 Dear students, in this module we shall discuss the action potentials. Action potentials are the type of signals that neurons use to send information along the nervous system for long distances. The action potentials are brief but large changes in membrane potential that are propagated along the X-zone without decrement i.e., come, who, a, b, r, all the intensity equally move. The action potentials are very important as they control the effector responses, they control the activation of voltage-gated ion channels, they control muscle contractions and they also cause exocytosis. In fact, action potentials are responsible for every sensation, every memory, every thought, indeed every impulse. The first step is the stimulation. The next step is the rising phase in which depolarization occurs. The third phase is the peak phase and then fourth is the falling phase in which repolarization occurs. While the last phase of action potential is the undershoot also known as after hyperpolarization. We will study these phases in detail. First of all, let us talk about stimulation. When a stimulus is received at exon hillock, exon hillock is the place where action potential is generated and this place has voltage-gated ion channels in it. So the stimulus causes the sodium channels to open in the neuronal membrane. When sodium channels open, sodium ions start to influx. This results in change in membrane potential locally which is called depolarization. As a result of depolarization, membrane potential becomes positive, relatively positive. Then a depolarization of about minus 55 to minus 30 millivolt is known as threshold. That can trigger an action potential. If the intensity of the stimulus is less than this threshold, it is called a sub-threshold. It also causes depolarization but these depolarizations cannot initiate an action potential. After a threshold has acted on the exon hillock, sodium ions start to enter and membrane potential becomes less negative. This causes more sodium channel to open, causing an even greater influx of sodium ions. Here positive feedback occurs, which causes more sodium ions to enter and more sodium channels open. Because the potassium channels are still closed, that is why sodium current dominates and membrane potential starts to become positive. The sodium channels become maximally open. At this stage, the positive feedback slows down. The membrane potential reaches a maximum, that is close to the sodium equilibrium potential of positive 55 millivolts. At this stage, further depolarization stops and it is known as the peak. The voltage-gated sodium channels start to close. So no further influx of sodium ions occurs. The voltage-gated potassium channels start to open. As a result, potassium ions start to outflow and this continues towards the potassium equilibrium potential of minus 58 millivolts. Result is repolarization, that is reversal of membrane potential. However, the membrane repolarization initiates closing of potassium channels. When potassium channels start to close, then the repolarization process slows down. But, potassium channels are delayed rectifiers and they are activated late. That is why the potassium ions keep outflow. As a result, the membrane potential dips even below the normal resting membrane potential. This means that membrane has become hyperpolarized. This brief hyperpolarization is known as undershoot or after hyperpolarization. It persists till the potassium permeability returns to the normal value.