Action Potentials (AQA A Level Biology): Revision Note

Generating an action potential

• In a resting axon the potential difference across the membrane is around -70 mV

  • This is the resting potential

• When a neurone is stimulated the membrane potential changes; if a stimulus is large enough this can lead to an action potential

• Action potentials can be transmitted along neurones; this is a nerve impulse

• An action potential consists of several stages

  • Depolarisation

  • Repolarisation

  • Hyperpolarisation, or the refractory period

Depolarisation

• When a neurone is stimulated, the following process occurs:

  1. sodium ion channels in the axon membrane open

  2. sodium ions pass into the axon down an electrochemical gradient

  3. the inside of the axon becomes less negative; this is depolarisation

     • This initial depolarisation is sometimes known as a generator potential

  4. 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

  5. enough sodium ions enter the axon for the membrane potential to reach around +30 mV; this is an action potential

Repolarisation

• Once membrane potential reaches around +30 mV, the following sequence of events occurs:

  1. all the voltage-gated sodium channels close, stopping any further sodium ion influx

  2. voltage-gated potassium ion channels open, allowing the diffusion of potassium ions out of the axon, down their concentration gradient

  3. the inside of the membrane becomes more negative

Hyperpolarisation

• 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

The all-or-nothing principle

• 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

• The all-or-nothing principle means that action potentials are always the same size, at around +30 mV; there is no such thing as a large or small action potential

• A strong, or long-lasting, stimulus will result in the generation of multiple action potentials in quick succession; this allows the brain to distinguish between large and smaller stimuli

  • A stronger stimulus = a high frequency of action potentials

  • A weaker stimulus = a lower frequency of action potentials

The refractory period

• 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