Action potential

An action potential is a depolarisation of the electrical potential difference across the axon membrane, caused by the rapid influx of sodium ions (Na⁺) followed by the outflow of potassium ions (K⁺).

Depolarisation Repolarisation Hyperpolarisation, or the refractory period

sodium ion channels in the axon membrane open sodium ions pass into the axon down an electrochemical gradient the inside of the axon becomes less negative; this is depolarisation This initial depolarisation is sometimes known as a generator potential if the membrane potential reaches around -50 mV, voltage gated sodium ion channels open and more sodium ions enter the cell -50 mV is known as a threshold potential enough sodium ions enter the axon for the membrane potential to reach around +40 mV; this is an action potential

Resting potential = -70mV Action potential threshold = -55 mv

Once membrane potential reaches around +40 mV, the following sequence of events occurs: all the voltage-gated sodium channels close, stopping any further sodium ion influx voltage-gated potassium ion channels open, allowing the diffusion of potassium ions out of the axon, down their concentration gradient the inside of the membrane becomes more negative

The outward movement of potassium ions during repolarisation continues until the inside of the membrane becomes more negative than resting potential; this is known as hyperpolarisation The hyperpolarised membrane is said to be in a refractory period Eventually the voltage-gated potassium ion channels close, and the action of sodium-potassium pumps restores the membrane to resting potential

If a stimulus is too weak then threshold potential will not be reached and there will be no action potential, while a stimulus that is strong enough for threshold potential to be reached will always result in an action potential This is the all-or-nothing principle (ACTION POTENTIAL ALWAYS SIME SIZE OF 40 mV)

A stronger stimulus = a high frequency of action potentials A weaker stimulus = a lower frequency of action potentials

During repolarisation the voltage-gated potassium ion channels remain open for longer than needed to restore resting potential, and the axon membrane becomes hyperpolarised The time during which the membrane is hyperpolarised is known as the refractory period The refractory period is very important as it ensures that: new action potentials are generated ahead, rather than behind, the original action potential, so nerve impulses are only ever transmitted in one direction nerve impulses are separate events, rather than merging together