动作电位，又称阈电位指的是处于静息电位状态的细胞膜受到适当刺激而产生的，短暂而有特殊波形的，跨膜电位搏动。细胞产生动作电位的能力被称为兴奋性，有这种能力的细胞如神经细胞和肌细胞。动作电位是实现神经传导和肌肉收缩的生理基础。

动作电位的形成与细胞膜上的离子信道开关相联系。一个初始刺激，只要达到了阈电位（而不论超过了多少，这就是全有全无律 (en:All-or-none law)），就能引起一系列离子信道的开放和关闭，而形成离子的流动，改变跨膜电位。而这个跨膜电位的改变尤能引起临近位置上细胞膜电位的改变，这就使得兴奋能沿着一定的路径传导下去。

In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and endocrine cells, as well as in some plant cells. Neurons play a central role in cell-to-cell communication. In other types of cells, their main function is to activate intracellular processes. In muscle cells, for example, an action potential is the first step in the chain of events leading to contraction. In beta cells of the pancreas, they provoke release of insulin. Action potentials in neurons are also known as "nerve impulses" or "spikes", and the temporal sequence of action potentials generated by a neuron is called its "spike train". A neuron that emits an action potential is often said to "fire".