 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 egg zone without decrement, that is, comehueh baghair poori intensity ke saath equally move karthi. 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. Abhum, action potential ki phases jeen phases me ye action potential mukammal hota hai, ye takriban five steps me mukammal hota hai. The first step is the stimulation. Second 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, hum in phases ko baribari detail ke saath study karenge. Sbse pehle hum stimulation ki baat karthe. When a stimulus is received at egg zone hillock, it, egg zone hillock woh jaga hai jahaan se action potential generate hota hai 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 egg zone 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. Yaha pa positive feedback hota hai. Jiski wajah se sodium ions juju enter hota hai, utre hi zyada sodium channels open hota hai. Chunke ish wak tak abhi potassium channels bandh hota hai ish liye sodium current dominate kati hai aur membrane potential positive hona shuru ho jata hai. 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. Yeh ek thoda sa time jis daraan meh membrane potential positive range meh rata hai, ishe hain overshoot kati hai. Iske baat peak phase ke baat falling phase shuru hoti hai. Membrane dubara se repolarize hona shuru ho jati hai. The voltage-gated sodium channels start to close. So no further influx of sodium ions occurs. Voltage-gated potassium channels start to open. As a result potassium ions start to outflow. Ja potassium ions ka outflow shuru ho jata hai to membrane dubara se polarize hona shuru ho jati hai. 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. Ja potassium channels kloz hona shuru hoti hai to repolarization ka process slowdown hota hai likin. Potassium channels delayed rectifiers hain yeh dir se activate hoti hai. Isli yeh potassium ions ka outflow jari rata hai. 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.