Titin stabilises sarcomeres as an elastic spring, while antagonistic muscles produce opposite movements because muscles actively contract but cannot push.
Coverage 2018–2025 · Updated 15 Jul 2026
A motor neuron triggers skeletal muscle contraction at the neuromuscular junction. An action potential reaches the motor end plate, acetylcholine is released and the sarcolemma is depolarized; the signal causes the sarcoplasmic reticulum to release Ca²⁺. Ca²⁺ exposes actin binding sites and cross-bridge cycling shortens the sarcomere. Acetylcholinesterase removes acetylcholine and Ca²⁺ pumps return calcium to the sarcoplasmic reticulum, so tropomyosin covers the binding sites and the fibre relaxes. A motor unit is one motor neuron plus all the fibres it controls. Titin provides elastic recoil, centres myosin and limits overstretching; antagonistic muscles are needed because muscle contraction produces force in one direction, not active extension.
This objective is assessed through structured response, commonly using Label / Identify / Outline.
Label / Identify / Outline / Describe / Explain
Build the answer around this relationship: Titin helps keep myosin centred in the sarcomere.
Saying antagonistic muscles both contract together for the same movement.
Representative question
Explain the role of the protein titin in muscle relaxation.
A strong answer links movement benefit, muscle contraction mechanism, force transfer, joint range, and locomotion adaptations. For contraction, use calcium-troponin-tropomyosin and ATP-driven myosin cross-bridge cycling. For movement, use antagonistic muscles, tendons, ligaments, skeletons as levers, and joint type. For locomotion, link body form to survival or swimming advantage. Locomotion can improve survival and reproductive success.
