Nerve cells generate electrical signals to convey information over substantial distances and to transmit it to other cells by means of synaptic connections. These signals ultimately depend on changes in the resting electrical potential across the neuronal membrane. A negative membrane potential at rest results from a net efflux of K+ across neuronal membranes that are predominantly permeable to K+. In contrast, an action potential occurs when a transient rise in Na+ permeability allows a net influx of Na+. The brief rise in membrane Na+ permeability is followed by a secondary, transient rise in membrane K+ permeability that repolarizes the neuronal membrane and produces a brief undershoot of the action potential. As a result of these processes, the membrane is depolarized in an all-or-none fashion during an action potential. When these active permeability changes subside, the membrane potential returns to its resting level because of the high resting membrane permeability to K+.