Transcript Diagram the chemical and mechanical steps in

What to Know About Skeletal
Muscle Physiology
1. Diagram the chemical and mechanical
steps in the cross-bridge cycle and explain
the effect on the muscle fiber length.
2. Describe the end of contraction
mechanisms.
3. 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 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
Be able to:
Draw and label a diagram to show the following
stage of the cross-bridge cycle:
1. At rest, when the muscle is not stimulated.
End of contraction
• ACh destroyed by ACh-esterase in synapse
• Muscle repolarizes
• Ca2+ returned to SR by Ca2+ active transporter
• ATP hydrolysis (+Mg) re-extends 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.
Be able to:
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 sevenmonths old, has a genetic mutation
that boosts muscle growth.
At age 14?
Excitation-Contraction Coupling
• action potentials, generated at neuromuscular
junction travel around sarcolemma and through
T-tubules
• 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 – contracts maximally or not at all