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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
• Ca2+ released from sarcoplasmic reticulum
• Ca2+ binds to troponin, cross-bridge cycling
between actin & myosin begins, filaments slide
http://www.sci.sdsu.edu/movies/actin_myosin_gif.html
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.
End of contraction
• ACh destroyed by ACh-esterase in synapse
• Muscle repolarizes
• Ca2+ returned to SR by Ca2+ active transporter
• ATP hydrolysis (+Mg) reextends myosin head
• Muscle elastic
elements recoil,
muscle returns to
resting length.
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 Ttubules
• T-tubules signal SR to release Ca2+ into
sarcoplasm (cytosol)
• Ca2+ 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, Ca2+
will continue to be released
• when action potentials cease, SR Ca2+
pumps return Ca2+ ceasing contractions
• skeletal motor units follow “all or nothing”
principle