Skeletal muscle contraction Contraction force and tension Sliding

Skeletal muscle contraction • • Contraction, force and tension Sliding Filament Theory of Contraction cycle Regulation of the contraction cycle

Muscle contraction • Movement or resist a load (force) • Load is the weight or force that opposes the contraction of a muscle • Tension is the force created by a muscle • Need ATP to generate tension

Observations during muscle contraction Muscle shortens when it moves a load. (When muscle contracts, it does not always shorten)

Observations during muscle contraction: A band does not shorten during contraction.

Sliding filament theory of contraction: movement and force Resting length How about force without movement?

During contraction • • • Z discs move closer together Sarcomere shortens A band same length I band reduced H band reduced

What pushes the actin filaments into the myosin? • Cross-bridges link myosin to actin • Power stroke: myosin head binds to actin myosin head release actin. Repeated many times. • Myosin molecules are flexible • ATP causes movement of myosin molecules

Myosin • A motor protein • Converts chemical bond energy of ATP to mechanical energy of motion • Each myosin as ATPase • Energy from ATP hydrolysis is stored as potential energy in the myosin molecule, and is used to create the power stroke.

Why don’t actin and myosin continuously bind together? • ATP is usually available • Actin’s binding site for myosin is revealed only during cross-bridge (binding). • During relaxation, actin’s binding site for myosin is concealed