Skeletal Muscle Physiology cont. Diagram the chemical and mechanical steps in the cross-bridge cycle and explain the effect on the muscle fiber length. Describe the end of contraction mechanisms. Muscle excitation and energy sources. Three roles of ATP in muscle function. Three sources of ATP for muscle function.

Sliding myofilaments shorten sarcomeres

Excitation -contraction coupling ACh binds to, opens nicotinic Na + /K + channels Muscle depolarizes Ca 2+ released from sarcoplasmic reticulum Ca 2+ binds to troponin, cross-bridge cycling between actin & myosin begins, filaments slide

Draw and label a diagram to show the following stage of the cross-bridge cycle: 1.At rest, when the muscle is not stimulated.

Skeletal Muscle Physiology cont. Diagram the chemical and mechanical steps in the cross-bridge cycle and explain the effect on the muscle fiber length. Describe the end of contraction mechanisms. Muscle excitation and energy sources. Three roles of ATP in muscle function. Three sources of ATP for muscle function.

Muscle elastic elements recoil, muscle returns to resting length. End of contraction ACh destroyed by ACh-esterase in synapse Muscle repolarizes Ca 2+ returned to SR by Ca 2+ active transporter ATP hydrolysis (+Mg) reextends myosin head Titin is the largest polypeptide known (34,350 amino acids in length). It spans from the M to Z lines.

Draw and label a diagram to show the following stage of the cross-bridge cycle: 2. At death when the muscle has depleted ATP. (rigor)

Genetic mutation turns tot into superboy 4-year-old is first documented human case, scientists say A German boy, seen here at seven-months old, has a genetic mutation that boosts muscle growth.

Excitation-Contraction Coupling action potentials, generated at neuromuscular junction travel around sarcolemma and through T- tubules T-tubules signal SR to release Ca 2+ into sarcoplasm (cytosol) Ca 2+ saturates troponin (in non-fatigued state) troponin undergoes conformational change that lifts tropomyosin away from actin filament

E-C Coupling (cont.) myosin head attaches to active site on actin filament forming cross-bridge after forming cross-bridge, myosin head moves actin-myosin complex forward and ADP and Pi are released ATP binds with myosin head, which releases actin, and returns to original position in resting state, myosin head contains partially hydrolyzed ATP (ADP and Pi)

E-C Coupling (cont.) entire cycle takes ~50 ms although myosin heads are attached for ~2 ms a single cross-bridge shortens 10 nm as long as action potentials continue, Ca 2+ will continue to be released when action potentials cease, SR Ca 2+ pumps return Ca 2+ ceasing contractions skeletal motor units follow “all or nothing” principle