+ is called hyperkalemia. When hyperkalemia occurs, more K+ can enter the cell. In terms of the Nernst equation, the ratio [K+o]/[K+J is decreased. This reduces the membrane potential (brings it closer to zero) and thus interferes with the proper function of the heart. For these reasons, the blood electrolyte concentrations are monitored very carefully in patients with heart or kidney disease.
Remember that Jessica's medical tests revealed hyperkalemia. What is hyperkalemia and why might Jessica have this condition? What is the relationship between the hyperkalemia and her abnormal EKG?
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.