excitabiliteten

Excitabiliteten (excitability) is the property of certain cells and tissues to respond to environmental or internal stimuli by producing electrical signals. It is a defining feature of excitable tissues, notably neurons and muscle cells, but also certain secretory and endocrine cells. The phenomenon arises from the presence of specialized membranes equipped with ion channels that respond to changes in voltage, chemical signals, or mechanical forces.

The mechanism relies on resting membrane potential established by ion gradients and the activity of ion pumps and leak channels. In neurons, this resting potential is typically around -70 mV, and the threshold for an action potential is about -55 mV, though values vary by cell type. When a stimulus reaches threshold, voltage-gated sodium channels open, causing rapid depolarization and the generation of an action potential. This is followed by potassium efflux that repolarizes the membrane. In many cells, calcium ions also contribute to signaling and muscle contraction. Propagation of the action potential along a fiber changes the local membrane potential as the signal travels.

Excitability is characterized by the all-or-none nature of action potentials, a refractory period that limits firing frequency, and the speed of signal propagation along a fiber. In neurons, myelination and axon diameter influence conduction velocity, with saltatory conduction in myelinated fibers increasing efficiency. The intensity of a stimulus is encoded in the rate of action potential firing rather than the size of each spike.

Abnormal excitability can produce clinical conditions. Hyperexcitability may result in tremors or seizures, while hypoexcitability can weaken reflexes or reduce responsiveness. Genetic mutations in ion channels (channelopathies) are common underlying causes. Understanding excitability is central to neuroscience, physiology, and medicine, and is routinely assessed in diagnostic tests such as nerve conduction studies and electromyography.