📈 Action potential
What happens in a resting state where no impulses are being sent through a neuron?
This is the resting potential where the membrane permeability differentiates between sodium (Na+) and potassium (K+) ions so that at any given time there are more Na+ ions outside than inside and more K+ ions inside than outside.
According to these electrochemical gradients, Na+ ions should move back inside to balance out their concentration (equilibrate) while K+ ions should move back outside the membrane until the concentrations are equal inside and out. This clearly isn’t the case, so what gives?
Found on the membrane there are Na+/K+ pumps which carry out active transport against the electrochemical gradient of these ions. The resting potential of the membrane is negative on the inside and positive on the outside – but how? Aren’t both sodium and potassium ions positively charged? This is achieved by the pump transferring 3 Na+ ions out while taking only 2 K+ ions in. This is where the difference comes from.
Now we know that in the absence of an action potential the resting potential of the neurone membrane is negative (about -70 mV; millivolts). What precedes an action potential and how does it unfold?
A stimulus may depolarise the membrane by opening up Na+ channels for those ions to rush into the axon. An action potential will occur only if the depolarisation passes a certain threshold. For example, if it reaches -60 mV up from -70 mV it will not trigger an…