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Skeletal muscles are one of the three types of vertebrate muscle tissue, the other two being cardiac and smooth muscle. Skeletal muscles are made up of muscle fibres, also known as muscle cells, which are long and cylindrical in shape. These muscle fibres are multinucleated contractile muscle fibres (myocytes) and are usually very large, measuring about 2-3 cm in length and 100 μm in diameter. The muscle fibres are composed of myofibrils, which are made up of actin and myosin filaments called myofilaments. The myofibrils are repeated in units called sarcomeres, which are the basic functional, contractile units of the muscle fibre necessary for muscle contraction.
| Characteristics | Values |
|---|---|
| Shape | Cylindrical |
| Diameter | 105 to 106 Ångstrom (1 Å = 10^-10 m) |
| Length | Up to 500 mm long |
| Cell Nuclei | Several hundred cell nuclei located at regular intervals near its surface |
| Composition | Sarcolemma, fibrils, sarcosomes, sarcoplasm, and sarcoplasmic reticulum |
| Connective Tissue | Endomysium, perimysium, and epimysium |
| Contraction | Contraction is initiated when the motor nerve fiber makes a junction with the sarcolemmal membrane |
| Types | Slow-twitch (type I) and fast-twitch (type IIa and IIb) |
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Skeletal muscle
Inside each skeletal muscle, muscle fibers are organized into fascicles. Skeletal muscle myocytes are usually very large, being about 2–3 cm long and 100 μm in diameter. By comparison, the mononuclear cells in muscles are much smaller. Some of the mononuclear cells in muscles are endothelial cells, macrophages, and neutrophils. In terms of nuclei present in skeletal muscle, myocyte nuclei may be only half of the nuclei present, while nuclei from resident and infiltrating mononuclear cells make up the other half. Having many nuclei allows for the production of the large amounts of proteins and enzymes needed for maintaining normal function in these large, protein-dense cells. In addition to nuclei, skeletal muscle fibers also contain cellular organelles found in other cells, such as mitochondria and endoplasmic reticulum. However, some of these structures are specialized in muscle fibers. The specialized smooth endoplasmic reticulum, called the sarcoplasmic reticulum (SR), stores, releases, and retrieves calcium ions (Ca++). The plasma membrane of muscle fibers is called the sarcolemma, and the cytoplasm is referred to as sarcoplasm.
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Myofibrils
The myofilaments are organised into repeated subunits along the length of the myofibril, with each subunit called a sarcomere. Sarcomeres are the basic functional contractile units of the muscle fibre, necessary for muscle contraction. They are approximately 1.6–2.0 μm in length and give the muscle its striped or striated appearance due to their alignment and repeating pattern of light and dark transverse bands.
The sarcomere structures give skeletal muscles their ability to contract. The shortening of individual sarcomeres leads to the contraction of the individual muscle fibres, resulting in muscle contractions. This contraction is powered by the oxidation of fats and carbohydrates, as well as anaerobic chemical reactions, particularly in fast-twitch fibres. These chemical reactions produce adenosine triphosphate (ATP) molecules, which are used to power the movement of the myosin heads. Under the influence of ATP, actin and myosin form a contractile compound, actomyosin, which is required for muscle contraction.
The number of myofibrils varies, ranging from 50 per myocyte in the muscles of a fetus to approximately 2000 per myocyte in the muscles of an untrained adult. They make up a significant proportion of the volume of a whole muscle, comprising approximately 80% of the volume of an individual muscle.
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Sarcomeres
Skeletal muscle is one of the three types of vertebrate muscle tissue, the others being cardiac and smooth muscle. Skeletal muscle cells are long and cylindrical and are commonly referred to as muscle fibres.
The interaction between actin and myosin filaments in the A-band of the sarcomere is responsible for muscle contraction. This is based on the sliding filament theory, which proposes that the active force is generated as actin filaments slide past the myosin filaments, resulting in the contraction of an individual sarcomere. The contraction of myosin's S1 region is called the power stroke, which requires the hydrolysis of ATP (adenosine triphosphate), which breaks a high-energy phosphate bond to release energy, resulting in force generation and shortening of an individual sarcomere.
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Connective tissue
Skeletal muscle cells, or muscle fibers, are long and cylindrical in shape. They are soft and fragile and are covered by connective tissue, which furnish support and protection to the cells. This connective tissue also allows the cells to withstand the forces of contraction and provides pathways for the passage of blood vessels and nerves.
Each skeletal muscle is an organ that consists of various integrated tissues, including skeletal muscle fibers, blood vessels, nerve fibers, and connective tissue. Each muscle is wrapped in a sheath of dense, irregular connective tissue called the epimysium, which is one of the three layers of connective tissue (along with the perimysium and endomysium) that enclose the muscle. The epimysium allows the muscle to contract and move powerfully while maintaining its structural integrity. It also separates the muscle from other tissues and organs in the area, allowing the muscle to move independently.
The perimysium is a middle layer of connective tissue that surrounds the bundles of muscle fibers, called fascicles. The endomysium is a thin layer of connective tissue that encases each individual muscle fiber. It is composed of collagen and reticular fibers and surrounds the extracellular matrix of the cells. The endomysium plays a critical role in transferring force produced by the muscle fibers to the tendons.
The three layers of connective tissue (epimysium, perimysium, and endomysium) are collectively called mysia. Mysia may also fuse with a broad, tendon-like sheet called an aponeurosis, or with fascia, the connective tissue between skin and bones. The connective tissue layers work together to transmit the tension created by the contraction of the muscle fibers to the tendon and then to the periosteum, pulling on the bone to create movement of the skeleton.
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Muscle contraction
Skeletal muscle, commonly referred to as muscle, is a type of vertebrate muscle tissue. It is part of the voluntary muscular system and is attached by tendons to bones of a skeleton. Skeletal muscle cells are long and cylindrical and are known as muscle fibres.
The physiological concept of muscle contraction is based on excitation-contraction coupling. This complex process begins when an action potential causes depolarization in the myocyte membrane. This causes a conformational change in the dihydropyridine receptors, which opens the nearby ryanodine receptors on the sarcoplasmic reticulum (SR). The SR is the storage site for calcium within muscle cells. When calcium is released from the SR, it binds to troponin C, which initiates muscle contraction.
The actin and myosin filaments within each skeletal muscle fibre slide past each other to produce a contraction, which is explained by the sliding filament theory. This can be described as a twitch, summation, or tetanus, depending on the frequency of action potentials.
Connective Tissue and Muscle Contraction
A muscle contains a non-contractile part of dense fibrous connective tissue that makes up the tendon at each end. The tendons attach the muscles to bones to give skeletal movement. The connective tissue covering furnishes support and protection for the delicate cells and allows them to withstand the forces of contraction.
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