P1 Skeletal Muscle

Question 1

What is the function and structure of skeletal muscle?

Answer 1

• Skeletal muscle is attached to bone, enabling limbs to move.
• It is made of striated muscle cells, which are tube-shaped and have many nuclei (multinucleated).
• Each tube-like structure is a bundle of muscle cells. A single muscle cell is an elongated cell that looks like a thread (a muscle fibre).
• Inside muscle fibres there are a series of more tube-like structures called myofibrils.
• Skeletal muscles work in antagonistic pairs to move bones.

Question 2

What is the function and structure of cardiac muscle?

Answer 2

• To pump blood.
• Made of striated muscle cells, which are branched, and only contain one nucleus (uninucleated).

Question 3

What is the function and structure of involuntary muscle?

Answer 3

• Found in internal organs and blood vessels, allowing substances to move through organs and blood vessels.
• Made of non-striated muscle cells, which are spindle-shaped and only contain one nucleus (uninucleated).

Question 4

Structure of a myofibril

Answer 4

• Contains many protein filaments (smaller, thread-like structures), containing thick filaments and thin filaments.
• Thin filaments are made of two polypeptide chains wrapped around each other. The polypeptide chain is actin, meaning these are actin filaments.
• Thin filaments also have another protein chain wrapped around the actin filaments, called tropomyosin.
• Thick filaments are made up of two myosin filaments wrapped around each other, creating a tail and a head. These are then bundled together with other myosin filaments to form the thick filaments.

Question 5

How are filaments arranged in a myofibril?

Answer 5

• Arranged in vertical patterns, with overlap between thick and thin filaments called sarcomeres.
• Myofibrils contain many repeating sarcomeres.
• A single sarcomere is one single stack of thin and thick filaments (at least one thick filament and 4 thin filaments).

Question 6

What is the M-line?

Answer 6

• In the middle of one sarcomere, positioned at the thick filaments, which consist of myosin filaments.

Question 7

What is the Z-line?

Answer 7

• The midpoint of two M-lines, positioned at the thin filaments that consist of actin filaments.

Question 8

What is the I-band?

Answer 8

• The area under a light microscope that appears lighter, since it contains thin filaments that let more light through.

Question 9

What is the A-band?

Answer 9

• The area under a light microscope that appears darker, since it contains thick and thin filaments that allow less light through.
• This creates a two-toned band (thick and thin filaments overlap, then just thick filaments in the centre).

Question 10

What is the H-zone?

Answer 10

• Found in the middle of the A-band.
• It is the slightly brighter part of the A-band under a light microscope, because it is made of only thick filaments.

Question 11

What happens to sarcomeres during muscle contraction?

Answer 11

When an action potential is triggered inside the muscle fibre (from neurotransmitters) the actin filaments and myosin filaments move inwards.

Question 12

What occurs when an action potential reaches a sarcomere?

Answer 12

1. The action potential spreads out across the whole muscle fibre, and reaches the sarcoplasmic reticulum, which is wrapped around the muscle fibre’s myofibrils.
2. The action potential causes the sarcoplasmic reticulum to release calcium ions into the myofibrils.
3. Actin (thin) filaments contain binding sites for myosin filaments heads. However the tropomyosin chain that is wrapped round the actin filament blocks this binding site.
4. When calcium ions arrive at the sarcomere, this triggers the tropomyosin to change shape, causing it to move away from the binding site and allowing the myosin head to bind to actin, forming a bridge between actin and myosin (actinomyosin bridge).
5. Myosin heads containing binding sites for ATP, and hydrolyses ATP to ADP. This release of energy causes the myosin head to move into it’s starting position (the power stroke).
6. Another ATP molecule attaches to the myosin head, causing the actinomyosin bridge to break, so the myosin head detaches from the actin.
7. The ATP molecule is then hydrolysed into ADP and a phosphate, releasing energy which allows the myosin head to return to it’s stating position.

Question 13

What are the steps of the sliding filament mechanism?

Answer 13

1. Calcium ions cause tropomyosin to move away from actin binding sites.
2. Myosin head attaches to a binding site on actin.
3. ADP and a phosphate group are released from the myosin head.
4. ATP binds to myosin head and myosin head detaches from actin.
5. The myosin head hydrolyses ATP and returns to it’s starting position. This then repeats in a cycle.
6. This occurs with many myosin heads and actin filaments, causing the whole sarcomere to contract, moving the actin filaments closer to the end line of the sarcomere. When lots of sarcomeres contract, the whole muscle contracts.

Question 14

How does muscle relaxation occur?

Answer 14

1. When the nerve impulse ceases, calcium ions are actively transported back into the sarcoplasmic reticulum.
2. In the absence of calcium ions, the tropomyosin molecule changes shape and blocks binding sites on actin again.
3. This means myosin heads are unable to attach to actin.
4. Muscle contraction stops, meaning the sarcomeres on the muscle’s myofibrils expand again.
5. This leads to the muscle becoming relaxed.

Question 15

What is the evidence for the sliding filament mechanism?

Answer 15

• During muscle contraction the A band stays the same length, proving that actin filaments slide along the myosin filaments.
• If the actin filaments shortened, the A band would also shorten.