Myofibril Definition

Myofibrils are components of animal skeletal muscles. Myofibrils are long filaments that run parallel to each other to form muscle fibers (myo). It is possible for myofibrils and resulting myofibers to be several centimeters long. Single multinucleated cells make up muscle fibers. Sarcomeres, which are repeating subunits of myofibrils, are the building blocks of myofibrils. Sarcomeres are responsible for muscle contractions.

Myofibril Structure

Two types of filaments make up myofibrils: thin filaments and thick filaments. A thin filament consists of strands of actin and a regulatory protein coiled together, whereas a thick filament consists of myosin strands. A sarcomere is a functional unit made up of thin and thick filaments that form partially overlapping layers. 

A myofibril appears to have dark and light bands as a result of how the myofilaments are arranged, giving muscles a striated appearance. There are some thin filaments and some thick filaments in the dark bands, known as A bands. During the energy metabolism process, enzymes are located in the M-line and in the H-zone, which are only thick filaments. 

In between the A bands lie the light bands, known as the I bands, which contain thin filaments only. Sarcomere subunits are divided by the I bands, centered on a region called the Z line, which anchors thin actin filaments and serves as a boundary between the subunits.

Myofibril Function

In muscles, myofibrils are made up of sarcomeres, which function as functional units. Muscle contraction is performed by the myofibril via the sliding-filament model. During rest, there is incomplete overlap between thin and thick filaments, with some areas containing only one type. 

Sarcomeres shorten when muscles contract because thick and thin filaments slide over each other, causing greater overlap between the filaments and shortening of the H-zone and I band. Sarcomere length decreases during muscle contraction, but myofilaments remain the same length.

Inorganic phosphate and ATP are hydrolyzed by myofilament proteins to power myofilament movement. When the ATP molecule is hydrolyzed, the myosin head on the thick filament changes conformation and forms a cross-bridge with the thin filament. 

In response to the release of the ADP and phosphate molecules, the myosin head again changes conformation and pushes the thin filament towards the center of the sarcomere. 

In response to a new ATP molecule binding to the myosin head, the head returns to its original conformation and releases the thin filament. Myosin heads change conformation, and the thin filament is pulled towards the center of the sarcomere as the new ATP molecule is hydrolyzed into ADP and inorganic phosphate. About five times per second, a thick filament can form cross-bridges with a thin filament by containing several hundred myosin heads. Muscle contractions are caused by continuous contractions of myofibrils.

ATP is responsible for powering muscle contractions. Since muscle fibers only contain a very small amount of ATP, energy must be derived from creatine phosphate and glycogen stored in the muscles. For short-term bursts of energy, muscle fibers store ATP and creatine phosphate can form ATP to produce ATP. 

Muscle fibers and creatine phosphate can produce energy for up to 15 seconds. As glycogen breaks down into glucose, which is converted to ATP through glycolysis and aerobic respiration, glycogen can serve as a long-term energy source.

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