Ion

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A molecule or ATOM that is electrically charged, having either gained or lost electrons. A CATION has a positive charge, an ANION has a negative charge. In solution many substances break down into their component parts: common salt—sodium chloride (NaCl)—for example breaks down in water into positively charged sodium ions (Na⁺) and negatively charged chloride ions (Cl⁻).

The relative balance of positively and negatively charged ions on either side of a neuronal MEMBRANE determines the RESTING POTENTIAL of a NEURON. The movement of ions across the membrane is critical for ACTION POTENTIAL generation. Movement of ions across a membrane can be a RECEPTOR-mediated process, occurring via the receptor-regulated ION CHANNEL system, or a process mediated by specialized transport mechanisms such as the SODIUM-POTASSIUM PUMP and the CALCIUM PUMP.

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