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Lecture

MUSCLES AND MOVEMENT 2

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Department
Biological Sciences
Course Code
BIOB34H3
Professor
Jason Brown

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MUSCLE & MOVEMENT 2 SLIDE 2 MUSCLE ANATOMY: SKELETAL MUSCLES: FUNCTION AS LEVERS  Cells of muscles are part of a functional level system  Levers have 3 components: o Load, fulcrum, effort  We can arrange these 3 components in levers in a number of ways to generate different kinds of levers.  First class lever has a fulcrum on the centre, and the effort and load on the sides o A non-biological example of this is scissors. A biological example of a first class lever is the head, where the load is your head, the fulcrum is the joint between the skull and the first vertebra, and the effort is exerted by the trapezius muscle which pulls on the region at the base of your skull.  Second class lever has a fulcrum on one end, effort on the other, and load in the middle. o Non-biological example is the wheel barrel. A biological example is standing up on your tip-toes or jumping. The fulcrum is at the joint between your toes and foot, the load is every part of the body, and the effort is the muscle on the base of the heel.  Third class levers have the fulcrum at one end, load in the other end, and effort in the middle. o Non-biological example is tweezers. Biologically, third class levers are the most common, example a bicep muscle. SLIDE 3 MUSCLE ANATOMY: SKELETAL MUSCLES: FUNCTION AS LEVERS  Mechanical advantage is defined mathematically as the length of the effort arm divided by the length of the load arm.  The effort arm is the length of the fulcrum to the effort. The load arm is the length of the fulcrum to the load.  If you know the lengths of these 2 arms you can calculate mechanical advantage, which tells us how efficiently muscular force (effort being exerted) is translated into leverage, in other words how effectively it’s used to lift that load.  If you look at the first class lever, for scissors the effort arm and load arm seem to be relatively equal so the mechanical advantage is 1.  How can you modify the system so you can use muscular force more efficiently with the same amount of effort? o You either decrease the load arm, or increase the length of the effort arm. Example a crowbar, if the load is very close to the fulcrum it’s much easier to lift the load, or you can get a crowbar with a longer arm.  Take a cheetah being chased by a lion as a biological example. One animal is built for power, and one is built for speed.  Mechanical advantage tells us how efficiently muscular force is translated into leverage, but the opposite of mechanical advantage which is mechanical disadvantage is how fast that lever can operate.  In third class levers, the load arm is much greater than the effort arm. So in these third class levers, which are common in biological systems, have very low mechanical advantage but they have the advantage of being speed levers.  In a third class lever arrangement, you can move a load over a distance than in first class levers. So when you have a mechanical disadvantage, you have a speed advantage.  Going back to the cheetah and lion, we look here at the Teres Major muscle which originates at the scapula and inserts in the humorous. The lion uses the effort arm for power, the cheetah is using the same muscle to move its limb as fast as possible.  In these 2 systems which have third class levers, the picture on the left has the lower mechanical advantage therefore it’s built for speed. We know this because the effort arm is much shorter here. By changing the insertion of the Teres Major muscle to make it closer to the shoulder, you can move this limb over a greater distance by compromising the power you can generate. Similarly for the lion, by moving the insertion of this muscle away from the fulcrum, you compromise the ability for this limb to go over a distance but you produce more power. SLIDE 4 MUSCLE ANATOMY: HILL MODEL  Prior to 1954, Archibald Hill proposed the Hill Model of muscle anatomy before we had an understanding how muscle contraction worked.  He proposed muscles can be thought of consisting of 3 components o Contractile component: the actin and myosin myofilaments. The part of the muscle that generates force during muscle contraction. o Series elastic components: largely made of tendons.  Muscles are attached to bones with tendons.  The series elastic component means an elastic component of the muscle is directly in line with the contractile component, where the sarcomeres that are arranged in a linear fashion and the tendons are arranged in the same line as those actin and myosin.  During the course of muscle contraction, the series elastic component has to get stretched out before the muscle can translate its full force on whatever load it’s trying to lift.  As the contractile component shortens, it pulls on the elastic component until it’s stretched out completely. Much of the force that’s being generated by the contractile component is being used initially to stretch out the elastic component rather than being fully translated on the load the muscle is trying to lift. o Parallel elastic component: it’s called the parallel elastic component because it’s not in line with the contractile component, it’s parallel to it. This component is largely made of titin.  When you stretch a muscle, it resists stretching and snaps back to its original length because you stretch out that titin, or in other words, you stretch out the parallel elastic component.  The parallel elastic component gets compressed when the muscle contracts and stretched out when the muscle gets stretched out and helps restore the muscle to its original length.  So if you stretch a muscle out, you stretch out titin and if you let the muscle go the muscle will contract passively and snap back due to the parallel elastic component. SLIDE 5 MUSCLE CONTRACTION: ISOMETRIC VS. ISOTONIC  Understanding these 3 components helps us explain the idea of isometric contraction  Muscles can contract without changing length, this is called isometric contraction.  You contract your muscles, but the length of the muscle doesn’t change and you don’t shorten any distance therefore the load can’t be lifted.  You can’t explain isometric contraction without understanding the elastic components present in the muscles which can be stretched out.  So for isometric contraction in pictures 1 and 2, the contractile component is shortening, but the length of the muscle hasn’t changed. Why? Because you’ve only stretched out the series elastic component so far. All this contraction is stretching are the tendons, which keep the muscles at the same length.  Once the series elastic component (SEC) is fully stretched, the force exerted is used to lift the load, but if that force is insufficient to lift the load it won’t move.  The key point is that energy is still being consumed. Why is energy still being consumed? o If you stop using energy, the myosin heads will unattach and the contractile component will stop operating and the SEC will snap back to its original length. Therefore it eliminates all the tension generated by the contractile component. o To keep the SEC being stretched out, you need to continue operating the cross bridge cycle and not allow the myosin heads to fully detach from actin.  In isotonic contraction the muscle length decreases. The SEC is fully stretched out, the tendons, we continue to generate force and when the amount of force exceeds the load, it gets lifted and the muscle shortens. Energy is consumed in this also.  Most natural muscle movements involve isometric and isotonic contractions. Initially muscle contraction will be isometric as the tendons stretch, and when they’re stretched out fully and you start lifting the load, the muscle starts becoming shorter in length in an isotonic contraction, unless you lift something too heavy in which case it will me isometric throughout.  The graph at the bottom shows you 2 types of tension. External tension is the one we commonly measure. External tension is the force being applied to anything outside the muscle (the load).  Internal tension is the force being generated by the contractile component.  When muscle begins contracting, internal tension rises quickly but much of that force is used to stretch out SEC rather than the load. As a result external tension, the force being applied to the load, rises very slowly. The difference between these 2 forces is the amount of tension being used to stretch out the SEC. SLIDE 6 REGULATION OF CONTRACTILE FORCE: TWITCH, SUMMATION, AND TETANUS  This mode explains 2 basic types of muscle contractions: a twitch, and tetanus.  When there’s an action potential in the muscle, there’s a latent period before the force begins to be applied  If you contract a muscle a single time with a single stimulation, you get a brief contraction and relaxation and this is called a twitch.  Notice that the length of the contraction and relaxation occurs at a slower pace than depolarization and repolarization of the membrane. You can stimulate the muscle to contract again even before it hasn’t begun to relax.  So you stimulate the muscle once and it contracts and before it can relax completely you stimulate it again, and when it contracts this time it will generate a little bit more force than it did last time.  If you keep stimulating the muscle quickly, the muscle doesn’t have a chance to relax, and it has a sustained and much more forceful contraction which is called tetanus. This occurs in response to frequent stimulation. SLIDE 7 REGULATION OF CONTRACTILE FORCE: TWITCH, SUMMATION, AND TETANUS  How do you explain twitch, and tetanus and the force generated by these contractions? You have to come back to the Hill Model.  During a single twitch of a muscle, there’s a single action potential. Calcium channels open up in response to that depolarization but then due to depolarization quickly turning to repolarization, the calcium channels quickly close.  Calcium levels inside the cell rise a little bit but quickly fall and the calcium pump begins to pump that calcium out.  There’s a small transient increase in calcium in response to a single twitch. This means 2 things: o One it means we haven’t released enough calcium in response to a single opening of the calcium channels to have calcium binding to all the troponin C. Therefore some troponin C stay bound to actin and keeps the tropomyosin in place and prevents myosin from making contract to actin and generating any force. o Secondly and more importantly, in this brief period of time you have a contractile apparatus operating (the calcium), it may not have been enough time to fully stretch out the SEC (the tendons).  If you keep stimulating the muscle over and over again through successive action potentials, more calcium channels open again or may not have a chance to close so more calcium gets out of the sarcoplasmic reticulum and this means 2 things: o For one, every troponin C is likely to become fully saturated and all the troponin moves out of the way and every myosin can bind to actin and can generate maximum force o At the same time there’s enough time allowed for the SEC to be fully stretched out.  By frequent stimulation, it reaches a maximum force and then the muscle relaxes. SLIDE 8 REGULATION OF CONTRACTILE FORCE: FORCE-VELOCITY CURVE  Force velocity curve tells us the faster the muscle contracts, the faster the rate at which the muscle shortens, the less force it can produce.  One way to effect the rate at which muscle contracts is to apply a heavier or lighter load to it.  Example if you stick an empty hand and contract you can do it quickly, but if you add a 50 pound weight to your hand, it takes more effort and the rate of contraction becomes slower. If you put a 1000 pound weight there won’t be any shortening at all.  If we give a muscle nothing to pick up, no load, it contracts very quickly but generates no force. If we give it a much heavier load, it begins to contract less quickly but generates much more force.  On the flip side if we give something extremely heavy for our muscles to lift, it won’t shorten therefore it can’t contract at all but it will generate the maximum amount of force, this will be an isometric contraction.  So you have a compromise between how quick you contract a muscle and how much force is generated. SLIDE 9 REGULATION OF CONTRACTILE FORCE: FORCE-VELOCITY CURVE: HUXLEY-HYPTHESIS  If a muscle shortens very quickly why doesn’t it generate any force?  If a muscle generates force think about what has to happen. We need to have a myosin head while it’s bound to actin in its high energy confirmation, it has to convert back to the low energy confirmation and in doing so it pulls the actin filament thus generating the force.  Imagine a lot of people are pulling on a rope on the side of you. You can just hold on to the rope without actually applying any effort and be pulled back along with the rope. Similarly, if a muscle is contracting very quickly, it’s possible that a high energy conformation myosin head binds on to an actin but doesn’t get to contribute to the power stroke, meaning it doesn’t go into its low energy conformation on its own but it does so because other myosin tug on the actin filament and pull the other myosin in its low energy conformation.  So the faster a muscle contracts, the more likely it is that some myosin can’t contribute to force generation  At the same time there’s a possibility for some actin bound in their rigor state to myosin heads in low energy states and get pulled into high energy state in the opposite direction. When this happens it generates negative force.  So when a muscle contraction is happening as quickly as possible both negative forces and positive forces equally balance each other out and no force is produced.  This idea is known as the Huxley Hypothesis.  The amount of force a muscle produces depends on the force exerted by myosin head and the number of myosin heads that are present. SLIDE 10 REGULATION OF CONTRACTILE FORCE:POWER-VELOCITY CURVE  Often times when it comes to animals moving, what’s more important than force is how much power the muscle can produce.  Power is force multiplied by velocity. It’s the measure of how quickly we can move a load over a distance.  You can generate a power-velocity curve by taking the data
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