INTRODUCTION:

Skeletal muscle attaches to the skeleton in our bodies. The contraction of skeletal muscles allow our bones to move which enable us to perform various motor activities.

 

LEVELS OF ORGANIZATION:

As shown in the figure on the left, skeletal muscles are organized into various levels. The large muscles are made up of bundles, which are made up of muscle fibers (striated single cell), which are then made up of myofibrils. Myofibrils are then made up of protein filaments that are actin (thin filament) and myosin (thick filament).

CROSS-BRIDGE CYCLE:

During each cross-bridge cycle, the cross bridge binds with an actin molecule, bends to pull the thin filament (actin) inward during the powerstroke, then detaches and returns to its restin conformation, ready to repeat the cycle.

When the muscle fiber is relaxed, there is no cross-bridge binding because the actin binding site is covered by the troponin-tropomyosin binding complex.

Once excited, released Ca2+ binds with the troponin, pulling the troponin-tropomyosin complex aside. This exposes the actin binding site allowing cross-bridge binding to occur.

Binding of the actin was energized by the splitting of ATP into ADP and Pi by the myosin ATPase site on the cross-bridge. Myosin molecules triggers a power stroke that pulls the thin filament inward during contraction.

 

MUSCLE CONTRACTION AND RELAXATION:

During muscle contraction, each sarcomere shortens as the thin filaments slide closer together between the tick filaments so that the Z lines are pulled closer together.