Nerve Impulses

A nerve impulse is a self-propagating wave of electrical activity that travels along the membrane of a neurone. It is not a flow of electrons as in a wire, but a change in the potential difference across the cell surface membrane caused by the movement of ions.The ions involved are mainly sodium ions (Na⁺) and potassium ions (K⁺). Being charged, they cannot diffuse through the phospholipid bilayer, so they move through specific channel proteins and are moved by carrier proteins in the membrane. Opening and closing these channels changes the charge inside the axon relative to the outside.When a neurone is not transmitting an impulse, the inside of the axon is negative compared with the outside: this is the resting potential. When the neurone is stimulated strongly enough, the potential difference briefly reverses so that the inside becomes positive: this is an action potential. An action potential at one point on the membrane triggers one at the next point, so the impulse travels along the axon.Action potentials follow the all-or-nothing principle. A stimulus must reach a threshold to produce an action potential, and any stimulus above threshold produces an action potential of the same size. A stronger stimulus does not make a bigger action potential. Instead, it produces a higher frequency of action potentials, and it may stimulate more neurones.The brain therefore works out the strength of a stimulus from how often impulses arrive, and the type of stimulus from which neurones carry them.