now let's look at the response of a muscle fiber to stimulus and this is indicating on the y-axis the amount of tension that this muscle fiber is going to create and then we have time on the x-axis so the stimulus for skeletal muscle as you recall is going to be an action potential and that action potential then results in changes within that muscle fiber so initially after the muscle fiber is stimulated we seem to have no tension developing and this is the latent period now eventually we then are going to develop tension and this is called the contraction phase so it's during this time that calcium is bound to troponin which has caused the shift of the tropomyosin so that we have binding sites Exposed on actin and we're also going to have activated myosin present and so cross Bridges form sarcomere shorten and tension develops we then enter the relaxation phase and it's during this time that now calcium is no longer being released from the smooth ER rather it's being sequestered back into the smooth ER so as we take calcium out of the cytosol we then are going to be covering up The Binding sites for actin and myosin and therefore the muscle can no longer create tension and that creates the relaxation phase so a single stimulus within a given muscle fiber will create this twitch and within a given muscle fiber it will always create the same amount of tension now if we compare the length of time it takes for an action potential to occur with the length of time it takes for tension to develop into muscle fiber you'll see that the time for an action potential is much much shorter than the time it takes for the twitch to occur so here's some general numbers it says an action potential takes about one to two milliseconds whereas a twitch takes about 100 milliseconds so with this in mind notice what happens if we have a single twitch or a single stimulus we get our twitch however because the single stimulus is so quick and short in duration what happens if we were to stimulate the same fiber again before it relaxed so the dotted line here represents the amount of tension that this muscle fiber can create but notice what happens if we stimulate it and then we stimulate it again and again so each time we're stimulating it we're causing the release of more calcium which exposes more binding sites and therefore we get an increase in tension this is called summation and if we continue in this manner and increase the frequency to the point that we are releasing all calcium so that all binding sites are exposed and all cross bridges are formed we eventually max out at a certain amount of tension that we can develop and that is called tetanus so let's look at some factors that can determine the strength of muscle fiber contraction and initially we're going to be talking about individual muscle fibers so one that we've just covered is related to the frequency of stimulation so we saw that summation can increase the amount of tension that a given muscle fiber can create we can also look at the thickness of the fiber so the thicker the fiber the more actin and myosin we have which means we have the potential to form our cross Bridges and the more cross Bridges we form the more tension we will create the last Factor related to individual muscle fibers has to do with the length of the sarcomere prior to stimulation so this is a diagram which is demonstrating sarcomeres and I've drawn these intentionally to show varying degrees of overlap between actin and myosin across the x-axis is the percent sarcomere length and the y-axis has the percent maximum tension so here where we're creating a hundred percent maximum tension we're going to call that 100 sarcomere length so this is the amount of overlap prior to stimulation that allows us to fill our maximum cross Bridges as we continue to shorten and create tension but notice what happens if we shorten the sarcomere before stimulating it and if we go all the way to the extreme down here we'll see that these thick and thin filaments begin to bump into each other so they can no longer shorten so as we shorten the sarcomere length before stimulation that will result in decreased tension likewise if we stretch the sarcomere before stimulating it and again we can take the extreme here where we've stretched these sarcomeres so far apart that there's no overlap of actin and myosin and therefore no tension can occur now within our body the way our muscles are inserted and originate with these skeletal muscles they are going to be living around what we call Optimal length so you don't have to consciously try and make your muscle an optimal length before you stimulate it that's just the way the skeletal muscles live in our body so in our body we're going to be around optimal length which will get us around 100 maximal tension when we activate a skeletal muscle now those three factors have to do with individual muscle fibers but let's bring in A New Concept and this is called a motor unit and a motor unit is comprised of a single motor neuron and all of the muscle fibers that it innervates so in this case we have the simplest of motor units which would be a motor neuron and a single muscle fiber if we're to activate this motor unit what we're going to see is that this muscle fiber will be stimulated and will reach tension our tetanus with regards to the amount of tension it creates now muscle motor units are not that simple with a single muscle fiber they typically have multiple muscle fibers so in this example this is a motor unit a single musk somatic motor neuron and all the fibers that it creates and when this is activated every fiber within this motor unit will be activated which will result in a greater amount of tension so again we're going to typically stimulate to the point that we are at tetanus so with regards to individual muscle fibers we said summation thickness of fiber and length of the fiber prior to stimulation will all affect the amount of tension that it can create but let's look at factors that would relate to the muscles as a whole and this just adds one more to these units these Concepts here and this is going to be the recruitment of motor units so realize that a muscle does not consist of just a single motor unit a muscle consists of lots of motor units so in this case we have a our red motor unit with five fibers and if we activate that we're going to see it developing that amount of tension if we have another motor unit in that same muscle then we can activate that motor unit as well so this would be recruiting the blue motor unit and that results in the blue motor unit Contracting to tetanus and we then add those together and if for some reason we needed more tension then we can activate the green motor unit that's also a part of that muscle and I'll add all of those motor units tensioned together so our nervous system is wired in such a way that it's going to activate the requisite number of motor units to perform a function so we can divide the load that we are lifting into what we call a sub maximal load so this highlight should actually be including sub here so sub maximal load is a load less than what the muscle is capable of moving so imagine that we're trying to lift a five pound object so we're going to stimulate those the requisite motor units and we're going to generate five pounds of tension and we can maintain that tension for a long time and what we do is we actually are going to rotate the motor units that are activated and what that allows us to do is to keep fresh motor units available so they can be the ones creating tension and before they tire out we turn those off and turn another one on and that's called asynchronous recruitment and that's a way that we can lift sub-maximal loads for long periods of time now compare that to a trying to lift a maximal load so with a maximal load we have to recruit every motor unit possible within that muscle and so what's going to happen here is let's imagine the muscle could lift 40 pounds initially we're able to do that and we can last for a while but eventually that muscle weakens or what we call fatigues and fatigue is is a complex topic it can be related to running out of energy to build up of local metabolic byproducts that affect muscle function it could be psychologic fatigue all sorts of different causes of fatigue which go beyond what we're going to talk about this semester but when we're lifting a maximum load eventually fatigue will occur now the last thing with motor units is our ability to precisely control movement so the preciseness with which we can con uh control our movement is going to be related to the size of the motor units that we are utilizing so when we use the term size with motor units we're describing the number of fibers within a motor unit so when we say a smaller motor unit that's referring to a motor unit with relatively few fibers a large motor unit has lots of fibers so the smaller the motor unit the greater the ability we're going to be able to precisely control our movement