Nerve Impulses

Nerve Impulses

In a nerve cell membrane, an action potential causes a local bioelectric current to reach other portions of the membrane. This stimulates the adjacent mem-brane to its threshold level and another action poten-tial is triggered, stimulating yet another region. These action potential waves move down to the end of the axon, constituting nerve impulses (­FIGURE 11-9).

TABLE 11-1 describes the steps in the conduction of a nerve impulse. A membrane’s resistance is defined as the level at which it restricts signal or ion movement.

Impulses are conducted over the entire surface of unmyelinated axons. Myelinated axons are insulated by their myelin content, reducing impulse ­conduction. The myelin sheath is interrupted by nodes of Ranvier between the Schwann cells, meaning action poten-tials occur at the nodes. Therefore, nerve impulses on myelinated axons appear to move from node to node. This saltatory impulse conduction is much faster than unmyelinated axon conduction­. The greater the diam-eter of the axon, the quicker the impulses are con-ducted. For example, the extremely fast conduction on a skeletal muscle­ as compared with much slower conduction on a sensory neuron.

Nerve impulses are conducted either completely or not at all, known as the all-or-none response. All impulses on an axon are of the same strength. If stimula-tion is raised, the impulses remain the same in strength but occur more rapidly. After each nerve impulse, a very brief refractory period limits the frequency of further nerve impulses. Most of the time, axons con-duct impulses at the speed of 100 per second, although speeds of as high as 700 per second are possible.