3.6.3 Skeletal muscles are stimulated to contract by nerves

• Damage to myelin sheaths of neurons can lead to problems controlling the contraction of muscles. Suggest one reason why. (2)


  • Action potentials travel more slowly
  • So delay in muscle contraction / muscles don’t contract / muscles contract slower
• Cannabinoid receptors are found on the pre-synaptic membrane of neuromuscular junctions. When a cannabinols binds to its receptors, it closes calcium ion channels. (4)

  • Prevents influx of calcium ions into pre-synaptic membrane
  • Synaptic vesicles don’t fuse with membrane / vesicles don’t release neurotransmitter
  • Neurotransmitter does not diffuse across synapse / does not bind to receptors on post-synaptic membrane
  • No action potential / depolarisation (of post-synaptic membrane) / sodium ion channels do not open / prevents influx of sodium ions.
• Describe the role of ATP in muscle contraction. (2)

  • to break actinomyosin bridges
  • to move / bend the myosin head
  • so actin filaments are moved inwards
  • for active transport of calcium ions into the sarcoplasmic / endoplasmic reticulum
• Fast twitch (skeletal) muscles are used during short-term or intense exercise.
• Slow twitch (skeletal) muscle fibres are used during long term exercise.
• What is the role of ATP in myofibril contraction (2)

  • reaction with ATP allows the binding of myosin to actin / breaks actinomyosin bridge
  • provides energy to move myosin head
• Describe the role of calcium ions and ATP in muscle contraction (5)

  • Ca2+ uncovers binding site on actin
  • Ca2+ binding to troponin / moving tropomyosin
  • allows myosin heads to attach to actin filaments
  • allows sliding of actin and myosin filament
  • binding of ATP causes myosin head to detach from actin filament
  • hydrolysis of ATP releases energy
  • which changes the configuration of myosin head
• Actin and myosin are two protein filaments that make up the structure of a sarcomere.
• Calcium ions bind to troponin, causing tropomyosin to move and expose myosin-binding sites on actin filaments.
• Phosphocreatine is a molecule that provides a rapid source of energy by regenerating ATP during intense muscle activity.
• Use your knowledge of how myosin and actin interact to suggest how the myosin molecule moves the mitochondrion towards the presynaptic membrane. (2)

  • myosin head attaches to actin and performs power stroke
  • pulls mitochondria past/along the actin
  • next myosin head attaches to actin and performs power stroke
• Suggest and explain one advantage of the movement of mitochondria towards the presynaptic membrane when nerve impulses arrive at the synapse. (2)

  • mitochondria supply ATP / energy
  • to move vesicles / for active transport of ions / for myosin to move past actin
  • re-synthesis and absorption of neurotransmitter
• Slow-twitch muscle fibres contract slowly and for longer periods of time, so they fatigue slowly. They are used for posture and endurance activities.
• Slow-twitch muscle fibres have a high density of mitochondria so rely on aerobic respiration for energy. They have a high concentration of myoglobin, a red protein which stores oxygen so appear dark in colour.
• Slow-twitch muscle fibres have small stores of glycogen and phosphocreatine, while fast-twitch muscle fibres have large stores, which are used for quick bursts of energy.
• Fast-twitch muscles contract quickly but relax rapidly. They fatigue easily and are used for rapid movements, such as sprinting.
• Fast-twitch muscle fibres have a low density of mitochondria so rely on anaerobic respiration for energy. They have a low concentration of myoglobin.
• Skeletal muscles, which contain both slow-twitch and fast-twitch fibres, work in antagonistic pairs to allow movement.
• Myofibrils are composed of two types of protein filaments: thick filaments made of myosin and thin filaments made of actin
• The repeating unit within a myofibril is called a sarcomere.
• The Z line is located at the end of the sarcomere and it is where sarcomeres are joined together.
• A band: The dark band under a microscope, representing the entire length of the myosin filament.
• I band: The light band, consisting of actin filaments only.
• H-zone: A lighter region within the A band, containing only myosin filaments and no overlapping actin.
• M-line: Located at the centre of the sarcomere, it serves as an attachment point for myosin filaments.
• Myofibrils are located in the sarcoplasm, the cytoplasm of muscle fibres.
• When sarcomeres contract, the A band remains the same length since myosin filaments do not change length during contraction.
• When sarcomeres contract, the I band shortens in length as the actin filaments slide towards the centre of the sarcomere, overlapping more with the myosin filament.
• When muscle fibre (sarcomeres) contract, the H band shortens in length because the actin filaments slide inwards during contraction, they fill in the H-zone, causing it to shorten and eventually disappear at full contraction.
• The sarcoplasm is the muscle fibre cytoplasm.
• The sarcolemma folds inwards to the sarcoplasm at certain points. The inwards folds are called transverse (T) tubules.
• The sarcoplasmic reticulum (SR) is an organelle in the sarcoplasm. It is a store of calcium ions.
• Muscle fibres have many mitochondria and nuclei. The mitochondria provide lots of ATP to power muscle contraction.
• Slow twitch fibres are found in muscles in the back and neck.
• Fast twitch fibres are found in muscles in the arms and legs.
• A neuromuscular junction is a synapse between a motor neuron and a muscle fibre.
• During intense exercise, the sarcoplasmic reticulum of muscle fibres becomes less efficient at reabsorbing calcium ions. Explain how this could affect muscle contraction and relaxation. (4 marks)

  • Calcium ions remain in the cytoplasm for longer, binding to troponin continuously.
  • Tropomyosin remains displaced, keeping actin-binding sites exposed.
  • This prolongs muscle contraction and delays relaxation.
  • Muscle fatigue occurs as the fibres cannot reset properly.
• Explain the importance of acetylcholinesterase in the neuromuscular junction. (2 marks)

  • Breaks down acetylcholine in the synaptic cleft.
  • Prevents continuous stimulation of the muscle fibre, ensuring muscle relaxation.
• Describe the sequence of events that occurs at the neuromuscular junction leading to muscle contraction. (4 marks)

  • Action potential arrives at the presynaptic membrane.
  • Voltage-gated calcium ion channels open, and calcium ions enter the presynaptic neuron.
  • Synaptic vesicles release acetylcholine into the synaptic cleft via exocytosis.
  • Acetylcholine binds to receptors on the sarcolemma, opening sodium ion channels, leading to depolarization of the muscle fibre.
• What is the role of calcium ions in muscle contraction?

  • They bind to troponin, causing tropomyosin to move.