Contract (Muscle sarcomere)
18" h x 47" w
Contract - a joining together. Muscle contraction depends on the sliding together of two types of protein filaments - myosin (purple) and actin (green), the principal components of the sarcomere - the unit of muscle shown in this piece. The process requires energy as myosin pulls on and moves the actin filaments to cause the shortening process.
"Contract." The word has so many meanings, from the gravitational collapse of matter to form stars, to the formal bond between two or more parties - the Ten Commandments, the marriage certificate, the informed consent form. These too require energy.
The sliding filament model, proposed in 1969 by Hugh Huxley, is the basis of all muscle contraction. "Thick" filaments (purple) are composed of a line of myosin molecules with "arms." These filaments line up facing in opposite directions on either side of the sarcomere. Myosin filaments are anchored at the ends by the large protein, "titin" (clear) and in the center by the "M" band made of myomesin (light amber) and M protein (red.) Thin filaments (green) are composed of two chains of actin molecules twisted into a double helix. Thick and thin filaments are interspersed in a regular array. The ends of thin filaments (actin) are anchored by Z line protein (red, at the two ends of the sarcomere). At rest, tropomyosin (aqua blue) chains block the interaction of myosin and actin. A nerve impulse triggers a muscle to contract. First, the impulses trigger release of calcium ions into the muscle cells. These calcium ions interact with the protein, troponin (dark blue) which is attached to the tropomyosin chains, resulting in a displacement of tropomyosin chains. This displacement movement allows myosin arms access to contact the actin filaments, and using the energy derived from ATP, to pull on the actin filaments. The result is the sliding of actin and myosin filaments with respect to each other, and contraction of the muscle.
In this work, most myosin molecules are shown interacting with the actin thin filaments. Those whose interaction with actin is blocked by the "aqua blue" chain of tropomyosin are shown in a "relaxed" state, not contributing to the contraction process.