Action Potentials and Saltatory Conduction

Revision notes

Resting membrane potential

A resting neurone has an electrical potential difference across its membrane; a typical teaching value is about −70 mV inside relative to outside. Potassium leakage and unequal ion distributions contribute. The sodium–potassium pump uses ATP to move three sodium ions out for every two potassium ions in, maintaining concentration gradients over time. The pump does not directly produce each rapid rising and falling phase of an action potential.

A sequence of permeability changes

1. A stimulus depolarises the membrane. If threshold is reached, voltage-gated sodium channels open rapidly.

2. Sodium ions enter down their electrochemical gradient. Increased sodium permeability produces rapid depolarisation and a brief positive membrane potential.

3. Sodium channels inactivate and voltage-gated potassium channels open more slowly. Potassium ions leave, repolarising the membrane.

4. Potassium channels close with a delay, so the potential can temporarily become more negative than its resting level: hyperpolarisation.

5. Channels return to their resting states. Leakage currents and maintained ion gradients support recovery; the pump maintains those gradients over repeated activity.

Refractory period and direction

During the absolute refractory period, sodium-channel inactivation prevents another action potential in that membrane region. During the relative refractory period, a stronger stimulus may trigger one. Refractoriness limits firing frequency and helps ensure forward propagation in an intact pathway because the membrane behind the impulse is temporarily less excitable.

An action potential is all-or-none once threshold is reached in a given axon. A stronger stimulus normally increases firing frequency rather than making each action potential taller. Local currents depolarise neighbouring membrane to threshold; the signal is regenerated and does not simply fade away.

Myelinated axons

Current spreads beneath the insulating sheath and depolarises the next node. New action potentials occur at nodes, giving saltatory conduction. Myelin reduces current loss and changes membrane electrical properties, increasing speed. Sodium ions do not travel all the way from the cell body to the terminal to carry one signal.

Worked application and data skills

A model axon fires 10 impulses in 0.5 s: frequency = 10/0.5 = 20 Hz. If a stronger stimulus causes 20 impulses in the same interval, frequency doubles to 40 Hz. The action-potential amplitude in that axon remains approximately the same; the information is represented by firing pattern, not a doubled peak voltage.

Common misconceptions

Na⁺ influx depolarises; K⁺ efflux repolarises. The pump maintains gradients rather than generating the fast falling phase. Stronger stimuli do not usually increase individual spike height.

Examination tips

Explain a graph using channel state, ion direction and effect on potential in that order.

Check your understanding

Five original questions. Use Tab to move between controls, arrow keys within a radio group, and Enter or Space on buttons. Feedback appears after Check Answer. Try Again clears the question. Attempts are not saved.

Sources and scope

Scope: Unit 1 A3.4.2. Original BTECRevision notes, schematics and questions. Independent resource; not endorsed by Pearson.

Official Pearson specification. Scientific review references are listed in the accompanying QA report.