For you see, neurons sit in salty water—outside a neuron’s skin are lots of sodium (which has a positive charge: +) and lots of chlorine (which has a negative charge: −). But inside the neuron is a little sodium, a little chlorine, and lots of potassium (another +). Because the concentrations of each type of charged ion—particularly the potassium—are different either side of the neuron’s skin, this creates a voltage across the skin. By playing with the concentrations of ions outside the neuron, Hodgkin and Huxley were thus mucking about with the neuron’s voltage. And crucially they could find
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