Excitability

Excitability is the ability of a cell or tissue to respond to a stimulus by generating an electrical signal, typically an action potential. In biology, excitability is most characteristic of nerve and muscle cells, though other tissues may show excitability under certain conditions. It arises from the activity of ion channels embedded in the plasma membrane.

Most excitable cells maintain a resting membrane potential via uneven distribution of ions and selective permeability. Depolarization opens voltage-gated sodium or calcium channels, causing rapid inward current and membrane potential rise. Repolarization involves opening potassium channels to restore the resting potential. In muscle and some neurons, Ca2+ influx links excitation to contraction or neurotransmitter release.

Excitability is often described by thresholds, all-or-none action potentials, and rate of firing. The strength and duration of stimuli influence whether a cell reaches threshold. Conduction along a cell is influenced by cable properties, channel density, and myelination in neurons and by t-tubule systems in muscle.

Alterations in excitability are central to several disorders. Hyperexcitability can underlie epilepsy, neuropathic pain, or fasciculations, while hypoexcitability can contribute to weakness or slowed reflexes. Pharmacological agents, electrolyte balance, temperature, and disease states modulate excitability.

Excitability is studied with electrophysiological methods such as patch-clamp recordings and extracellular measurements. Concepts such as resting potential, threshold, rheobase, and chronaxie are used to describe responsiveness in different tissues.