okay this video going to talk about just some of the concepts that are associated with whole muscle contraction so it turns out the same principles apply to contraction of both single fibers and entire muscles well so this entire muscles are made of lots of single fibers you know make sense and the contraction produces muscle tension in tension is basically the force that's exerted on a lauder object to be moved now the contraction may or may not shorten the actual muscle an example of this is an isometric contraction where our muscles can produce tension without a significant amount of shortening and this is actually where the tension increases but does not exceed the load that way the muscles don't change in length isotonic contraction is where the muscle shortens because the muscle tension it does exceed the load and so force and duration of contraction can vary in response to the amount of stimuli as well as the frequency of stimuli intensities and each muscles served by at least one motor nerve motor nerves contains axons of up to hundreds of different motor neurons and these axons can branch new terminals each of which forms a neuromuscular Junction with single muscle fibers now motor units are the nerve to muscle functional unit so an example of a motor unit is basically where we have a motor neuron and all the muscle fibers it supplies so the small or the fiber number the greater the fine control and the weaker the amount of contraction that's going to be able to be produced so larger motor units have larger fiber numbers and gross motor control small motor units have smaller numbers of muscle fibers they connect to and therefore more fine motor control so muscle fibers from a motor unit are spread throughout the whole muscle and stimulation of a single motor unit usually causes very weak muscle contractions so what this is showing are two different motor units we have motor unit one and two one is the purple one two is the is the red one and we see that the axon from motor unit one goes to only two muscle fibers whereas the axon 4 motor unit two branches and connects the three so fibers so motor unit ones give me the smaller motor unit because it connects to less muscle fibers but that means it's also going to be under more fine motor control because it you know it connects the less fibers and therefore provides a you know a weaker contraction which is better for more fine muscle movements motor unit - because it connects to more muscle fibers won't be as fine in fact it's gonna connect to more muscle fibers which actually causes a more strong contraction and what's pretty cool is our nervous system can control motor units specific to the type of action that's necessary so let's say if you just want to like take notes by you know using your pen you're gonna use smaller motor units like that of you know I'm wearing of one otherwise if you want to lift like a burden car off of somebody you know you might want to use motor unit one and two that way you can actually can contract a lot of different muscle fibers to produce a more maximal amount of force now muscle twitches are the simplest contraction that result from a muscle fibers response to a single action potential muscle fibers can track quickly and then relax a little more slowly now these twitches can be actually being observed and recorded as a Maya Graham my omitting muscle gram meaning graph and we get a trace or a line recording of contraction activity so these Maya grams occur in three phases we have a latent period a period of contraction in a period of relaxation the latent period is that is the period of delay between the stimulus and the onset of a muscle contraction this is where you don't see any tension produced now the period of contraction is when the crossbridge is formed in tension starts to increase in the period of relaxation is when calcium reentry back into the sarcoplasmic reticulum causes a relaxation and tension starts to decline towards zero it turns out that muscles contract faster than they relaxed because it's easier to release calcium from the sarcoplasmic reticulum causing contraction and tension than it is to pump that calcium back into the SR that takes longer which is why the relaxation period is longer on the mile gram so this is showing a myelogram where we have time in milliseconds over maximum tension here so this showing one muscle twitch where we have a stimulus at time zero in that period of delay between the stimulus and the onset of tension is basically our latent period this latent period is the period of time it takes for action potentials to spread down the t tubules you know and also causing calcium to be released from the SR and then eventually leading to enough crossbridges we can have a measurable amount of tension and all that occurs within this latent period which is in the order of several milliseconds now this period here where you get a rapid increase in tension is the contraction phase this is actually going to be due to that rapid rise in calcium from SR and that calcium then allows for lots of myosin heads to form cross bridges with actin which is why we see a rapid rise in tension now this period of contraction continues until eventually we start to get a period of relaxation where calcium pumps back into the SR and because that calcium is being pumped back in the SR you know troponin is gonna start to relax and allow for triple madison to cover the actin binding sites which allow for the myosin heads to you know relax and you know prevents crash bridges from being formed which is why we see tension and start to decline over time and here until eventually tension goes back towards zero now the reason why the period of relaxation is longer than the period of contraction is that it takes longer for calcine to be pumped back into the SR than it is for calcium to be released and that's why we see a difference in the slope of these lines in the period of contraction versus relaxation now the differences in strength and duration of twitches are due to variations and the metabolic properties of enzymes between muscles so it turns out that not all muscles twitch the same way some twitch more rapidly some twitch more slowly and all thing that depends on the types of molecules you find in those cells so for instance eye muscles have a rapid type of twitch which is brief whereas larger fleshy muscles like your calf muscles contract more slowly and can hold the contraction longer so this is showing that the twitch duration and different muscles on your body like the soleus and gastrocnemius are the sum of your calf muscles and you can see that these calf muscle have much longer twitches then say the extraocular muscles of your eyes because for one they're gonna be involved in kind of longer and slower gross motor movements whereas the extraocular muscles because they need to produce pretty rapid eye movements their twitches are rapid and ultimately what underlies the difference between these the twitch duration is how quickly calcium is released from the SR and also how quickly it's pumped back in so we're seeing here is that with the slower twitch fibers you know calcium is gonna be released more slowly and it's actually going to be pumped back in to the SR also more slowly which results in a much longer muscle twitch and it actually makes sense for this the way this muscle needs to be used now in terms of muscle responses we find that you know normal muscle contractions are pretty smooth and the strength varies with the needs of that muscle and so muscle twitches which you've seen a lab are gonna be you know just individual myelogram muscle twitches but you don't find this in normal muscle normally in living muscle that's you know in an organism you find that that muscles are going to be a more of a graded response where multiple muscle twitches add together which can kind of you know provide different properties for the demands of that muscle now the responses are graded by changing the frequency of stimulation of that muscle and changing the strength of stimulation so if you have more frequent stimulation you're gonna get stronger muscle contraction or if you also have a stronger stimulus you're also gonna get a stronger muscle contraction so what this is showing is you know one stimulus and then one muscle twitch like we saw in the myelogram earlier and this can actually start to add together when you have wave or temporal summation where muscle twitches can start to some a or add together as long as you have stimuli in rapid succession in fact muscle fibers don't have the tend to completely relax which means that these twitches increase in force with each stimulus this is because calcium is released more and more so with the second stimulus now this produces a smooth continuous contraction that adds up or some eights and further increases in stimulus frequency can cause the muscle to you know lead to a quivering type of contraction that's sustained we call this unfused or incomplete tetanus so what this is showing is more of a low stimulation frequency which we call unfused or incomplete tetanus so each little arrow here showing one stimulus and you see we'd a stimulus and a twitch but before this first twitch can fully relax we get another stimulus which causes another twitch and because it hadn't fully relaxed that second twitch added on the first which causes your maximal tension to be higher than the first and because the second twitch didn't fully relax before our next stimulus here then it adds out of that one and adds on to that one until eventually we get tension that's or even greater than the initial you know twitch itself because of the summation or addition of the tension that's produced by each stimulus on the muscle and so this is going to get us to you know a period of you know even greater tension or a muscle force that's able to be produced now muscle responses can also be due to a change in in seemless frequency and if stimulus frequency increases muscle tension can reach a maximum and we call this fused or complete tetanus because contractions fuse into one smooth sustained contraction plateau and what determines this contraction plateau is basically just the number of sarcomeres you find that muscle eventually going to run out of sarcomeres that are completely contracted which is going to cause the maximal amount of tension that's able to be produced by that muscle so what this is showing is if you actually have lots of stimuli or action potentials in rapid succession we see that there's a really rapid increase in tension up to a plateau and this plateau is the maximal amount of tension that you can produce in this muscle until once you have the last stimulus which then allows for relaxation to occur so this is an example of complete or fused tetanus so muscle responses to a change in stimulus can also be due to a recruitment in recruitment is where actually have multiple motor units that can lead to summation now there's different types of stimulus that can lead to recruitment like sub threshold threshold and maximal stimulus in a sub threshold stimulus is a stimulus that is not strong enough to lead to a contraction that's why it's sub threshold because you get a stimulus but no corresponding muscle contraction threshold stimuluses this is the minimum amount of stimulus required to cause a first observable muscle contraction in a maximal stimulus is the strongest stimulus that increases the maximum contractile force now that is illustrated well by this graph where these arrows represent the size of the stimulus and you can see that 1/2 are basically sub-threshold because yes they are stimulus however they don't actually recruit any muscle fibers to contract which is why we don't see any tension produced now simulus number 3 is called a threshold stimulus because this is the first observable sign of contraction where you do recruit some fibers and you get a little bit of muscle contraction that's that's you know produced now upon subsequent stimulation increases we do see an increase in tension until eventually we get to a point where there's a maximal stimulus and the maximal stimulus means that you've recruited as many muscle fibers as you can in this muscle which means that increasing stimulus intensity doesn't cause an increase in an amount of tension that's produced by that muscle this is why it's a maximal stimulus we're getting stronger stimuli it doesn't lead to even stronger tension because ultimately you've reached the peak amount of tension that's able to be produced by this muscle because you've recruited all the muscle fibers in that muscle so one and two are examples of sub-threshold stimuli number three is an example the threshold stimulus which is the minimum amount of stimulus required to cause a muscle to contract and then number seven here is an example of maximal stimulus which is then the last type of stimulus that can lead to the greatest amount of muscle contraction produced by that muscle now muscle changes can also sorry muscle responses can also change due to assimilation strength and it turns out that fibers are recruited based on size principle so that the smaller muscle fibers are recruited first larger muscle fibers have recruited last and so motor units with the smallest muscle fibers are the ones that are recruited first and the motor units with larger and larger fibers are recruited last because of you know a resistance to stimulation now what we see then it actually makes sense is that motor units with the smaller muscle fibers are contributed first are stimulated first which means you get smaller contraction initially but as you start to increase the stimulus you get larger and larger muscle contractions because larger muscle fibers are therefore recruited and it makes sense with respect to how muscles should be used because you don't want to have the maximal amount of tension being produced initially you want to start with the smaller motor units and then work your way up to a maximal amount of you know tension so physiologically this idea of muscle tone is a constant slightly constructed state of all muscles it's actually due to spinal reflexes where groups of motor units are alternatively activated in response to input from stretch receptors and it helps keeps muscles firm healthy and readily ready to respond now earlier we talked about the isotonic and isometric contractions remember isotonic contraction is basically where you have a change in the muscle length because of the load moving and causing a change in the length of your muscle now isotonic contractions can either be concentric or eccentric concentric contractions where the muscle shortens as it does work example this is like when your biceps contract to pick up a book where your biceps can actually produce more force than the load or that the weight of that book and that way the muscle shortens at Purdue as it reduces work that's concentric and Annie center contraction is like the muscle lengthens as it generates this force this would be like letting a book down as the biceps relaxes it actually has a little bit less tension or force than the load of that book so an example of this like an isotonic contraction is basically where the muscle contracts and it shortens as it contracts which allows you to kind of pick up that load and this example is a three kiloton kilogram block so if you can measure this you know experimentally we see that you know tensions developed we get a peak amount of tension that's developed by that muscle and that actually causes you know muscle length to decrease and in response to that tension and this is an example of a centric isotonic contraction opposite if we actually have isometric contractions and isometric contraction is basically where the load is greater than the maximum tension that the muscle can generate so the muscle neither shortens nor lengthens and we find then is that you know isometric contractions would be like where if you can just hold a book out in air and your arms not changing length at all and the muscles not changing length at all and therefore you're just adequately resisting that load the muscles not changing length and it isn't the muscle isn't shorten or lengthen so the way we get get isotonic and isometric contractions is that in isotonic contractions actin filaments shorten and cause moment in isometric contractions the cross bridges generate force but the actin filaments don't change their position they don't change their length and therefore don't shorten and this is where the myosin heads kind of just spin their wheels in the same position which keeps them the sarcomere is the same length so with an isometric contraction you're helping to resist the load but this muscle is not changing length so you actually help you can hold this this block up in space but the muscles doesn't change its length which is isometric or same distance so with an isometric contraction yes you're producing a peak amount of tension but you're not changing a length of that muscle