The impulse: lit nodes, or the moving band without myelinNeuron (GFP)Myelin internodesNot to scale: the axon stands for 10 mm of nerve, compressed; all speeds are true and shown 2,000× slower. At rest the membrane sits at −70 mV. A stimulus at the soma crosses threshold (about −55 mV) at the axon hillock; sodium channels open and the membrane swings to about +40 mV in half a millisecond, then potassium brings it back past −80 mV. Myelin insulates the internodes, so the impulse is regenerated only at the nodes of Ranvier and jumps between them at about 40 m/s; without myelin (a thin C fibre) it must regenerate everywhere and crawls at about 2 m/s.
−70 mV静息膜电位resting potential+40 mV峰值peak~1 ms一次动作电位one action potential40 m/s有髓纤维传导myelinated conduction0.5–2 m/s无髓纤维传导unmyelinated conduction~86 billion人脑神经元数（~860 亿）neurons in a human brain