Question

1. In one paragraph (or more), discuss the role of Actin and Myosin in a muscle contraction. Include the proteins troponin and tropomyosin in your discussion.

2. In the second paragraph, discuss the major events of muscle contraction and relaxation.

3. Discuss what an agonist, antagonist, and synergists mean and give an example for each.  

Answer 1

1. Role of Actin and Myosin in a muscle contraction:-

• Contraction cycle begins when Calcium ions arrive at sarcomere
• Calcium ions bind to troponin. Troponin changes its position, moving tropomyosin and exposing myosin binding sites on actin
• Myosin heads bind to exposed active sites on actin, forming cross bridges
• Stored energy within myosin head is released and is used to pivot myosin head towards the middle of sarcomere. This is called power stroke
• ADP and P are released from myosin head
• Cross bridges detach
• Attachment of new ATP to myosin head causes release of myosin from actin
• Active sites now available to form another cross bridge
• Free myosin head splits ATP into ADP and phosphate
• Released energy helps in muscle contraction

2. Major events of muscle contraction

• A nerve impulse travels down a motor neuron axon
• The motor neuron terminal releases the neurotransmitter, Acetylcholine(ACh)
• ACh binds to ACh receptors
• The sarcolemma is stimulated
• Muscle impulse reaches the sarcoplasmic reticulum and calcium channels open
• Calcium ion diffuses and binds to troponin C
• Tropomyosin molecules move and expose specific sites of actin
• Actin and myosin form linkages
• Actin filaments are pulled towards the center of the sarcomere by myosin cross bridges
• Contraction occurs as muscle fibre shortens

Major events of muscle relaxation

• Relaxation occurs when stimulation of nerve stops.
• Calcium ions are pumped back into the sarcoplasmic reticulum
• This breaks the link between actin and myosin
• Actin and myosin returns back to their unbound site
• This causes muscle relaxation

3. Agonist- The muscles that are directly involved in producing a certain joint movement.

Antagonist-  The muscle that opposes the agonist.

Synergist- The muscles that are indirectly involved and stabilizes a joint around which movement is occurring.

Example, biceps(agonist) causes the muscle contraction to move the arm up. Triceps(antagonist) opposes this action. Brachialias and brachioradialis (synergists) supports this action.