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Question #1
1. Describe the sliding filament theory of muscle contraction and why it is called the
sliding filament theory?
 The sliding filament theory explains how the movement of thick and thin
filaments relative to each other leads to the contraction and relaxation of whole
muscles. The myosin heads on the thick filaments "hook" onto, and pull, the thin
filaments towards the center of each sarcomere. The appearance of this action is
transitioned from a "relaxed" to a "fully contracted" muscle. As the thin filaments
slide over the thick filaments, the I bands and H zones become narrower and
narrower until they disappear when the muscle reaches its fully contracted state.
 It is called the sliding filament theory because it involves the sliding of thick and
thin filaments.
2. Describe the action of the cross bridges that causes a power stroke.

Cross-bridge cycling forms the basis for movement and force production in
muscle cells. Each cycle of myosin binding to actin and movement of the thin
filament involves the hydrolysis of one ATP molecule. A myosin cross-bridge
attaches to actin. ATP binding causes the dissociation of myosin from actin. In the
absence of ATP (as occurs after death), myosin cannot dissociate from actin, and
the muscles become stiff. This is known as rigor mortis. The state where the lowenergy myosin head is bound to actin is known as the rigor configuration. ATP
hydrolysis causes a shape change so that the myosin head is cocked. The products
of ATP hydrolysis (ADP and inorganic phosphate) remain bound. Cocking of the
myosin head puts it in line with a new binding site on the actin filament. Myosin
binds to actin and the powerstroke occurs.
3. What is the role of calcium and ATP in muscle contraction and relaxation?
 Calcium ions released from the sarcoplasmic reticulum regulate when crossbridge cycling can occur. In resting muscle, the myosin binding sites on the actin
monomers are blocked by a filamentous protein called tropomyosin.
Tropomyosin is held in place by troponin, which is actually a complex of three
proteins, one of which binds Ca++. Troponin is part of the family of Ca++-binding
regulatory proteins that includes calmodulin.