10.4 Passage of an action potential

When an action potential is generated by a receptor, it rapidly 'moves' down the neurone, in what is called an nerve impulse. In reality, nothing physically 'moves' down the neurone. what happens is that the reversal of electrical potential is reproduced again and again, and this wave of depolarisation travels down the neurone towards the CNS, effectors or other neurones.

The passage of an action potential can be likened to a Mexican wave, where a wave can pass all around a stadium with no one actually physically moving.

• Resting potential. All parts of the neurone maintain a constant potential of negative 60-70 mV. There is a higher concentration of sodium ions outside the cell, and a higher concentration of potassium ions inside the cell. Overall though, there is a higher concentration of positive ions outside of the cell
• Stimulus. Part of the neurone is stimulated. This causes sodium ions to flow into the cell, down their chemical and electrical gradient. This is the action potential, and themembrane at this point is depolarised
• Passage. The influx of sodium ions at that point of the membrane causes sodium ions gates a little further down to open. This causes the membrane at this point further along to be depolarised. The sodium gates at the original site of depolarisation close, and the potassium ion gates open, causing potassium ions to move out of the cell along its chemical gradient
• Propagation. The action potential travels down the neurone by the continued depolarisation of adjacent parts of the membrane
• Re-polarisation. As potassium ions move out of the cell at the original site of depolarisation, the inside of the cell starts to become negative again. The membrane at this point starts actively transporting sodium ions out of the cell, and regains its resting potential

In myelinated axons, the fatty sheath of myelin around the axon acts as an electrical insulator, preventing actino potentials from forming. At intervals of 1-3mm there are breaks in this myelin insulation, called nodes of ranvier.Action potentials can occur at these points. The localised circuits therefore arise between adjacent nodes of ranvier and the action potentials in effect 'jump' from node to node in a process known as saltatory conduction.

As a result, an action potential passes along a myelinated axon a myelinated neurone faster than along the axon of an unmyelinatedone. In out maxican wave analogy, this is equivalent to a whole block of spectators leaping up simultaneously, followed by the next block and so on. Instead of the wave passing around the stadium in hundreds of small stages, it passess around in 20 or so large ones and is consequently more rapid.