What are the major parts of a skeletal muscle fiber, and what is the function of each part?
Muscle cells are also called as myocytes and are present in muscle tissue. They are rich in the proteins actin and myosin and have ability to contract and relax providing movements.
Skeletal muscle cells (fibers) are very different from typical cells.
They develop through the fusion of mesodermal cells (myoblasts) until they become very large and contain hundreds of nuclei.
The cell membrane of a muscle cell is called the sarcolemma, which surrounds the sarcoplasm or cytoplasm of the muscle fiber.
Because the whole muscle fiber must contract at the same time, the signal (action potential) is conducted through the cell by transverse tubules (T tubules) which have the same properties as
Within each muscle fiber are hundreds of lengthwise subdivisions called myofibrils.
The 2 types of myofilaments are:
thin filaments: made of the protein actin, and
thick filaments: made of the protein myosin.
Sarcoplasmic Reticulum: Surrounding each myofibril is a membranous structure called the sarcoplasmic
reticulum, which is involved in transmitting the action potential to the myofibril.
Ion pumps concentrate calcium ions (Ca++) in the cisternae. The calcium ions are released into the contractile units of the muscle (sarcomeres) at the beginning of a muscle contraction.
Two transverse tubules encircle each sarcomere near the 2 zones of overlap. When calcium ions are released by the sarcoplasmic reticulum, thin and thick filaments interact.
The complex interactions of thick and thin filaments which cause muscle contraction are determined by the structures of their protein molecules.
Thick Filaments contain twisted myosin subunits. The tail binds to other myosin molecules. The free head, made of 2 globular protein subunits, reaches out to the nearest thin filament.
During a contraction, myosin heads interact with actin filaments to form cross-bridges. The myosin head pivots, producing motion.
Thick filaments contain titin strands that recoil after stretching.