Nerve impulse propagation. Animation showing how an nerve impulse travels along a nerve fibre. A nerve impulse is a spike in voltage (action potential) caused by the movement of ions across the nerve cell membrane. The interior of a resting nerve fibre is negative relative to its exterior due to the presence of a high concentration of sodium ions (red) outside of the cell. When stimulated, voltage-gated sodium channels (blues) in the cell membrane open, causing in influx of sodium ions that causes a positive charge in the nerve fibre. This change in charge causes the sodium channels to close and potassium channels (green) to open. Potassium ions (blue) exit the cell, causing the return of a negative charge inside the cell. The opening of sodium channels in one part of the membrane triggers their opening further along the fibre, propagating the signal along the fibre.
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