BIOL122 skeletal muscle

Why is skeletal muscle referred to as "striated" and "voluntary"?

Striated: Visible bands or striations can be seen under a microscope due to the alignment of sarcomeres.

Voluntary: It is under conscious control by the nervous system to voluntarily initiate movement.

Name five functions of skeletal muscle besides producing movement.

1. Maintaining posture.

2. Supporting soft tissues.

3. Controlling "openings and exits" (e.g., swallowing, urination, defecation).

4. Maintaining body temperature (generating heat through energy use, like shivering).

5. Serving as a nutrient reserve (amino acids/protein source during fasting).

What are the three layers of connective tissue that organize a skeletal muscle, from internal to external?

1. Endomysium: Surrounds each individual muscle cell (muscle fibre).

2. Perimysium: Wraps a bundle of muscle fibres called a fascicle.

3. Epimysium: Surrounds the entire packed collection of fascicles and connects to the tendon.

Why are skeletal muscle fibres multinucleate?

They are multinucleate because several embryonic myoblast cells fuse together early in development to form a single long muscle fibre.

What are T-tubules and what is their primary function?

Transverse tubules (T-tubules) are narrow tubes continuous with the cell membrane (sarcolemma). They propagate the action potential deep into the muscle fibre so the entire cell contracts simultaneously

What is the function of the sarcoplasmic reticulum (SR)?

It is a specialized endoplasmic reticulum that surrounds myofibrils and stores the calcium ions Ca2+ necessary for muscle contraction.

What are the two types of myofilaments that make up a myofibril, and what proteins form them?

Thick filaments: Made of the protein myosin.

Thin filaments: Made of the protein actin.

Define the Sarcomere.

The repeating functional unit of a myofibril; it is the basic unit of muscle contraction

Match the following structural regions of a sarcomere to their definitions: A band, I band, H band, M line, and Z line.

A band: The dark region running the full length of the thick (myosin) filaments, including their overlap with thin filaments.

I band: The light region containing only thin (actin) filaments and titin.

H band: The narrow center area containing only thick filaments.

M line: The central line that connects thick filaments together.

Z line (Z disc): The boundary line that anchors thin filaments and joins adjacent sarcomeres.

What protein prevents the over-extension of a sarcomere and holds thick filaments in place?

Titin, which is bound to the ends of thick filaments.

When a muscle is relaxed, why can't myosin bind to actin?

The myosin-binding sites on the actin molecules are physically covered and blocked by the regulatory protein strand tropomyosin.

How are the myosin-binding sites on actin uncovered?

When stimulated, calcium Ca2+ released from the sarcoplasmic reticulum binds to troponin. This causes a shape change in troponin, which shifts tropomyosin away, exposing the binding sites.

During contraction, what happens to the length of the thick and thin filaments themselves?

Nothing. The filaments do not change length; instead, they slide past each other, which brings the Z lines closer together and shortens the overall sarcomere.

What are the 6 structural steps of the Crossbridge Cycle?

1. Exposure: Ca2+ binds to troponin, exposing active sites on actin.

2. Crossbridge Formation: Energized myosin heads bind to the active sites on actin.

3. The Power Stroke: Myosin releases stored energy, pivoting from an "open" to a "closed" position, pulling the thin filament toward the M line. ADP and phosphate are released.

4. Detachment: A new molecule of ATP binds to the myosin head, causing it to detach from actin.

5. Myosin Reactivation: Myosin hydrolyses (splits) the ATP into ADP and P, using the energy to "re-cock" the head back into the open, high-energy position.

6. Relaxation or Repeat: If Ca2+ is still present, the cycle repeats. If the impulse stops, Ca2+ is pumped back into the SR, and tropomyosin re-covers the sites.

What 4 steps cause the contraction cycle to end and the muscle to relax?

1. Acetylcholine (ACh) is broken down in the synaptic cleft.

2. Calcium stops being released from the sarcoplasmic reticulum.

3. Calcium ions are actively pumped from the cytoplasm back into the SR.

4. Calcium detaches from troponin, allowing tropomyosin to block the myosin-binding sites again.