 Hello and welcome to physiology open. The excitability of nerve is studied using two properties cronaxi and rheobase which are determined by plotting strength duration curve. For plotting this curve we need to stimulate the nerve with an electrical stimulus. When the stimulus is of threshold intensity the nerve responds by generation of an action potential which can be recorded. Now when we deliver a stimulus basically we are delivering some charge to the tissue. Charge will change the potentials and if the charge quantity is enough the potential change will reach the threshold. This is known as threshold stimulus and since charge is equal to current into time any stimulus has two features that is our stimulus strength and the other the duration for which it is given. Now you see same amount of charge can be delivered by changing either the current or the time. So suppose I have to deliver a strength of say 40 charge to generate a threshold stimulus. We are just taking some arbitrary numbers here so please forgive me for units and all okay. Anyways for a charge of 40 either I can keep the current as 2 and time as 20 or 4 into 10 or maybe 5 into 8 or 10 into 4 and so on. So same charge can be delivered by multiple combinations of current amount and duration. Fundamentally if the stimulus strength is kept more the duration for which it needs to be given is kept less and if strength is less duration is more. We represent this stimulus by this kind of diagram where height represents the strength of current and plateau represents the duration. So now let's see the strength duration graph of a nerve. So in this graph x axis shows a stimulus duration while y axis shows a stimulus strength. Now this graph represents all the stimulus strength and stimulus duration combinations which will produce a response in nerve. You can notice here that as we decrease the stimulus strength duration for which it is given is increasing. Same as what we have discussed earlier this occurs to a point that if the stimulus strength is kept this much just below this strength no matter how long it is given does not generate an action potential. Why does it happen? Remember some charge always leaks through the membrane due to leaky channels. So if we give very small current for very large duration that small amount of current has ample time to leak through the membrane and hence it never causes the potential to reach the threshold. Or one may argue that if you increase the duration for unlimited time that is infinite time it may lead to generation of action potential. So this current is strength which needs to be given for infinite time to generate an action potential is known as Rio base. But in practical terms is it really possible to give a current for infinite duration? Will you wait till infinity to get that one value? So in practical terms it is generally considered that minimum current strength which given for 300 millisecond duration produces an action potential is Rio base. Minimum because if you use above this strength obviously lesser time will be required below this strength it will not produce any action potential and the time for which the Rio base is given is known as utilization time. Now as already told that practically it's very difficult to know true Rio base. So what we do is instead of Rio base we determine something known as Chronaxi. Chronaxi is the duration for which double the strength of Rio base current will produce an action potential. So we get an approximate Rio base say here double its strength so it will be till here and Chronaxi is duration for which this strength is given. So we just extend this line to the graph drop up a pendicular and see how much duration is needed. So this much duration is Chronaxi. So why do we want to know all this? You will understand it better if I show you graphs of two different norms. By seeing this graph can you tell which represents a more excitable norm A or B? You can pause the video and think about the answer before moving on. Okay here B represents more excitable tissue. See if for any particular duration we check the strength of current required. So I will draw a line from here to the graphs and then draw a line from here to the current required. So we see that here B requires less strength. So obviously lesser charge is excitable to B and hence it is more excitable. In simple terms compared to B A is having more Rio base and Chronaxi. This happens in nerve injuries. When a nerve is injured it becomes less excitable and the strength duration curve shifts right towards and outwards and if we follow it up with recovery of the nerve excitability improves and the curve gradually shifts towards normal. Well thanks for watching the video. If you liked it do press the like and share button and don't forget to subscribe to the channel Physiology Open. Thank you.