The Muscle Fiber

The Muscle Fiber

We don’t call muscle cellsThe basic structural and functional units of life. muscle cells we call them muscle fibers. And this is explained in the next slide but just trust me here. Thousands of individual muscle fibers are embedded within one of your skeletal muscles. Let’s say, for example, in the biceps. Within each muscle fiber, there are tons of proteinsLarge molecules made of amino acids with various functions in the body. called myofibrils.

About 80% of a muscle fiber is the cylinder shaped proteins myofibrils. The remaining 20% consists of a small number of organellestructures within a cell that perform specialized functions.. Most of these organelles are designed to contribute to making a contraction.  Start there are tons of mitochondria because all we’re doing in muscle fibers are using ATPThe energy currency of cells used for muscle contraction. to make contractions. In factA statement based on direct observation that is repeatedly confirmed. you never stop making contractions. In the background all of your muscles are maintaining a low level of contraction.

If you haven’t watched the muscle wrappings mini lecture I suggest you do that now. Every muscle fiber is attached to the endomysium. The endomysium is attached to the paramecium surrounding the muscle faciles. This is then attached to the epimysium surrounding the entire muscle. The epimysium fades into the tendons or aponeuroses. These structures connect the muscle to a bone in the skeleton. If we were able to shorten the myofibrils, we could shorten the muscle fibers. The entire skeletal muscle would also shorten, bringing one bone closer to another in a contraction.

Why so many nuclei?

When your muscles are forming in utero there are stem cells generating two different types of cells. You can see those two populations here one in a Peach type color and one in yellow. The peachy cells here are myoblasts. These are precursor muscle cells. What happens is that the myoblasts fuse together to create one muscle fiber. This accounts for the many nucleiClusters of neurons in the CNS responsible for processing information. in one muscle fiber.

And it also accounts for the fact of why we call it a fibre and not a cell. Muscle fibers are usually incapable of doing mitosis unless there’s a mutationA change in DNA sequence that can affect gene function. that causes that to happen. However you can change the size of your muscle fibers and increase the myofibril proteins in the muscle fiber. If you damage your muscle, you may need a new muscle fiber. Mitosis occurs by copying another population of cells called myo satellite cells. These myo satellite cells will tap into the instructions of the nucleusThe control center of the cell that contains DNA and directs cellular activities. to do mitosis. Muscle fibers themselves will not tap into the nucleus to do mitosis.

Myofibrils

MyofibrilsCylindrical structures within muscle fibers that contain myofilaments. are the most abundant proteins contained in a muscle fiber. As we progress through these mini lectures, you will start to confuse the cylinder shaped muscle fibers. You will also confuse them with the cylinder shaped myofibrils. Before you even look at a cylinder shaped structure, ask yourself a question. Am I looking at a muscle fiber? Or am I looking at a myofibril?

There is another mini lecture that more in detail explains the structure of a myofibril. But you can see here on this myofibril that there are repeating stripes of dark and light bands. This is what creates the striations that we can see in a microscopic picture of muscle. 

The Sarcolemma

We give a muscle cell a specific name: a muscle fiber. Therefore, we will also give the cell membrane a specific name: sarco lemma. As we learn from learning about osteonsThe structural unit of compact bone, consisting of concentric lamellae around a central canal. the suffix lemma means sheet. For muscle we can use prefixes myo, myself, and sarco. The circle lemma is not just a sheet that covers a muscle fiber. There are tubes piercing through the sheet running through the muscle fiber and connecting parts of the circle lemma.

Basically these tubules which are called T tubules or transverse tubules they increase the surface area of the sarcolemma. As we know structures displayed longitudinally can look extraordinarily different from structures when they are cut in a transverse section A cut or slice of the body or an organ for study.. If you look at the very right edge of this muscle fiber, you will see a web-like or honeycomb-like structure. It wraps around each individual myofibril contained in this one muscle fiber. The need for this increased surface area arises because muscle fibers do not act on their own. They are controlled by action potentials or thoughts from your brain. So if an action potentialA rapid, temporary electrical charge that travels along neurons, allowing signal transmission. leaves your brains travels down your spinal cordThe central nervous system structure that relays signals between the brain and body. comes out your hand into muscle on your index finger when that action potential or electricity is moved from the nerve to the muscle fiber we want the action potential to quickly spread through your muscle fiber to contribute a coordinated contraction

The Sarcoplasmic Reticulum

The other significant organelle besides mitochondria and nuclei that’s present in a muscle fiber is something called the sarcoplasmic reticulum.  The sarcoplasmic reticulum is basically just a modified endoplasmic reticulum. Recall from previous chapter that we talked about the rough endoplasmic reticulumA membrane network covered in ribosomes that produces and processes proteins. and the smooth endoplasmic reticulumA membrane network involved in lipid synthesis, detoxification, and calcium storage.. Here the sarcoplasmic reticulum has a specific function in storing and releasing calcium.

This was something that neither the rough nor the smooth ER did does in any other cells. The sarcoplasmic reticulum wraps around each myofibril so it wraps around each individual protein within a muscle fiber. The sarcoplasmic reticulum actually comes into contact with all of those T tubules. These T tubules pierce through the sarcolemma and the muscle fiber. In this picture on the left the T tubules are these two thin stripes right here.

The portion of the sarcoplasmic reticulum that comes up against these T tubules are called terminal cisternae. The word terminal just means end and the word sister name means sync or basin. When the action potential comes through the T tubule as a wave, it triggers the terminal cisternae. This results in the release of calcium into the sarcoplasm of the muscle fiber. I’m sure that you could infer that sarcoplasm is just a fancy word for cytoplasmThe gel-like substance within a cell that contains organelles and cytosol. of a muscle fiber.