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A sarcomere lies between two Z-lines. A Z-line is a thin plate that joins the sarcomeres together. (striations) The H-zone contains only the thick myosin filaments. The A-band includes the H-zone plus the overlapping myosin and actin filaments on both sides of the H-zone. (length of thick filaments) The I-band is a region with actin filaments only and it lies on both sides of the Z-line of two adjoining sacromeres. Actin is a globular protein. Balls of globular protein are connected together in two long chains that are wound around each other like two twisted ropes. The globules of the actin filament have myosin binding sites on their surfaces. Another kind of protein called tropomyosin, has the shape of two ropes twisted around each other. Tropomyosin is wrapped around the actin filaments in a way that covers all the myosin binding sites. Troponin is a small globular protein connected to the tropomyosin at certain intervals. Each troponin molecule has a calcium binding site. 1.When a nerve impulse reaches the nerve endings of a motor neuron, the neuron releases acetylcholi ne, the neurotransmitter for the skeletal system. 2. Acetylcholine combines with receptors on muscle fiber membranes. 3.This causes an action potential to travel in the muscle fibers. 4.The action potential triggers the release of Ca 2+ from the sarco plasmic reti culum . 5.Ca 2+ combines with troponin at the Ca 2+ binding sites. 6.This cause troponin to flip over, pulling the tropomyosin with it. 7.This exposes the myosin binding sites on the actin filaments. 8.Myosin heads that are attached to ADP bind to the actin filaments at the myosin binding sites (cross bridge). 9.Myosin releases ADP, causing the myosin to change in shape. This shape change causes myosin to pull the actin filament toward the center of the sacromere. 10.Myosin binds ATP, causing another shape change. Myosin releases actin. 11.ATP is hydrolyzed to ADP and Pi. 12.Now the myosin and ADP are ready to start the cycle again if another nerve signal arrives.
