Muscle Fiber Composition

A muscle fiber is a muscle cell!

So everything inside of a muscle fiber was stuff that you learned about back in chapter 3. Now all you have to do is to try to understand the different arrangement of these things that exists in muscle fibers. You are also going to have to learn many new terms, since the odd qualities of the organelles in muscle fibers has lead to them getting new names.

First of all, a muscle fiber is huge! It is much larger (mainly longer) than other cells that you have learned about. It is also multinucleated, meaning that it has more than one nucleus. This is also different from other cells that you have thought about. However, since cells have to be able to supply themselves with proteins, all of which are planned for in the genetic code which lies within the nucleus, and muscle fibers are huge, you can imagine that these cells need more nuclei to be able to make all the proteins they need. A muscle fiber nucleus is a myonucleus. It sits in the muscle fiber cytoplasm, which is called sarcoplasm. To see a photograph of skeletal muscle tissue, go to the muscle section of the JayDoc HistoWeb, and check out slide #1 in the "expanded view."

The cytoskeleton within the muscle fiber has to allow the muscle fiber to contract (in other words, go from a long condition to a short one), and then to relax again. In order to accomplish this, the cytoskeleton is organized along the long axis of the muscle fiber. That way, when the contractile apparatus contracts, so does the muscle fiber. The contractile apparatus is thus arranged in long packets within the muscle fiber, called myofibrils. Since the muscle fiber has to contract as its main function, it has to have many myofibrils. It gets so packed with myofibrils that everything else has to squeeze in around them.

Mitochondria are jammed into muscle fibers in high numbers! Remember, mitochondria produce ATP whenever cells have oxygen gas available to them. And ATP is cellular energy, which is necessary for movement to occur.

A couple of other surprising items are found in muscle fibers. The cell membrane, which is specifically called the sarcolemma in muscle cells, is not merely like a sleeve over the muscle fiber. Instead, it dives into the cell, forming tubes of cell membrane within the muscle fiber. This can be seen clearly in Figure 9.6 on page 285. These tubes of membrane are called t-tubules. Why do muscle fibers need t-tubules? Because muscle fibers carry their electrical signals for contraction on their sarcolemma. Any time that a muscle fiber has to contract, an electrical signal must run along its sarcolemma. That works fine, but because the muscle fibers have such a large diameter, the electrical signal could never reach the middle of the muscle fiber quickly if it weren't specifically carried into the cell interior. The t-tubules are little channels to draw the electrical activity into the depths of the muscle fiber.

Meanwhile, the smooth endoplasmic reticulum (sER) has a special function in muscle fibers, so there's lot of it. It also gets a special name in muscle fibers, so rather than calling it the sER, it is called the sarcoplasmic reticulum (SR). The SR is simply many packets of membrane, like a regular sER, and it functions to hold calcium ions within it. You see, in order to a muscle fiber to contract, it needs the electrical signal, but then it also has to do something with that signal. Right? I mean, if you were handed a note at work that said, "your boss is on the phone and wants to talk with you," you would need to respond to that note (the signal)... if you didn't, you would risk getting fired!

Muscle fibers respond to the electrical signal (called an action potential) by changing the concentration of calcium ions within them. The presence of calcium ions is what the myofibrils need to contract. Normally, the sarcoplasm lacks calcium ions. That is because the SR hoardes the calcium ions within it. But, in response to an action potential the SR releases its calcium from storage. Shortly after, it sucks it all back up again. You'll be learning more about this soon, but until then, I wanted you to know something about why there is so much SR!

Now that you know why there's a lot of SR and what t-tubules are, you should understand that they have to be intimately associated with one another. When t-tubules are carrying the action potential, they have to signal the SR immediately to release calcium ions. Therefore, they should run side-by-side. This is indeed the case. The SR makes a sandwich around the t-tubule. This sandwich is called a triad, because when it is cut, you can see three regions (1. SR, 2. t-tubule, 3. SR). Your textbook does not define a triad, but the A.D.A.M. CD does.

That is all the special stuff on muscle fibers. The next thing is to understand the myofibrils within the muscle fiber. For that, go on to the myofibril composition page.