M.Ed., Stanford University
Winner of multiple teaching awards
Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.
The muscular system includes all the muscles in an organism's body and allows organisms to move. A lot of the muscles in the muscular system of vertebrates are controlled by the nervous system. There are three types of muscle tissue. Skeletal (also known as striated) muscle provides body movement. Smooth muscles control automatic functions like breathing. Cardiac muscles make up the heart and allow it to pump blood through the circulatory system.
I know when you look at me you think about muscles. Well that's because the muscle system is such an important system in the body. It's involved in movement and we know that. It's involved in support. Sometimes we don't realize that but imagine if you just stop using muscles you just go brrr and flop on the ground. It's also used to provide heat. We don't think about that but just remember what happens when you go outside and it's cold? You start to shiver. Why? To generate the heat. Most of the energy used by your muscles roughly 80 percent or so is given off as heat and this one of the ways that we mammals are able to maintain our high body temperature by using muscle contraction to generate a lot of heat.
Now within the muscle system, there's actually three different types of muscle tissue. Not just the muscles that make up things like your biceps. Now your bicep is agood example of a muscle made out of skeletal muscle tissues. Skeletal muscle tissue as the name implies are the ones that are attached to the bones and they're involved in pulling our bones and moving them around. Now skeletal muscle tissue is considered voluntary which means that when my arm goes up, because I chose to make it do that. Not just because it's going up and attacking me for some random reason.
Now, there are some involuntary things within this. If you sit down on a tack you'll pop up. And you didn't say, I wish to jump up in the air. But that's considered still a voluntary movement even though it's a reflex.
Now, skeletal muscles are striated which means they have a stripped appearance and I'll go into when I discuss skeletal muscle anatomy. I'll discuss why they have those stripped appearances. But not only are they voluntary and striated, they're also of the three types, they're the ones that are typically considered the fastest and strongest. But they're relatively quick to fatigue. If you doubt me, go running with a suitcase in either hand and run at your top sprint speed for 15 minutes. You'll discover how easy it is for skeletal muscles to fatigue.
Cardiac muscles make up the walls of your heart and unlike the skeletal muscles, they do not fatigue until you're dead. Because, obviously once your heart stops beating, you're offline forever. So they are designed so that they don't fatigue. They, actually that's one reason why your heart does not go squeeze. My bicep muscles I can squeeze and keep them locked in contraction. Your heart though does not lock in contraction because that would be kind of dumb. Instead it goes beat, relax. Beat. Relax. And that period even though it seems so short between a heartbeat, is a long enough time to recover all the necessary ATP energy in order to continue doing the beating for the next 80 odd years of your life, hopefully 100. Cardiac muscles much like skeletal muscle tissue is also striated. It is considered involuntary. In fact your cardiac muscle cells have the ability to generate their own signal to contract. they are pretty fast. Maybe not as fast as the muscles that make up your eyes for looking around or maybe not as fast as, if you've ever seen somebody who's really good at those fast personal shooter games their fingers blur in the air, they're maybe not as fast as the skeletal muscles but they're still pretty fast. They're not quite as strong as the skeletal muscles and if you stopped to think about it, the amount of force your heart needs to pump to push blood up to the top of your heart, is nowhere near as much as your need to say toss a football 30 feet up in the air. You just don't need your heart to be that strong.
Smooth or visceral muscles, they're given this order name smooth because they don't have that stripped appearance called striations. So they're called smooth muscles. A lot of people are starting to switch to the term visceral muscles because they line your visceral, your guts. These are the non-striated muscles that are involved in moving food through your digestive system, adjusting the diameter of your arteries and veins, they are the things that help make your hairs go up when you're scared or cold and because of this non-striated arrangement of the proteins within them, they aren't nearly as fast. But on the other hand, do you really need to digest your food really super fast? Mmm and it's out? No. So you can go with slow contractions, they also don't fatigue. And that's another good thing. You don't want to be walking down on the street and the little sphincter muscle at the end of your digestive tract, you don't want that to go "I'm tired." Hmm, crap.
Last, they're involuntary. You don't choose to think, I must squeeze, I must squeeze. No. It just happens. And actually you took about three years of intense training to learn to control some of these muscles. We call that potty training and it took you three years to learn how to keep those close and basically, you can just override the signals to open and if you've ever had somebody tell you such a great joke or somebody who just jumps out and startles you, sometimes, people lose that control.
Moving on to skeletal muscle anatomy. Muscles are made up of multiple layers but I can think of them as bundles of bundles of bundles. A muscle is made up of these bundles called fascicle. Each fascicle is made up of muscle cells and for stupid historical reasons, muscle cells are called muscle fibres. Each muscle fibre is made up of bundles of protein called myofibrils. Each individual protein is called a myofilament. Myo in case you don't know is a root word that means muscle. Now the myofilaments are arranged in repeating patterns at least in skeleton and cardiac muscles called sarcomeres.
Now, I can just tell you all this but it might work better to show you. so let's take a quick look at this Youtube video. Now, I'll go ahead and make it larger. And now here we see somebody's mighty muscles. If we zoom in on this bicep muscle and slice it open and taking a cross section, we can zoom in on that and we can see these bundles called fascicles. If we extend one of these fascicles out, we can see that it is wrapped in a layer called the paramecium and here we've got one muscle cell. You can see the multiple nuclei that make it up. Sticking out of it is a myofibril. We zoom in on the myofibril we can see these red and blue lines arranged in this repeating patern. Those red and blue lines are the myofilaments that make up the myofibril. If we extend out some of these myofilaments we can see the red thick filaments and the blue thin filaments. The red filaments are made out of a protein called myoin, the blue ones are made oput of a protein called actin. And it's the myosin that has these little extensions that it's not appearing on this video called cross bridges that reach out and grab the actin that's around it. And they can pull when they do that. If you see this repeating pattern of thick filaments in a stack with blue thin filaments around it, that's called a sarcomere and it's their movement, that if we add it up leads to the contraction that we call muscle contraction.
Now, let's switch over to powerpoint and we'll take a quick look and I'll go through very quickly, the process of muscle contraction. So, this little red thing at the top that's got a little bumpy thing sticking down, that's one of those myosin cross bridges. The silvery balls here, those are the actin thin filaments. Now, there's these green things here that are controlling proteins. One's called troponin and the other one's called tripomyosin.
When a nerve cell gives a signal to a muscle, it causes that muscle cell to release an ion of calcium. Those calcium ions bind or stick to the green one. Now the actin is blocking the cross bridges from grabbing. But with calcium as we can see in the upper right hand corner, the calcium helps the actin move out of the way so that the tripomyosin that was covering up the actin binding cite can is now out of the way. That red myosin as we can see in the lower right, reaches down and grabs a binding cite on the actin. When it does that, the cross bridge actually had an ATP or energy molecule attached to it. The ATP falls off as the cross bridge, that's called the power stroke. And it grabs and pulls the actin and that is what causes the shortening of the sarcomere as the actin is pulled inwards sliding across the myosin.
Now, if there is an ATP available, the cross bridge will let go of the actin as it grabs it is grabbed by the ATP and reach back. It's kind of like resetting a mouse trap. The myosin cross bridge is like the part of the mouse trap that snaps. You have to put energy into it, to make it let go of the mouse. But as soon as the trigger is set, if the myosin and calcium and tripomyosin the long green skinny thing is still combined, then it'll grab again. However, if the calcium is removed back into a part of the muscle cell called the edoplasmic reticulum, then the troponin and tripomyosin cover up actin and the cross bridges can no longer grab. And that is muscle contraction.