A neuron, or nerve cell. In the right foreground you can see a nerve cell fiber in cross section. The upper left photo gives a more realistic picture of the shape of neurons. Nerve impulses usually travel from the dendrites and soma to the branching ends of the axon. The nerve cell shown here is a motor neuron. The axons of motor neurons stretch from the brain and spinal cord to muscles or glands of the body.
Fig. 2-4, p. 51
The interior of an axon. The right end of the top axon is at rest. Thus, it has a negative charge inside. An action potential begins when ion channels open and sodium ions (Na+) rush into the axon. In this drawing, the action potential would travel from left to right along the axon. In the lower axon, the action potential has moved to the right. After it passes, potassium ions (K+) flow out of the axon. This quickly renews the negative charge inside the axon so that it can fire again. Sodium ions that enter the axon during an action potential are pumped out more slowly. Removing them restores the original resting potential.