Nerve Impulses and Conduction

A neurone has unequal ion distributions across its membrane and selective permeability. Sodium–potassium pumps help maintain gradients by moving sodium out and potassium in using ATP. Potassium tends to diffuse out through leak pathways, leaving the inside relatively negative. Selective permeability and opposing electrical forces contribute to a stable resting potential.

The pump maintains conditions for signalling, but a rapid action potential is not simply the pump suddenly running backwards. A motor neurone has a cell body, dendritic input regions and an axon ending in terminals; myelin can insulate sections of the axon, leaving nodes of Ranvier.

If depolarisation reaches threshold, voltage-gated sodium channels open and sodium enters down its electrochemical gradient, causing rapid depolarisation. Sodium-channel inactivation and increased potassium permeability then contribute to repolarisation as potassium leaves. Continued potassium movement can briefly produce hyperpolarisation.

Action potentials are all-or-nothing once threshold is reached. Local currents depolarise the next suitable region to threshold, regenerating the signal. Channel states during the refractory period limit immediate re-excitation, helping produce separate impulses and limiting maximum firing frequency.

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In a non-myelinated axon, successive nearby membrane regions regenerate action potentials. In a myelinated axon, current spreads beneath insulated regions and action potentials are regenerated mainly at nodes: saltatory conduction. Ions do not literally leap through empty space from one node to another.

Greater axon diameter can reduce internal resistance and increase conduction speed. Temperature affects channel and enzyme kinetics within a viable range, while extreme conditions can impair function. Conduction speed in m s⁻¹ is different from impulse frequency in s⁻¹; a shorter refractory period can raise a theoretical frequency limit without itself specifying the speed along an axon.

Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.