A&P CHAPTER 9

troponin

A protein monomer associated with the thin actin filaments involved in striated muscle contraction, a regulatory protein that is a component of the thin filament. When calcium ions (Ca2+) bind to troponin, it undergoes a change in shape; this conformational change moves tropomyosin away from myosin-binding sites on actin molecules, and muscle contraction subsequently begins as myosin binds to actin

elastic filament = titin = connectin

An enormous ELASTIC PROTEIN found in the sarcomere of striated muscle cells

thin filaments = actin fibers

The 7-9 nm diameter contractile PROTEINS attached to the Z discs of striated muscle sarcomeres, arranged with opposite polarity in each half sarcomere

myosin

The contractile protein monomer comprising myosin fibers which consists of a pair of intertwined strands forming the "tail" and the active "head" of each strand which is the specific site for both attachment of myosin to actin and for the repeated power strokes, due to its ATPase activity, which achieve the sliding of actin fibers over the myosin fibers during muscle contraction

thick filaments = myosin fibers

The larger diameter contractile proteins anchored to the M lines of striated muscle sarcomeres, arranged with opposite polarity in each half sarcomere; composed of the protein subunit myosin; these contractile proteins are found in the cytoskeleton of smooth muscle and some other tissues as well; their activity is regulated by Ca++ ions released from the sarcoplasmic reticulum.

myosin heads

The pair of active subregions of each myosin protein monomer which have the capacity to bind and to hydrolyze ATP, to form covalent cross-links with actin filaments, and to perform the power strokes which produce the sliding of filaments that achieves contraction of the sarcomere, the unit of contraction in the myofibrils (organelles) of straiated muscle tissues; their activity is regulated by Ca++ ions released from the sarcoplasmic reticulum.

myosin tails

The pair of subregions of each myosin protein monomer which align with other myosin monomer's tail regions to produce the huge polymer which is the myosin fiber or thick filament observed in the myofibrils of striated muscle tissues.

myosin-binding site

The specific location on each G-actin molecule where a myosin head can covalently bond to the actin filament during muscle contraction; at rest the myosin-binding site is covered by the tropomyosin molecule, but during contraction, the presence of calcium ions causes a conformational change in the tropomysoin molecule so that it shifts position and exposes the myosin-binding site.

actin

A contractile PROTEIN which is the main constituent of the thin filaments

tropomyosin

A fibrous protein monomer associated with actin filaments in the sarcomere of striated muscle cells and in the cytoskeleton of most cells, A HELICAL protein that WINDS around actin helices in skeletal and cardiac muscle cells to form the thin filament of the sarcomere. In the absence of Ca2+, tropomyosin covers the myosin-binding sites on actin and prevents muscle contraction. When calcium is present, a conformation change in tropomyosin occurs so that the myosin-binding sites are exposed and muscle contraction can occur.