The first part of the axon, known as the initial segment, is often referred to as the trigger zone (see fig. 10.6) because it contains a great number of voltage-gated sodium channels. At the resting membrane potential, these sodium channels remain closed, but when threshold is reached, they open for an instant, briefly increasing sodium permeability. Sodium ions diffuse inward across that part of the cell membrane and down their concentration gradient, aided by the attraction of the sodium ions to the negative electrical condition on the inside of the membrane.
As the sodium ions rush inward, the membrane potential changes from its resting value (fig. 10.13a) and momentarily becomes positive on the inside (this is still considered depolarization). At the peak of the action potential, membrane potential may reach + 30mV (fig. 10.13b).
The voltage-gated sodium channels close quickly, but at almost the same time, slower voltage-gated potassium channels open and briefly increase potassium permeability. As potassium ions diffuse outward across that part of the membrane, the inside of the membrane becomes negatively charged once more. The membrane is thus repolarized (note in fig. 10.13c that it hyperpolar-izes for an instant). The voltage-gated potassium channels then close as well. In this way, the resting potential is quickly reestablished, and it remains in the resting state until it is stimulated again (fig 10.14). The active transport mechanism in the membrane works to maintain the original concentrations of sodium and potassium ions.
Axons are capable of action potentials, but the cell body and dendrites are not. An action potential at the trigger zone causes an electric current to flow a short distance down the axon, which stimulates the adjacent membrane to its threshold level, triggering another action potential. The second action potential causes another electric current to flow farther down the axon. This sequence of events results in a series of action potentials occurring sequentially all the way to the end of the axon without decreasing in amplitude, even if branches occur. The propagation of action potentials along an axon is the nerve impulse (fig. 10.15).
A nerve impulse is similar to the muscle impulse mentioned in chapter 9, page 304. In the muscle fiber,
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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.