A nerve impulse is a brief electrical change that travels along the axon membrane. It depends on ion gradients that the cell maintains at all times.
Resting potential
At rest, the inside of a neuron is about minus 70 mV compared with the outside. The sodium-potassium pump keeps sodium high outside the cell and potassium high inside.
Action potential
When a stimulus brings the membrane to threshold, sodium channels open and sodium rushes in, so the inside turns positive (depolarization). Potassium channels then open, potassium leaves and the membrane returns to negative (repolarization).
All or none
An action potential fires fully or not at all, so a stronger stimulus means more impulses per second, not bigger ones. A brief refractory period after each impulse keeps it moving forward.
Conduction speed
In unmyelinated axons the impulse moves slowly along the whole membrane. In myelinated axons it jumps from node to node (saltatory conduction), and thick myelinated fibers are the fastest.
At the bedside
Abnormal potassium, sodium or calcium levels change how easily nerves and muscles fire, causing weakness, cramps, tingling or seizures. Local anesthetics such as lidocaine block sodium channels so the impulse cannot pass.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).