Question

1. If you were to watch muscle tissue contract:

1. Under a light microscope, would you see the muscle fibers get narrower, or the striations get thinner? Explain.
2. 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)
   
   Please answer question (a).

Answer 1

Ans a- muscle fibers get narrower.

Each skeletal muscle fiber is a skeletal muscle cell. Within each muscle fiber are myofibrils—long cylindrical structures that lie parallel to the muscle fiber.The striated appearance of skeletal muscle tissue is a result of repeating bands of the proteins actin and myosin that are present along the length of myofibrils. Dark A bands and light I bands repeat along myofibrils, and the alignment of myofibrils in the cell causes the entire cell to appear striated or banded.

Each I band has a dense line running vertically through the middle called a Z disc or Z line. The Z discs mark the border of units called sarcomeres, which are the functional units of skeletal muscle. One sarcomere is the space between two consecutive Z discs and contains one entire A band and two halves of an I band, one on either side of the A band. A myofibril is composed of many sarcomeres running along its length, and as the sarcomeres individually contract, the myofibrils and muscle cells shorten.

During Muscle Contractions

• Action potential in a motor neuron triggers the release of Ca2+ ions from the sarcoplasmic reticulum
• Calcium ions bind to troponin (on actin) and cause tropomyosin to move, exposing binding sites for the myosin heads
• The actin filaments and myosin heads form a cross-bridge that is broken by ATP
• ATP hydrolysis causes the myosin heads to swivel and change orientation
• Swiveled myosin heads bind to the actin filament before returning to their original conformation (releasing ADP + Pi)
• The repositioning of the myosin heads move the actin filaments towards the centre of the sarcomere
• The sliding of actin along myosin therefore shortens the sarcomere, causing muscle contraction.

From above discussion it is clear that muscle fibers get narrower.