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A myofibril (also known as a muscle fibril or sarcostyle) [1] is a basic rod-like organelle of a muscle cell. [2] Skeletal muscles are composed of long, tubular cells known as muscle fibers, and these cells contain many chains of myofibrils. [3]
The unusual microscopic anatomy of a muscle cell gave rise to its terminology. The cytoplasm in a muscle cell is termed the sarcoplasm; the smooth endoplasmic reticulum of a muscle cell is termed the sarcoplasmic reticulum; and the cell membrane in a muscle cell is termed the sarcolemma. [9]
The protein complex composed of actin and myosin, contractile proteins, is sometimes referred to as actomyosin.In striated skeletal and cardiac muscle, the actin and myosin filaments each have a specific and constant length in the order of a few micrometers, far less than the length of the elongated muscle cell (up to several centimeters in some skeletal muscle cells). [5]
There are more than 600 skeletal muscles in the human body, making up around 40% of body weight in healthy young adults. [18] [19] [20] In Western populations, men have on average around 61% more skeletal muscle than women. [21] Most muscles occur in bilaterally-placed pairs to serve both sides of the body.
Each muscle cell contains myofibrils composed of actin and myosin myofilaments repeated as a sarcomere. [3] Many nuclei are present in each muscle cell placed at regular intervals beneath the sarcolemma. Based on their contractile and metabolic phenotypes, skeletal muscle can be classified as slow-oxidative (Type I) or fast-oxidative (Type II). [1]
The body mass of an average adult man is made up of 42% of skeletal muscle, and an average adult woman is made up of 36%. [ 6 ] Cardiac muscle tissue is found only in the walls of the heart as myocardium , and it is an involuntary muscle controlled by the autonomic nervous system .
In cardiac and smooth muscle an electrical impulse (action potential) triggers calcium ions to enter the cell through an L-type calcium channel located in the cell membrane (smooth muscle) or T-tubule membrane (cardiac muscle). These calcium ions bind to and activate the RyR, producing a larger increase in intracellular calcium.
A myocyte is composed of multiple myofibrils, which contain the “contractile units” of the muscle known as a sarcomere. [3] These sarcomeres are arranged in adjacent formations along the myofibrils. Similarly to the plasma membrane of other cells, the sarcolemma protects and surrounds the myocytes.