"Ions are electrically charged particles. [...] Opposite charges attract and like charges repel. [...] Electrical potential, also called voltage, is the force exerted on a charged particle, and it reflects the difference in charge between [either side of a membrane or barrier]. [...] The resting potential of a typical neuron is about -65 millivolts [...], and is an absolute requirement for a functioning nervous system."—from Mark F. Bear, Barry W. Connors, & Michael A. Paradiso in Neuroscience:  Exploring the Brain, Lippincott Williams & Wilkins,2001, pp. 60-62