Question

If you were to watch muscle tissue contract:

a. Under a light microscope, would you see the muscle fibers get narrower, or the striations get thinner? Explain.

b. At the EM level, focusing on one sarcomere, you would be able to see a region of thick filaments overlapping two regions of thin filaments. Use the structure of the thick filaments to explain how ONE region of thick filaments is able to pull in microfilament in two opposite directions (both toward the center of the sarcomere).

Answer 1

Sarcomere: Is the unit of muscle that contracts.

Z-line: It is seen at the end of each sarcomere .They define where one sarcomere begins and another ends like a border. The thin filaments connect to the Z-line.

M-Line: It is seen in the middle of each sarcomere which contains thick filaments. This is also the middle of the thick filament.

I band: The I band is a zone around the Z-lines which has thin filaments. This includes part of two separate sarcomeres. It gets smaller when the muscle contracts.

H band:  The H band is a zone around the M-line which includes only thick filaments. The H band gets smaller when the muslce contracts.

A band: The A band is a zone that includes the length of the sarcomere where thick filaments are. This does not change size. It can include only thick or both thick and thin filaments.

The sliding filament theory is put forward to explain the mechanism of muscle contraction based on muscle proteins( actin and myosin) that slide past each other. This sliding creates contraction movement. The myosin forms the thick filaments of muscle fibers slide past the actin the thin filaments during muscle contraction. Thus muscle contraction is brought by the sliding of the  two groups of filaments and therefore they remain at relatively constant length.

An action potential from motor neuron stimulate the muscle, not the entire muscle but only a number of muscle fibers with in a muscle. The individual motor neuron plus the muscle fibres it stimulates, is called a motor unit.

When an impulse reaches the muscle fibres of a motor unit, it stimulates a reaction in each sarcomere between the actin and myosin filaments. This reaction results in the start of a contraction by sliding

The incoming impulses creates a reaction in the  the 'heads' on the myosin filament to reach forward, attach to the actin filament and pull actin towards the centre of the sarcomere. This process occurs simultaneously in all sarcomeres, the end process of which is the shortening of all sarcomeres. ( answer to the first question)

Troponin, or the troponin complex, is composed  of three regulatory proteins troponin C, troponin I, and troponin T. This is integral to muscle contraction in skeletal muscle. Troponin is attached to the protein tropomyosin and lies within the groove between actin filaments in muscle tissue. In a relaxed muscle, tropomyosin blocks the attachment site for the myosin to the actin to form the crossbridge, thus preventing contraction. When the muscle cell is stimulated to contract by an action potential, calcium channels open in the sarcoplasmic membrane and release calcium into the sarcoplasm. Some of this calcium attaches to troponin, which causes it to change shape, exposing the active binding sites for myosin on the actin filaments. Myosin's binding to actin causes crossbridge formation and thus begins the contraction of muscle

For the initiation of muscle contraction, tropomyosin has to expose the myosin-binding site on an actin filament that allows cross-bridge formation between the actin and myosin microfilaments. In the first step of muscle contraction Calcium ion (Ca+2)  binds to troponin so that tropomyosin can slide away from the binding sites on the actin strands. This then allows the myosin heads to bind to the exposed binding sites to form cross-bridges. Then the thin filaments are pulled by the myosin heads to slide past the thick filaments toward the center of the sarcomere. Each head can only pull a very short distance before it has reached its limit so it must be “re-cocked” before it can pull again,this step requires ATP.

For thin filaments to continue to slide past thick filaments during muscle contraction process, the myosin heads should pull the actin at the binding sites, detach, re-cock, attach to more binding sites, pull, detach, re-cock. This is a repeated movement known as the cross-bridge cycle.. Each cycle requires energy, which is provided by ATP.

The active site on actin is exposed when calcium binds to troponin. The myosin head is attracted to actin, and myosin binds actin at its actin-binding site, forming the cross-bridge. During the power stroke, the phosphate generated in the previous contraction cycle is released. This results in the myosin head pivoting toward the center of the sarcomere, after which the attached ADP and phosphate group are released. A new molecule of ATP attaches to the myosin head, causing the cross-bridge to detach. The myosin head hydrolyzes ATP to ADP and phosphate, which returns the myosin to the cocked position.

Thus during cross-bridge formation thats is when the myosin head attaches to the actin the adenosine diphosphate (ADP) and inorganic phosphate (Pi) are still bound to myosin . The Pi is then released, causing myosin to form a stronger attachment to the actin, after which the myosin head moves toward the M-line, pulling the actin along with it. As actin is pulled, the filaments move approximately 10 nm toward the M-line. This movement is called the power stroke. It this power stroke that pulls the actin filaments on both sides toward the center of sarcomere (answer to the second question) In the absence of new ATP, the myosin head will not detach from actin.

One part of the myosin head attaches to the binding site on the actin, but the head has another binding site for ATP. When the ATP binds the myosin head detach from the actin. After this occurs, ATP is converted to ADP and Pi by the intrinsic ATPase activity of myosin. The energy released during ATP hydrolysis changes the angle of the myosin head into a cocked position. The myosin head is now in position for further movement.

When the myosin head is cocked, myosin is in a high-energy configuration. This energy is expended as the myosin head moves through the power stroke, and at the end of the power stroke, the myosin head is in a low-energy position. After the power stroke, ADP is released; however, the formed cross-bridge is still in place, and actin and myosin are bound together. Thus as long as ATP is available, it readily attaches to myosin, the cross-bridge cycle can recur, and muscle contraction can continue.

Answer to both the questions are explained detaily in the above data including diagrams.