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.
• The neurotransmitter released at the neuromuscular junction is acetylcholine
• The neuromuscular junction is where a motor neuron communicates with a muscle fibre.
• Steps involved in the neuromuscular junction process

  1️⃣ Action potential arrives at the axon terminal

  2️⃣ Acetylcholine is released into the synaptic cleft

  3️⃣ Acetylcholine binds to ACh receptors on the muscle fibre membrane

  4️⃣ Depolarisation of the muscle fibre initiates contraction
• The space between the motor neuron and muscle fibre is called the synaptic cleft.
• What is the purpose of the neuromuscular junction?

  Communication between neuron and fibre
• Match the component with its function in the neuromuscular junction:

  Axon Terminal ↔️ Releases acetylcholine

  Synaptic Cleft ↔️ Allows diffusion of $ACh$

  ACh Receptors ↔️ Bind $ACh$ to initiate depolarisation
• Excitation-contraction coupling links electrical excitation with mechanical contraction in muscle fibres.
• Steps in excitation-contraction coupling

  1️⃣ Action potential spreads along T-tubules

  2️⃣ DHPRs are activated by depolarisation

  3️⃣ Ryanodine receptors release calcium

  4️⃣ Calcium binds to troponin, exposing myosin-binding sites

  5️⃣ Myosin binds to actin, initiating contraction
• T-tubules are invaginations of the plasma membrane.
• What is the role of the sarcoplasmic reticulum in muscle fibres?

  Stores and releases $Ca^{2 + }$
• Excitation-contraction coupling is followed by ATP hydrolysis and actin-myosin interaction.
• Steps in the sliding filament theory

  1️⃣ Calcium ions are released from SR

  2️⃣ Calcium binds to troponin, exposing myosin-binding sites

  3️⃣ Myosin heads bind to actin, forming cross-bridges

  4️⃣ Myosin heads pivot, pulling actin filaments

  5️⃣ ATP binds to myosin, causing detachment

  6️⃣ ATP hydrolysis re-energises myosin heads

  7️⃣ Cycle repeats as long as calcium and ATP are available
• Calcium ions bind to troponin on the actin filament.
• ATP is the main energy source for muscle contraction.
• ATP binding to myosin helps detach it from actin.
• Match the ATP use with the process in muscle contraction:

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

  Detaching myosin from actin ↔️ Binding of ATP to myosin
• What are the two types of filaments found in sarcomeres?

  Actin and myosin
• The A band in a sarcomere contains myosin filaments.
• 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