Last time we looked at an action potential happening across a neurone cell-surface membrane. The membrane was depolarised and repolarised. However, that depolarisation must be propagated along the whole length of a neurone in the right direction. Today we will look at how an action potential works it’s way along in a wave of depolarisation.
The wave of depolarisation
When an depolarisation occurs, sodium (Na+) ions diffuse into the neurone through the voltage-gated Na+ ion channels. Once inside, some of the Na+ ions can diffuse sideways (in either direction) to the next part of the cell-surface membrane. On the side closer to the axon terminal, this brings the membrane potential to the threshold, triggering voltage-gated Na+ channels open and depolarisation occurs. Even though the Na+ ions also diffuse sideways to the previous section of membrane, the voltage-gated ion channels Na+ channels there are in the refractory period and are currently inactivated. They are not able to open again until resting potential has been re-established at the end of the action potential. This makes sure that the action potential is unidirectional. It also means that action potentials are discrete (do not overlap).
All-or-nothing
Action potentials are all-or-nothing. If the threshold is reached (see the last article) then an action potential is generated with the same change in potential difference every time (from -70mV up to about +30mV). If the threshold is not reached, an action potential is not conducted along the neurone. So how can the brain work out whether a stimulus is stronger or weaker? It depends on the frequency of the action potentials – a stronger stimulus means more action potentials are propagated along the neurone in a given time.
Speed of conductance
There are three main factors which affect the speed at which an action potentials are conducted along a neurone:
- Axon diameter – a wider axon diameter means less resistance to sideways diffusion of Na+ ions in the cytoplasm, so depolarisation can spread to the next part of the membrane more quickly.
- Temperature – a higher temperature means that the ions have more kinetic energy and diffuse more quickly. Don’t forget that proteins are denatured by high temperatures, so the speed would decrease if the ion channels are denatured.
- Myelination of neurones – many neurones have a myelin sheath. This is a coating of the axon made up of Schwann cells which acts as an electrical insulator. The gaps between the Schwann cells are called the nodes of Ranvier. Na+ ions can only enter the neurone at these points because the voltage-gated Na+ channels are concentrated there. The wave of depolarisation jumps between the nodes rather than having to travel the whole length of the axon membrane. Therefore the nerve impulse is conducted more quickly. This is called saltatory conduction.
Summary
- Action potentials are unidirectional and discrete.
- Action potentials are all-or-nothing: the change in potential difference is always the same. It is the frequency of the action potentials which tells the brain the size of the stimulus.
- Axon diameter, temperature and myelination all affect the speed of conduction.
