Muscles are made up of muscle cells, called muscle fibers. However, these muscle fibers are long, tube-like cells that run long lengths of the muscle. Also, muscle fibers work electrically, which means that each one has to be somehow insulated from others; if they were not insulated, any time one muscle fiber were activated, others would also get activated... and you would have no control over your muscles.
Therefore, muscles have very nice insulation schemes. They get insulated by connective tissue wrappings. Even the entire muscle needs its own insulation, so that individual muscles can contract in isolation (for example, your biceps can contract without also activating your triceps).
The connective tissue wrapping around the entire muscle is called the epimysium. Remember, "epi-" means upon. And "mys" stands for muscle. So this is the outermost wrapping of connective tissue, since it is upon the muscle. To see this, and the items that follow, open up your book to Figure 9.2 while you read this.
If you looked inside of the muscle, you would certainly see muscle fibers. But these fibers are bundled together into large tube-like packages of fibers, called muscle fascicles. You see, the connective tissue wrapping not only insulates the muscle and its fibers, but it also is well vascularized and helps to bring blood vessels to all the muscle fibers of the muscle. By getting wrapped up into fascicles, more blood vessels are able to access the fibers inside. And muscle fibers need a good blood supply since they have to get lots of oxygen to make energy (ATP).
OK. So now you know that a muscle is wrapped by epimysium. And within the muscle, there are bundles of muscle fibers called muscle fascicles. You even know that these fascicles are bundled up by connective tissue. There's a name for the connective tissue around the fascicles-- it's called perimysium. Remember what "peri-" means? It means "around." So the perimysium runs around the muscle fascicles, bundling them up.
Within the muscle fascicles we find the muscle fibers. Each muscle fiber has its own connective tissue wrapping, insulating it from the other fibers. This individual fiber wrapping is called the endomysium, since it is the wrapping "within" ("endo-") the muscle. I have depicted this, as much as I could, in the schematic to the right here.
Conceptually, this is where most students think that this notion of a tube inside a tube should end. Yet there is one more level. You see, although the smallest living unit of muscle is the muscle fiber, it does contain within it more tube-like structures. The structures within a cell must be made out of something that you already learned about in Chapter 3 on cells-- and it is!
Muscle fibers need to be experts at movement, since it is there contraction that causes all the movements we make. Therefore, they need to have a highly organized cytoskeleton, perfectly adapted for contraction. They have this, and the organized units of the cytoskeleton are shaped like long tubes and are called myofibrils. Keep in mind that myofibrils are not cells. They are simply organized units of cytoskeleton within the muscle fiber. What does that mean? Cytoskeleton is made out of protein, so a myofibril is a tube-like configuration of this protein. This is described in more detail in the myofibril composition web page in this unit. However, first, you may want to go on to the page that describes muscle fibers in more detail.
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