7.1.2 Mechanism of Muscle Contraction

• What allows skeletal muscle fibres to synthesize proteins rapidly?

  Multinucleated structure
• The main structural component within muscle fibres is the myofibril
• The A band in a sarcomere contains only actin filaments.

  False
• What is the function of troponin in muscle contraction?

  Binds to calcium ions
• Match the component of muscle fibres with its function:

  Myosin ↔️ Generates force

  Sarcoplasmic Reticulum ↔️ Stores calcium ions

  T-tubules ↔️ Carry action potentials

  Actin ↔️ Binds to myosin
• Sarcomeres are repeating units within myofibrils that contain actin and myosin filaments.
• What marks the boundaries of each sarcomere?

  Z-line
• The neuromuscular junction is where a motor neuron communicates directly with a muscle fibre.
• Which neurotransmitter is released at the neuromuscular junction?

  Acetylcholine
• Steps in excitation-contraction coupling

  1️⃣ Action potential spreads along T-tubules

  2️⃣ DHPRs in T-tubules are activated

  3️⃣ Ryanodine receptors in SR open

  4️⃣ Calcium ions are released into sarcoplasm

  5️⃣ Calcium binds to troponin

  6️⃣ Tropomyosin shifts from actin-binding sites

  7️⃣ Myosin binds to actin
• DHPRs activate ryanodine receptors (RyR) in the sarcoplasmic reticulum (SR) to release calcium ions into the sarcoplasm.
• What is the role of T-tubules in excitation-contraction coupling?

  Carry action potentials
• Match the component of excitation-contraction coupling with its role:

  DHPR ↔️ Activates RyR

  RyR ↔️ Releases calcium ions

  SR ↔️ Stores calcium ions

  T-tubules ↔️ Carry action potentials
• Acetylcholine ($ACh$) is released into the synaptic cleft.
• What is the first step in the sequence of events at the neuromuscular junction?

  Action potential arrival
• Acetylcholine ($ACh$) is released into the synaptic cleft
• The neuromuscular junction is where a motor neuron communicates with a muscle fibre.
• What happens after $ACh$ binds to its receptors on the muscle fibre membrane?


  Muscle fibre depolarisation
• The axon terminal releases acetylcholine
• What carries the action potential deep into the muscle fibre during excitation-contraction coupling?

  T-tubules
• Ryanodine receptors (RyR) are calcium channels in the sarcoplasmic reticulum.
• What protein complex does $Ca^{2 + }$ bind to in order to initiate muscle contraction?


  Troponin
• The sarcoplasmic reticulum stores and releases calcium
• Steps of the sliding filament theory

  1️⃣ Calcium ions are released from the sarcoplasmic reticulum.

  2️⃣ Calcium binds to troponin, exposing myosin-binding sites on actin.

  3️⃣ Myosin heads bind to actin, forming cross-bridges.

  4️⃣ Myosin heads pivot, pulling actin filaments towards the M-line.

  5️⃣ ATP binds to myosin, causing it to detach from actin.

  6️⃣ ATP is hydrolyzed to ADP and inorganic phosphate, re-energizing myosin heads.

  7️⃣ Cycle repeats as long as calcium and ATP are available.
• What protein blocks the myosin-binding sites on actin in the absence of calcium ions?

  Tropomyosin
• ATP is the primary energy source for muscle contraction.
• ATP hydrolysis produces $ADP$ and inorganic phosphate
• What happens when ATP binds to myosin during muscle contraction?

  Cross-bridge detachment
• Match the use of ATP with the corresponding process in muscle contraction:

  Re-energising myosin heads ↔️ Hydrolysis of ATP to ADP + $P_{i}$

  Detaching myosin from actin ↔️ Binding of ATP to myosin
• What are the repeating units in a skeletal muscle fibre called?

  Sarcomeres
• The A band in a sarcomere contains only actin filaments.

  False
• The I band in a sarcomere contains only actin
• What are skeletal muscle fibres composed of?

  Multinucleated cells
• Skeletal muscle fibres contain cylindrical bundles of repeating units called sarcomeres
• A sarcomere contains both actin and myosin filaments.
• What are the two distinct bands found in a sarcomere?

  A band and I band
• The A band in a sarcomere contains the protein myosin
• The I band in a sarcomere contains only actin filaments.
• Match the component of a sarcomere with its function:

  Actin ↔️ Binds to myosin for contraction

  Myosin ↔️ Generates force for contraction

  A band ↔️ Darker area where actin and myosin overlap

  I band ↔️ Lighter area with no myosin present
• What is the boundary of each sarcomere called?

  Z-line