The direction that information travels through neurons, as this picture shows, is first through dendrites towards the cell body, and then along the axon towards the axon terminal.
During this process, the membrane potential within the neuron changes dramatically and goes through a cycle: this cycle is referred to as an action potential. As the picture below shows, the cycle has four distinct phases: a resting state, a polarising phase, a depolarising phase and an undershoot. I wasn’t sure of an everyday example that I could compare this to, until someone gave me the suggestion of a wave at the beach. I thought it was a damn good analogy, and I got their permission to use it.
- Resting phase
The action potential starts off at the beginning of the axon, which is referred to as the axon hillock. This occurs as a result of multiple stimuli hitting the dendrites: this adds up and travels along the dendrites towards the cell body. The cell body interprets the stimuli and if the additive stimuli reaches over a threshold, the next stages of the action potential takeplace. If not, the signal remains in the same area.
If you were at the beach right now, you wouldn’t see the resting phase. It is a flat calm which is associated with deep water.
2. Depolarising Phase
- The depolarising phase is the second stage of the action potential. What happens here is at the axon hillock, voltage-dependent sodium channels are opened as a result of the increased membrane potential. Sodium (Na+) then comes rushing in due to the difference in concentration (higher concentration of sodium particles on outside of axon hillock vs. lower concentration on the inside). This happens until the membrane potential is positive and all the sodium channels are open, as a result of a positive loop.
This would be the equivalent to a wave approaching a coastline where the depth of the water is shallower than the open sea. As a result, the height of the wave increases until it can no longer hold its shape.
3. Repolarising Phase
- Once the membrane potential becomes positive, the potassium channels start to open.
- This is essentially the opposite of the depolarising phase. The potassium rushes out of the axon hillock into the spaces outside of the cell, and consequently, quickly reduces the membrane potential back into the negative range.
This is the equivalent of the wave losing its shape and breaking on the shore line. This happens relatively quickly in comparison to the building up of the wave.
- The undershoot occurs because of the rapid rushing out of the potassium molecules during the repolarising phase. To correct this, small amounts of Na+ and K+ cross across the edge of the cytoplasm in their opposite respective directions (Na+ out, K+ in) and the potential eventually returns to that of the resting phase.
This is the equivalent of the foamy backwash that you see as the wave retracts.
This occurs across the whole of the axon. In order to increase speed, you may remember me taking about Schwann Cells and Nodes of Ranvier from a Structure of a Neuron. These are responsible for the signal ‘skipping’ across the axon rather than travelling the whole distance, as the picture below shows. The cycle of the action potential only takes place on the Nodes of Ranvier (i.e. the exposed part of the axon) and moves over the Schwann Cells.