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Muscle Contraction Notes

Pages 187-193. Muscle Contraction Notes. Microscopic Anatomy of Skeletal Muscle. Skeletal muscle tissue is made of cells called muscle fibers . Muscle fibers contain small cylinders called myofibrils . Myofibrils are made of sarcomeres linked end-to-end.

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Muscle Contraction Notes

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  1. Pages 187-193 Muscle Contraction Notes

  2. Microscopic Anatomy of Skeletal Muscle Skeletal muscle tissue is made of cells called muscle fibers. Muscle fibers contain small cylinders called myofibrils. Myofibrils are made of sarcomeres linked end-to-end. Sarcomeres are the basic unit of contraction in skeletal and cardiac muscle.

  3. Sarcomeres • Actin and Myosin filaments are arranged in an overlapping pattern of light ("I" bands) and dark ("A" bands). In the middle of each "I" band is a line called a "Z" line. The section of a myofibril from one Z-line to the next Z-line is the sarcomere. • The place where sarcomeres attach to each other is called a Z line. • Z lines and the filaments between them create light and dark bands that make skeletal and cardiac muscle appear striped.

  4. Myofilaments • There are two types of threadlike protein myofilaments: • The larger thick filaments (myosin filaments) are made mostly of bundled molecules of the protein myosin. The myosin heads can bind and detach from the thin actin filament. When bound it creates cross-bridges.

  5. Myofilaments cont. • The thin filaments (actin filaments) are composed of the contractile protein called actin.

  6. The area between two Z discs is a sarcomere • I band contains actin filaments only (light bands) • A band contain overlapping actin and myosin filaments (dark) • H zone contains myosin filaments only http://entochem.tamu.edu/MuscleStrucContractswf/index.html

  7. Sarcoplasmic Reticulum The major function is to store calcium and to release it on demand when the muscle fiber is stimulated to contract

  8. The overlapping arrangement of the protein filaments in a sarcomere enable muscles to contract. • When the muscle is stimulated, these filaments slide past each other, making the sarcomere to shorten. • Muscle contraction usually begins when a muscle fiber receives a signal from a nerve cell.

  9. Muscle Properties • Muscle cells have some special functional properties that enable them to perform their duties. • Excitability (aka responsiveness or irritability) – ability to receive and respond to a stimulus • Contractability– ability to shorten when adequately stimulated • Extensibility – ability of muscle cells to be stretched • Elasticity – ability to recoil and resume their resting length after being stretched

  10. Steps of Muscle Contraction • When a nerve impulse reaches the axon terminals, a neurotransmitter (specifically acetylcholine) is released. • Acetylcholine (ACh) diffuses across the synaptic cleft and attaches to receptors (membrane proteins) that are part of the sarcolemma. • An action potential travels through the T tubules • The action potential goes to the sarcoplasmic reticulum • The sarcoplasmic reticulum releases Ca2+.

  11. Steps of Muscle Contraction cont. • The calcium binds to the troponin on the actin filament • This opens up binding site for the myosin to attach • Now the myosin binds to the actin • ATP is needed for the myosin to slide past the actin

  12. Sliding Filament Theory • Step 1 Myosin attaches to a binding site on an actin filament. Calcium is required to make a binding site available for myosin. • Step 2 The myosin head rotates and causes the actin filament to slide along the myosin filament. This sliding causes the filaments to overlap more, and the sarcomere becomes shorter. 

  13. Step  After the myosin head has rotated as far as it can, it must let go of the actin fiber. ATP is required for myosin to detach from actin. The myosin head snaps back into its original position, using the energy in the ATP. The ATP becomes adenosine diphosphate (ADP) and releases a phosphate ion. Step  Calcium exposes a new actin binding site and myosin reattaches to actin. Steps 1 through 3 happen again. Sliding Filament Theory (cont.) • Step 3  After the myosin head has rotated as far as it can, it must let go of the actin fiber. ATP is required for myosin to detach from actin. The myosin head snaps back into its original position, using the energy in the ATP. The ATP becomes adenosine diphosphate (ADP) and releases a phosphate ion.  • Step 4 Calcium exposes a new actin binding site and myosin reattaches to actin. Steps 1 through 3 happen again.

  14. Sliding Filament Theory (cont.) • In this way, the myosin heads walk along actin filaments and step at available binding sites. • This grabbing and pulling action repeats, the sarcomere shortens, and the Z lines are pulled closer together.  • Myosin filaments bind to actin filaments, actin filaments move inward,and sarcomeres shorten to cause muscle contraction. • This whole process occurs many times in the instant that you snap your fingers!

  15. Muscle Contraction Animations • http://www.youtube.com/watch?v=RmQzIS9IGUE

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