Each muscle fiber is surrounded by its plasma membrane, or sarcolemma. Within this sarcolemma lie quiescent satellite cells that are essential for the repair of muscle after injury.
Following muscle injury, inflammatory substances are thought to stimulate these satellite cells to undergo proliferation and differentiation into new muscle fibers.
Fibers are further divided into smaller units called myofibers or myofibrils, which are made of sarcomeres arranged in series alignment. Sarcomeres are the basic unit of muscle contraction; they are made of myosin, actin, tropomyosin, and troponin proteins. Muscle fibers contain a number of other intracellular organelles that are important for normal function as well.
Each fiber has hundreds to thousands of nuclei that are found peripherally. Nuclei are the regulatory control centers of the cell, determining cellular material and distribution (e.g., synthesis).
The amount and type of proteins to be synthesized is determined by the nuclear DNA, and it is carried out by ribosomes in response to mRNA encoding. Because of their ability to quickly up- and down-regulate protein synthesis, muscles are fairly adaptable.
Depending on the type of muscle fiber, cellular mitochondria may account for nearly 20% of total cell volume, being most abundant in highly oxidative, slow-twitch fibers. Mitochondria are responsible for production of adenosine triphosphate (ATP) through oxidative metabolism.
This ATP is the crucial energy source for skeletal muscle force production as well as most other cellular tasks. Muscle fibers also contain various amounts of enzymes, lipids, and glycogen.
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