exciteability

Excitability is the ability of certain biological tissues to respond rapidly to a stimulus by generating electrical signals. In animals, excitable tissues include neurons, skeletal and cardiac muscle, and some smooth muscle. The response depends on the cell’s membrane potential and the function of ion channels and pumps that establish and regulate it.

At rest, the membrane potential is maintained by ion gradients across the plasma membrane, primarily via the Na+/K+ ATPase and selective permeability to potassium and other ions. A stimulus can cause depolarization; when the membrane potential reaches a threshold, voltage-gated ion channels open, producing an action potential characterized by rapid sodium entry followed by potassium efflux, then restoration of the resting potential through closing of channels and activity of pumps.

Action potentials are typically all-or-none: they occur fully or not at all, propagate along axons or muscle fibers, and may be saltatory in myelinated fibers. After an action potential, refractory periods limit immediate re-excitation, ensuring directionality of conduction. The duration and amplitude depend on ion channel kinetics, ion concentrations, temperature, and modulators.

Excitability can be modulated by neurotransmitters, hormones, toxins, and disease. Altered excitability underlies conditions such as epilepsy (increased neural excitability) or neuropathic pain, while reduced excitability occurs in certain channelopathies or after nerve injury. Pharmacological agents that block voltage-gated sodium or calcium channels can diminish excitability, whereas others that enhance depolarization can increase it.

Measuring excitability involves electrophysiological techniques such as patch-clamp or extracellular recordings and is central to neurophysiology and cardiology.