Last time we looked at an action potential happening in a section of a neurone cell membrane. The membrane was depolarised and repolarised. However, that depolarisation must travel along the whole length of a neurone in the right direction. Today we will look at how an action potential travels in a wave of depolarisation.
The wave of depolarisation
When an action potential 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 to the next part of the cell membrane. This alters the potential difference, and voltage-gated Na+ channels open resulting in depolarisation. Even though the Na+ ions can also diffuse sideways to the previous section of cell membrane, the ion channels there are in the refractory period and are not yet ready for another action potential. This makes sure that the wave of depolarisation only happens in one direction. It also means that nerve impulses are discrete (separate). There has to be a delay between action potentials and they cannot overlap.
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 +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 big or small? It depends on the frequency of the action potentials – a bigger stimulus means more action potentials travel along the neurone in a given time.
Speed of conduction
There are three main factors which affect the speed at which an action potentials are conducted along a neurone:
- Axon diameter – a bigger axon diameter means less resistance to diffusion of Na+ ions in the cytoplasm, so depolarisation can travel 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 travels more quickly. This is called saltatory conduction.
- A wave of depolarisation travels along a neurone in one direction in discrete impulses.
- Action potentials are all-or-nothing: the change in potential difference is always the same. It is the frequency of the waves which tells the brain the size of the stimulus.
- Axon diameter, temperature and myelination all affect the speed of conduction.