What is an action potential?

The action potential lies at the heart of how we think, move and sense, as it constitutes the language of the nervous system. The two main characters in the spectacle of the action potential are potassium ions (K+) and sodium ions (Na+). Notice that both of these ions are positively charged. Now, when a neurone is resting, i.e. during the resting membrane potential, there are many more Na+ ions on the outside than there are inside the cell and many more K+ ions on the inside than there are outside. Both ions experience a concentration gradient, meaning that all the Na+ ions want to get into the cell, while all the K+ ions want to get out of the cell, but can’t. This situation has been created by the sodium-potassium pump, a protein in the surface of the membrane, which chucks Na+ ions out of the cell and brings K+ ions into the cell. Another thing to consider is that apart from there being a chemical difference across the neuronal membrane there is also an electrical difference. The inside of the cell is electrically negative with respect to outside because the ratio of positive to negative ions inside the cell is smaller than on the outside, just like in a battery where you have a ‘plus side' and a ‘minus side’. This is why the resting membrane potential is given the value -70 millivolts (-70 mV), as it refers to the inside of the cell being negative in respect to the outside. In this state the membrane is referred to as polarised.When you get a kiss on the cheek, the stimulus raises the voltage across the membrane of the neurones in your cheek to around -55mV because of an influx of positively charged ions. Action potentials are all-or-nothing phenomena, meaning that -55mV is the threshold voltage below which the action potential will not fire. Once the voltage gets this high, an action potential will fire which will travel along the axon of the neurone all the way to your brain to tell it that you’ve been kissed. How does this happen? The threshold voltage causes sodium channels to open which allow Na+ to rush into the cell making the inside more positive. Because of this the voltage rises all the way up to around +40mV. This is the peak of the action potential and at this point the membrane of the cell is referred to as depolarised. After this Na+ channels close and in turn K+ channels open, allowing K+ to rush out of the cell. This makes the inside of the cell progressively more negative again and drives the voltage all the way down to -90mV undershooting the resting membrane potential. At this point the membrane is hyper-polarised. To get the voltage back up to -70mV the sodium-potassium pump steps in and the whole system is brought back to its original state. The time it takes for the sodium-potassium pump to restore the resting membrane potential is called the refractory period of the neurone and no action potentials can fire during this time. The action potential will propagate along the axon in the form of an electrical impulse, which we have learnt, simply means a wave of Na+ and K+ ions moving in and out of the axon.