Structure of a Muscle Fiber
- Muscle fibers are giant, multinucleate cells that make up human muscle tissue. Each fiber contains multiple nuclei for burning energy, chemical signaling channels, and cylinders of muscle tissue and protein called myofibrils. These myofibrils are made up of tiny pockets called sacromeres, and these use energy created by the cell to contract and carry out muscular work.
- Myofibrils run the length of the muscle fiber, and serve as the protein-containing building blocks of muscle fibers. A muscle is made up of thousands of little pockets of muscle fiber, each ranging from a few millimeters to a few centimeters long. Webs of protein bind all of these little lengths of muscle fiber to one another.
- A muscle fiber is surrounded by a specialized cell membrane that runs the entire length of a fiber known as a sarcolemma. Unlike normal cell membranes, a sarcolemma has extensions that connect to the sacromeres. This connection links all of the sacromeres in a muscle fiber together, allowing a muscle to tell each sacromere to contract simultaneously. A structure connected to the sarcolemma called the sarcoplasmic riticuluum releases stores of calcium ions to signal contraction.
- Human muscle fibers come in two main varieties and each one is used for a different type of motion. Type I fibers are known as slow-twitch because they contract much more slowly than other fibers. Slow-twitch fibers have much more endurance, and because of this they sustain low intensity muscle movements like keeping your spine erect throughout everyday activities.
Type II fibers, also called fast-twitch fibers, are those that power more intense muscular activity. Fast-twitch fibers are used for high-intensity activities like sprinting or dead-lifts; they are much thicker in diameter than slow-twitch fibers, and also fatigue much more rapidly. - Muscle fibers are grouped together in bundles known as fascicles, and these bundles are held together by a connective tissue called perimysium. A muscle like the biceps might be made up of a couple dozen or so fascicles, all held inside of a wrapping of tough tissue, the fascia. Inside of the fascicle, every muscle fiber is held together by a connective tissue known as the endomysium.