E-Figure 14-4.

Resting and Action Potentials.

A, An action potential most commonly arises via neurotransmitter-induced chemical messages (depolarization) at synapses between presynaptic and postsynaptic neurons. It travels through dendrites, the cell body, and then down the axon to axon terminals of the postsynaptic neuron. B, Nerve impulse conduction occurs because of an electric potential sustained across the cell membrane of the dendrite/neuron cell body/axon. A resting membrane potential is maintained by differences in concentrations of potassium ions inside and sodium ions outside the cell membrane. When a neurotransmitter or other stimulus depolarizes the cell membrane to a threshold of approximately −50 mV, an action potential will occur. Sodium and potassium ions leak across the cell membrane, and therefore concentration gradients and thus the electric potential are maintained by sodium-potassium pumps in the cell membrane. C, When sodium channels are opened, an action potential occurs locally in the cell membrane. It is propagated down the cell membrane by the successive opening of voltage-gated sodium channels in adjacent sections of the membrane. Depolarized segments repolarize as sodium channels close and potassium ions move out of the cell.

(Courtesy Dr. A.D. Miller, College of Veterinary Medicine, Cornell University; and Dr. J.F. Zachary, College of Veterinary Medicine, University of Illinois.)