Muscle Fiber Structure
including Myofiber and Myofibrils
Muscles are made of many individual muscle fibers. Muscle fibers are long, cylindrical cells giving skeletal muscles their striped or striated appearance.
Muscle fibers are the primary component responsible for muscle contraction. There are two basic types of muscle fibers. Slow-twitch fibers, also called ST or Type I fibers. ST fibers have a slow contraction time, but a high resistance to fatigue. These muscle fibers use aerobic respiration for energy. This provides for their high level of endurance. Slow-twitch muscles are used for aerobic activities requiring a low exertion level over a long period of time. You are using slow-twitch muscles for activities such as sitting or walking.
Fast-twitch fibers, also called FT or Type II fibers, have a quick contraction time, but are quick to fatigue. There are three different types of fast-twitch muscles. Type IIa muscle fiber has a moderately fast contraction time, and a relative long resistance to fatigue. Type IIx muscle fiber has a fast contraction time, and a moderate resistance to fatigue. Lastly, the type IIb muscle fibers have a very fast contraction time, but tire very quickly. Fast-twitch muscle fibers use anaerobic respiration for energy. Fast-twitch muscles are used for anaerobic activities requiring a high-force, such as sprinting or jumping.
Each muscle fiber is a single cell. Each cell consists of a structure including the sarcolemma, nuclei, sarcoplasm, motor nerve endings, myofibrils, sarcomeres, mitochondria, lysosomes, and golgi complex.
The sarcolemma is the membrane surrounding the muscle fiber. The nuclei is the brain of the cell, controlling its function. The sarcoplasm is the cytoplasm or fluid filling the cell. The myofibrils contain the proteins that produce the contraction force. These myofibrils are packed with proteins and energy sources to support muscle contraction. The motor nerve endings, under control of the brain and nervous system, excite the muscle and cause it to contract. For effective muscle contraction, the fiber must be excited along the whole length simultaneously. Sarcomeres are the basic contractile unit of the muscle. Mitochondria provides chemical energy to the cell, supporting all cellular activity. Lysosomes contain enzymes capable of digesting the cell contents, such as foreign bacteria or dying cells. The golgi complex transfers proteins, sugars, and enzymes into and out of the cell. Together these components form the muscle fiber cell, and provide for muscle contraction.