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StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-.
Matthew C. Gash ; Patricia F. Kandle ; Ian V. Murray ; Matthew Varacallo .
Last Update: April 1, 2023 .
The physiological concept of muscle contraction is based on two variables: length and tension. In physiology, muscle shortening and muscle contraction are not synonymous. Tension within the muscle can be produced without changes in the length of the muscle, as when holding a dumbbell in the same position or holding a sleeping child in your arms. Upon termination of muscle contraction, muscle relaxation occurs, which is the return of muscle fibers to a low-tension state.
Mammals have three types of muscles: skeletal, cardiac, and smooth. Skeletal muscles are attached to bones and give the body structure and strength. Cardiac muscle comprises the walls of the heart, allowing blood to be pumped through the vasculature. Smooth muscle is found throughout the blood vessels, gastrointestinal (GI) tract, bronchioles, uterus, and bladder.[1]
Muscle contraction throughout the human body can be broken down based on muscle subtype specialization.[2] In general, muscle fibers are classified into two large categories: striated muscle fibers and smooth muscle fibers.
Striated muscle fibers contain actin and myosin filaments that power contraction and are organized into repeating arrays, called sarcomeres, with a striated microscopic appearance.[1] Cardiac muscle tissue is a striated muscle fiber under involuntary control by the body's autonomic nervous system (ANS).[3] Skeletal muscle tissue is a striated muscle fiber under voluntary control.
Smooth muscle fibers do not contain sarcomeres but use actin and myosin contraction to constrict blood vessels and move the contents of hollow organs in the body. These fibers are under involuntary control by reflexes and the body's ANS.[4]
Striated Muscle
To understand the mechanism of striated muscle contraction, it is first essential to understand its structure. The striated muscles in our body comprise many individual muscle fibers. Inside these muscle fibers are smaller units called myofibrils made of parallel thin and thick filaments. These filaments are arranged longitudinally in small units known as sarcomeres, which give the muscle a striated appearance under microscopy.[5]
The thick filaments are made from the protein myosin, which has one pair of heavy chains and two pairs of light chains; these heavy and light chains differ from the thin and thick filaments of myofibrils. At the tail of the thick filament, the two heavy chains are intertwined in a helical formation. At the other end of the thick filament, each heavy chain is paired with two light chains, giving rise to two heads. The myosin heads have an actin-binding site that helps them attach to the thin filaments.[6]
The thin filaments are composed of actin, tropomyosin, and troponin. Actin is a globular protein that combines with other actin globules to form two intertwined strands with positive and negative ends. The double-stranded actin filaments are covered by tropomyosin, which blocks the interaction between myosin and actin when the muscle is inactive. The troponin group comprises troponins I, T, and C and is located along the actin filaments next to tropomyosin.[7]
The complex process leading to muscle contraction, called excitation-contraction coupling, begins when an action potential causes depolarization in the myocyte membrane. The depolarization is spread via the transverse (T) tubules, invaginations of the muscle cell membrane, that help spread depolarization signals to the entire muscle fiber. Depolarization of the T tubules causes a conformational change in the dihydropyridine receptors, which causes the opening of nearby ryanodine receptors on the sarcoplasmic reticulum (SR), the storage site for calcium within muscle cells. When calcium is released from the SR, it binds to troponin C. This causes a conformation change, which shifts tropomyosin, allowing the myosin heads to attach to the actin filaments, creating what is known as a cross-bridge.
Cross-bridge cycling begins when ATP binds to an ATP-binding domain on the myosin head. Myosin dissociates from the actin, breaking the cross-bridge. ATP is then hydrolyzed into ADP and P, which causes the myosin heads to change conformation and move toward the positive end of the actin, cocking the myosin head. The phosphate is released, and the ADP-bound myosin binds to a new location on the actin filament. ADP is then released, which causes the myosin to return to its original position, pulling on the actin filament and causing the sarcomere (and, therefore, the muscle fiber) to contract. These cycles continue until calcium levels in the myocyte fall, causing tropomyosin to cover the actin filaments' myosin-binding sites.[8]
Smooth Muscle
The same thin and thick filaments discussed in striated muscles are present in smooth muscles. However, in smooth muscle tissue, these filaments are not organized into sarcomeres. As a result, smooth muscle does not contain the troponin complex required for skeletal muscle contraction and, thus, has a different mechanism for controlling contraction. This difference is characterized by how calcium (Ca) enters the cell, with three mechanisms increasing intracellular concentration:
Voltage-gated Ca channels are activated by membrane depolarization, allowing Ca to enter the cell. Hormones or neurotransmitters can open ligand-gated channels on the cell membrane.Hormones and neurotransmitters such as norepinephrine and angiotensin II can, via the phospholipase-C (PLC) pathway, cause an increase in intracellular inositol triphosphate (IP3).
IP3 can bind to receptors on the SR and cause Ca to be released. Once Ca is released, it binds to a calmodulin protein instead of troponin C, as it does in striated muscle. Calmodulin then activates myosin light chain kinase (MLCK), which, as the name suggests, phosphorylates the myosin light chain. The phosphorylated myosin light chain has ATPase activity, which hydrolyzes ATP, increasing its affinity to actin. The myosin can then readily bind to actin. From this point, cross-bridge cycling is the same as in striated muscle. The smooth muscle will remain contracted as long as Ca is bound to calmodulin and the MLCK is phosphorylated. This allows for prolonged periods of vasoconstriction in blood vessels.[4]
A step-by-step outline of each type of muscle contraction is discussed in a later section.
Skeletal, cardiac, and smooth muscles are derived from the mesoderm. More specifically, skeletal muscle is derived from the paraxial mesoderm, cardiac muscle is derived from the lateral splanchnic mesoderm, and smooth muscle fibers differentiate from the splanchnic mesoderm.
Smooth muscle exceptions not derived from the mesoderm include the muscles of the iris (sphincter and dilator pupillae) and the myoepithelial cells in mammary and sweat glands, which are derived from mesenchymal cells originating from the ectoderm.
Organ Systems With Striated Muscle
