depolarizing

Depolarizing refers to a process or agent that reduces the electrical potential difference across a cell’s membrane, making the inside of the cell less negative relative to the outside. In excitable cells, such as neurons and muscle cells, depolarization is the initial phase of an action potential and is typically triggered by the opening of voltage-gated sodium channels, which allows Na+ to enter the cell. This influx raises the membrane potential from a resting value (about -70 mV in many neurons) toward zero and often beyond, to a positive level. If the depolarization reaches a threshold, usually around -55 mV, a rapid, self-sustaining spike of electrical activity occurs.

In neurons, depolarization propagates along the axon as an action potential, which underlies synaptic transmission and information processing. In cardiac tissue, depolarization coordinates contraction. Atrioventricular and ventricular myocytes exhibit fast Na+-driven depolarization, while pacemaker cells rely more on calcium currents to generate rhythmic depolarizations that set heart rate. The wave of depolarization is followed by repolarization, where potassium efflux restores the resting potential, and the cell returns to a ready state for subsequent signaling.

In pharmacology and medicine, the term often refers to depolarizing neuromuscular blocking agents, such as succinylcholine. These drugs initially cause muscle fasciculations followed by sustained depolarization that renders the motor endplate unresponsive to further stimulation, producing paralysis. This contrasts with non-depolarizing (competitive) blockers that prevent activation without initial depolarization. Depolarizing blocks can be distinguished by their phases and clinical considerations, including effects on potassium release and potential malignant hyperthermia risk.