excitabilité

Excitability refers to the ability of cells, particularly nerve and muscle cells, to respond to stimuli by generating and propagating electrical signals. This property is fundamental to the functioning of excitable tissues. In neurons, excitability allows for the transmission of information throughout the nervous system, enabling processes like thought, sensation, and movement. In muscle cells, excitability leads to contraction, which drives all forms of bodily motion.

The basis of cellular excitability lies in the cell membrane's selective permeability to ions, such as sodium (Na+), potassium (K+), calcium (Ca2+), and chloride (Cl-). These ions are distributed unevenly across the membrane, creating an electrochemical gradient. At rest, the cell membrane maintains a resting membrane potential, which is typically negative inside relative to the outside. This potential is established and maintained by ion channels and pumps embedded within the membrane.

When an excitable cell receives a stimulus, it can lead to a change in the membrane potential. If this change reaches a certain threshold, known as the threshold potential, it triggers a rapid and transient depolarization of the membrane. This depolarization event is called an action potential in neurons and a muscle action potential in muscle cells. The action potential is an "all-or-none" phenomenon, meaning it either occurs fully or not at all. After the depolarization, the membrane repolarizes, returning to its resting potential, often with a brief hyperpolarization phase. This sequence of electrical events allows for rapid communication and coordinated responses within the organism.