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SIGNALING IN THE BRAIN DEPENDS on the ability of nerve cells to respond to very small stimuli with rapid and large changes in the electrical potential difference across the cell membrane. In sensory cells, the membrane potential changes in response to minute physical stimuli: Receptors in the eye respond to a single photon of light; olfactory neurons detect a single molecule of odorant; and hair cells in the inner ear respond to tiny movements of atomic dimensions. These sensory responses ultimately lead to the firing of an action potential during which the membrane potential changes at up to 500 volts per second.
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The rapid changes in membrane potential that underlie signaling throughout the nervous system are mediated by specialized pores or openings in the membrane called ion channels, a class of integral membrane proteins found in all cells of the body. The ion channels of nerve cells are optimally tuned to respond to specific physical and chemical signals. They are also heterogeneous—in different parts of the nervous system different types of channels carry out specific signaling tasks.
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Because of their key roles in electrical signaling, malfunctioning of ion channels can cause a wide variety of neurological diseases (Chapters 57 and 58). Diseases caused by ion channel malfunction are not limited to the brain; for example, cystic fibrosis, skeletal muscle disease, and certain types of cardiac arrhythmia are also caused by ion channel malfunction. Moreover, ion channels are often the site of action of drugs, poisons, or toxins. Thus, ion channels have crucial roles in both the normal physiology and pathophysiology of the nervous system.
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In addition to ion channels, nerve cells contain a second important class of proteins specialized for moving ions across cell membranes, the ion transporters and pumps. These proteins do not participate in rapid neuronal signaling but rather are important for establishing and maintaining the concentration gradients of physiologically important ions between the inside and outside of the cell. As we will see in this and the next chapters, ion transporters and pumps differ in important aspects from ion channels, but also share certain common features.
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Ion channels have three important properties: (1) They recognize and select specific ions; (2) they open and close in response to specific electrical, chemical, or mechanical, signals; and (3) they conduct ions across the membrane. The channels in nerve and muscle conduct ions across the cell membrane at extremely rapid rates, thereby providing a large flow of electric charge. Up to 100 million ions can pass through a single channel each second. This current causes the rapid changes in membrane potential required for signaling (Chapter 10). The fast flow of ions through channels is comparable to the turnover rate of the fastest enzymes, catalase and carbonic anhydrase, which are limited by diffusion of substrate. (The turnover rates of most other enzymes are considerably slower, ranging ...