6. Nerve Impulse

When an action potential occurs in one region of a nerve cell membrane, it causes a bioelectric current to flow to adjacent portions of the membrane. This local current stimulates the adjacent membrane to its threshold level and triggers another action potential. This, in turn, stimulates the next adjacent region. A wave of action potentials moves down the axon to the end. This propagation of action potentials along a nerve axon constitutes the nerve impulse.

Figure 8- A nerve impulse. (a) An action potential in one region stimulates the adjacent region, and (b) and (c) a wave of action potentials (a nerve impulse) moves along the axon.

6.1 Impulse Conduction

An unmyelinated axon conducts an impulse over its entire surface. A myelinated axon functions differently because myelin insulates and prevents almost all ion flow through the membrane it encloses. The myelin sheath would prevent the conduction of a nerve impulse altogether if the sheath was continuous. However, nodes of Ranvier between Schwann cells interrupt the sheath. Action potentials occur at these nodes, where the exposed axon membrane has sodium and potassium channels. A nerve impulse traveling along a myelinated axon appears to jump from node to node. This type of impulse conduction, termed saltatory, is many times faster than conduction on an unmyelinated axon.

The speed of nerve impulse conduction is proportional to the diameter of the axon—the greater the diameter, the faster the impulse. For example, an impulse on a relatively thick, myelinated axon, such as that of a motor neuron associated with a skeletal muscle, might travel 120 meters per second. An impulse on a thin, unmyelinated axon, such as that of a sensory neuron associated with the skin, might move only 0.5 meter per second.

6.2 All-or-None Response

Nerve impulse conduction is an all-or-none response. That is, if a neuron responds at all, it responds completely. Thus, a nerve impulse is conducted whenever a stimulus of threshold intensity or above is applied to an axon, and all impulses carried on that axon are of the same strength. A greater intensity of stimulation does not produce a stronger impulse, but rather, more impulses per second.

For a very short time following a nerve impulse, a threshold stimulus will not trigger another impulse on an axon. This brief period, called the refractory period, limits the frequency of impulses in a neuron. It also ensures that the impulse proceeds in only one direction—down the axon. Although a frequency of 700 impulses per second is possible, 100 impulses per second is more common.