Conduction of Nerve Impulses

When stimulating electrodes artificially depolarize one point of an axon membrane to a threshold level, voltage-regulated channels open and an action potential is produced at that small region of axon membrane containing those gates. For about the first millisecond of the action potential, when the membrane voltage changes from -70 mV to +30 mV, a current of Na+ enters the cell by diffusion because of the opening of the Na+ gates. Each action potential thus "injects" positive charges (sodium ions) into the axon (fig. 7.17).

These positively charged sodium ions are conducted, by the cable properties of the axon, to an adjacent region that still has a membrane potential of -70 mV. Within the limits of the cable properties of the axon (1 to 2 mm), this helps to depolarize the adjacent region of axon membrane. When this adjacent region of

— Axon

Second action potential begins

Second action potential begins

Third action

Third action

. + potential begins K

. + potential begins K

□ = depolarization

□ = repolarization

□ = depolarization

□ = repolarization

■ Figure 7.17 The conduction of action potentials in an unmyelinated axon. Each action potential "injects" positive charges that spread to adjacent regions. The region that has just produced an action potential is refractory. The next region, not having been stimulated previously, is partially depolarized. As a result, its voltage-regulated Na+ gates open and the process is repeated. Successive segments of the axon thereby regenerate, or "conduct," the action potential.

membrane reaches a threshold level of depolarization, it too produces an action potential as its voltage-regulated gates open.

Each action potential thus acts as a stimulus for the production of another action potential at the next region of membrane that contains voltage-regulated gates. In the description of action potentials earlier in this chapter, the stimulus for their production was artificial—depolarization produced by a pair of stimulating electrodes. Now it can be seen that each action potential is produced by depolarization that results from the preceding action potential. This explains how all action potentials along an axon are produced after the first action potentials are generated at the initial segment of the axon.

Action potential now here

Action potential now here

I I = resting potential l l = depolarization I I = repolarization

I I = resting potential l l = depolarization I I = repolarization

■ Figure 7.18 The conduction of a nerve impulse in a myelinated axon. Since the myelin sheath prevents inward Na+ current, action potentials can be produced only at gaps in the myelin sheath called the nodes of Ranvier. This "leaping" of the action potential from node to node is known as saltatory conduction.

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