Introduction Skeletal muscle cells are composed of muscle fascicles muscle fascicles are composed of multi-nucleated muscle fibers muscle fibers are composed of myofibrils myofibrils contain sarcomeres, where actin and myosin filaments slide and produce contractions Muscle cell transverse tubules (T-tubules), invaginations of the plasma membrane, are juxtaposed with the sarcoplasmic reticulum’s terminal cisternae to contract T-tubules are responsible for conducting the action potential to the cisternae of the sarcoplasmic reticulum skeletal muscle triad 1 T-tubule and 2 terminal cisternae cardiac muscle dyad 1 T-tubule and 1 terminal cisterna Sliding filament theory sliding of thick and thin filaments form a contraction thick filaments composed of protein myosin anchored to M line thin filaments composed of actin, tropomyosin, and troponin actin contains binding sites for myosin tropomyosin covers these binding site grooves anchored to Z line Excitation-Contraction Coupling Action potential depolarizes plasma membrane presynaptic voltage-gated Ca2+ channels open and neurotransmitters are released from the neuron post-synaptic ligand is bound, causing depolarization of the motor end plate of the muscle Depolarization spreads to the T-tubule depolarization causes a conformational change in the voltage-sensitive dihydropyridine receptor this causes a conformational change in the ryanodine receptor (also a calcium channel protein) Ca2+ is released from the sarcoplasmic reticulum this initiates the contraction Contraction Free Ca2+ binds to troponin C this causes a conformational change and moves tropomyosin out of a myosin-binding groove Myosin releases ADP and Pi, causing displacement of myosin on actin filament this produces a power stroke and contraction shortens H and I bands, resulting in shortening between the Z lines (HIZ shrinkage) A band remains the same length Myosin head is detached from the actin filament because of binding of a new ATP molecule Hydrolysis of ATP to ADP causes myosin head to adopt a high-energy position, in preparation for the next contraction