actin

(noun)

A globular structural protein that polymerizes in a helical fashion to form an actin filament (or microfilament).

Related Terms

  • myosin
  • sarcomere

Examples of actin in the following topics:

  • Mechanism and Contraction Events of Cardiac Muscle Fibers

    • Calcium in the cytoplasm then binds to cardiac troponin-C, which moves the troponin complex away from the actin binding site.
    • This removal of the troponin complex frees the actin to be bound by myosin and initiates contraction.
    • The myosin head binds to ATP and pulls the actin filaments toward the center of the sarcomere, contracting the muscle.
    • Intracellular calcium is then removed by the sarcoplasmic reticulum, dropping intracellular calcium concentration, returning the troponin complex to its inhibiting position on the active site of actin, and effectively ending contraction as the actin filaments return to their initial position, relaxing the muscle.
    • This animation shows myosin filaments (red) sliding along the actin filaments (pink) to contract a muscle cell.
  • Rigor Mortis

    • Physiologically, rigor mortis is caused a release of calcium facilitating crossbridges in the sarcomeres; the coupling between myosin and actin cannot be broken, creating a constant state of muscle contraction until enzymatic decomposition eventually removes the crossbridges.
    • Diffusion of the calcium through the pumps occurs, facilitation binding of myosin and actin filaments.
    • Unlike muscular contractions during life, the body after death is unable to complete the cycle and release the coupling between the myosin and actin, creating a state of muscular contraction until the breakdown of muscle tissue by enzymes (endogenous or bacterial) during decomposition .
    • Diagram showing Actin-Myosin filaments in Smooth muscle.
    • The actin fibers attach to the cell wall and to dense bodies in the cytoplasm.
  • Adherens Junctions

    • The actin microfilaments of the cytoskeleton (internal skeleton of the cell). 
    • They link the actin microfilaments to the cadherins.
    • The latter two proteins are what link the cadherin-catenin complex to the cell’s internal skeletal framework (the actin microfilaments).
    • Actin filaments are associated with adherens junctions in addition to several other actin-binding proteins.
  • Microscopic Anatomy

    • The two most important proteins within sarcomeres are myosin, which forms a thick, flexible filament, and actin, which forms the thin, more rigid filament.
    • Myosin has a long, fibrous tail and a globular head, which binds to actin.
    • Actin molecules are bound to the Z-disc, which forms the borders of the sarcomere.
    • Together, myosin and actin form myofibrils, the repeating molecular structure of sarcomeres.
    • When ATP binds to myosin, it seperates from the actin of the myofibril, which causes a contraction.
  • Force of Muscle Contraction

    • When a sarcomere contracts, myosin heads attach to actin to form cross-bridges.
    • Then, the thin filaments slide over the thick filaments as the heads pull the actin.
    • Simply put,  the tension generated in skeletal muscle is a function of the magnitude of overlap between actin and myosin myofilaments.
    • The force generated by a muscle depends on the number of actin and myosin cross-bridges formed; a larger number of cross-bridges results in a larger amount of force.
  • Myocardial Thickness and Function

    • Two of the important proteins found in sarcomeres are myosin, which forms the thick filament, and actin, which forms the thin filament.
    • Myosin has a long, fibrous tail and a globular head, which binds to actin.
    • Together, myosin and actin form myofibril filaments, which are the elongated, contractile threads found in muscle tissue.
  • Muscular Dystrophy

    • Specifically the proteins dystrophin and the dystrophin protein complexes are altered and nonfunctional, unable to join with actin filaments .
  • Action of Thyroid Hormones

    • Thyroid hormones play a particularly crucial role in brain maturation during fetal development by regulating actin polymerisation during neuronal development.
  • Velocity and Duration of Muscle Contraction

    • The force generated by a muscle depends on the number of actin and myosin cross-bridges formed; a larger number of cross-bridges results in a larger amount of force.
  • Muscle Development

    • Serum response factor (SRF) plays a central role during myogenesis, being required for the expression of striated alpha-actin genes.
    • Expression of skeletal alpha-actin is also regulated by the Androgen Receptor.
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