 This graph here shows simple muscle twitch when the nerve is stimulated and muscle contraction and relaxation are recorded. Now for recording this graph in a mechanical device actually the paper is moving that is why we get the graph like this contraction is upwards and there is some space right and then relaxation is there. Now suppose the screen is not moving or in case of mechanical device the drum is not moving will we get the record like this no actually it will be like a straight line the muscle will contract because the screen is not moving the liver is going to go up and then the muscle is going to relax when the liver is going to come down so we will get a straight line for simple muscle twitch if the screen is not moving. Fine with this let's try to understand the concept of recruitment of motor units. Now if you saw the video on simple muscle twitch we said there that simple muscle twitch can be recorded even if only a single motor unit is contracting some muscle fibers are contracting so we will record the contraction and relaxation and maybe that whole muscle is not contracting. So suppose if we take out a nerve muscle preparation and just start stimulating the nerve with the minimal stimulus strength so here in this diagram the x-axis is showing the stimulus strength and we start stimulating with the minimal strength say suppose a five millivolt or say suppose seven millivolt so what we see initially there is no contraction recorded right so here we got blank why because maybe the stimulus strength is not strong enough and not even a single neuron in the nerve is stimulated and none of the muscle fibers have contracted right so we slowly increase the strength say suppose to 7.2 millivolt and still no contraction is there but when we increase the strength to say suppose 7.5 millivolt we see a simple muscle twitch so this is a simple muscle twitch recorded right so what we say is that the strength of the stimulus at which contraction just occurs that is the threshold stimulus right so this is the threshold stimulus strength and below that right where no contraction is occurring that is known as sub threshold stimulus so these are sub threshold stimulus that means that when the stimulus strength is below the threshold we will not get any contraction fine now what we do is that we slowly keep on increasing the stimulus strength so maybe suppose here it is 7.8 millivolt what we see is that you see the height here the strength of contraction has increased it was here this was maximum and here this is maximum right then maybe 8.2 millivolt slowly we have to increase 7.8 to 8 millivolt then 8.2 millivolt what we see is that again the strength of contraction has increased right but after a certain point if we increase so this is say suppose 8.6 millivolt 8.8 millivolt 9 millivolt what we see is that the strength of contraction has increased here also right so this is increasing but after this the strength of contraction is not increasing any further so if we consider in terms of stimulus we said that this is threshold stimulus where the contraction is just started and these are supra threshold stimulus right supra threshold stimulus then there is a stimulus at which a maximum strength of contraction is reached right so here 8.6 millivolt a maximum strength of contraction is reached so this stimulus at which we get the maximal contraction that is known as the maximal stimulus and above a stimulus that even if we increase further we see that no strength of contraction is not increasing then this is known as supra maximal stimulus so depending on the strength of stimulus which we are using and how the muscle is contracting we have sub threshold is stimulus where no contraction is seen threshold is stimulus then supra threshold is stimulus then maximal is stimulus where maximum contraction occurs and if above the maximum is stimulus if we increase the strength of stimulus we don't get increased strength of contraction so that is the supra maximal strength of stimulus so this is known as recruitment of motor units you see initially only maybe one or two motor units were excited and we got a small strength of contraction because few muscle fibers that is of those excited motor units only contracted and later on as more neurons are stimulated of other motor units we get increased strength of contraction why because more muscle fibers are contracting now why we need to know about this concept of recruitment of motor units first of all this is a concept which body uses for force gradation in our body what is this force gradation that means depending on the amount of force which is required the number of motor units are recruited that means suppose you want to lift a pen do you want the whole muscle to contract no maybe a very small motor unit contraction will be sufficient for lifting a pen however if you want to lift a 2 kg object or maybe 5 kg object then more motor units need to be recruited then maybe 10 kg maybe whole muscle needs to contract to lift that so what is happening with increase in the load which the muscle has to lift there is recruitment of motor unit so this is one mechanism of force gradation in our body second thing when we talk about experiments which will we will see in other videos on effect of temperature and other videos basically for doing those experiments we need to use supramaximal strength of stimulus it is very important that we use supramaximal strength of stimulus for recording simple muscle twitch in those experiments because we are seeing that if we are using lesser strength of stimulus we are getting recruitment of motor units right so whenever we do any experiment we have to make sure that the effect which is seen in that experiment is not due to any other thing rather it should be based on that particular intervention only for example if we are using the temperature increase temperature or decrease temperature whatever effect is seen it should be only because of that temperature and not because of the recruitment of motor units right so first we make sure that we recruit all the motor units so we use supramaximal strength of stimulus and we are sure that we are getting the maximum strength and then we do that intervention fine so before I end just a question for you all you can type the answer in the comment section below does this graph violate all or none law what does all or none law say all on law says that either the response will be maximum or there will be no response at all so my question here is does this graph violate all or none law type your answers in the comment section below thanks for watching the video if you liked it do press the like button share the video with others and don't forget to subscribe to the channel physiology open