Boðspennan

Boðspennan, or action potential, is a brief, all-or-nothing electrical signal that travels along the membranes of excitable cells such as neurons and skeletal muscle fibers. It enables rapid communication by converting a local stimulus into a transient depolarization that propagates along the cell.

In a resting state, the membrane potential is typically around -70 mV, maintained by ion gradients created by the Na+/K+-ATPase and leak channels. When a stimulus raises the membrane potential to a threshold (about -55 mV), voltage-gated Na+ channels open, and Na+ rapidly enters the cell. The positive feedback from further depolarization causes a rapid rise in membrane potential to roughly +30 mV. During the peak, Na+ channels inactivate, and voltage-gated K+ channels open, allowing K+ to exit and the cell to repolarize. The membrane may briefly hyperpolarize below the resting level before ion pumps and leaks restore the resting potential.

Propagation: In unmyelinated fibers, the depolarization spreads continuously along the membrane. In myelinated fibers, myelin reduces membrane capacitance and fosters saltatory conduction, with the impulse leaping between nodes of Ranvier and thus traveling faster.

Refractory periods: After an action potential there is an absolute refractory period during which no new spike can be initiated, followed by a relative refractory period during which a stronger stimulus can trigger another spike. These properties shape firing rate and timing.

Significance: Boðspennan is essential for nervous system function, enabling neurotransmitter release at synapses and information encoding through spike frequency and timing. Typical durations are on the order of 1 ms, with conduction velocities ranging from a few meters per second in small fibers up to over 100 m/s in heavily myelinated fibers.