Question 4: How does a muscle shorten during its contraction and return to its original form during relaxation?
Answer:
When a sarcomere contracts, myosin heads attach to actin to forms cross- bridges. Then, the thin filaments slide over the thick filaments as the heads pull the actin. Thus result in sarcomere shortening, creating the tension of muscle contraction. The steps followed during relaxation are:
- At the beginning when the neural signal reaches the neuromuscular junction then neurotransmitter acetylcholine which creates an action potential in the sarcolemma.
- Calcium ions are released and spread through muscle fibres due to this action potential.
- Calcium ions bind to the subunit of troponin on the actin filament due to the increase in the concentration of the calcium ions which further remove the active sites in the myosin.
- Cross- bridge formed when myosin bind with actin.
- This causes the attached actin filaments to move towards the center of A- band. A concentration of the sarcomere occurs in the muscles as the Z- line moves inwards, the I- band gets reduced while the A- band retains its original length.
- ADP and Pi are released when the cross bridge breaks and myosin goes back to its relaxed state and new ATP binds which breaks the cross- bridge.
- The muscle relaxation occurs after contraction when the calcium ions are pumped back to the active sites on actin myofilaments. The Z- line returns to original position, i.e. relaxation.
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