3.6.3 Skeletal muscles are stimulated to contract by nerves

• Fast twitch (skeletal) muscles are used during short-term or intense exercise.
• Slow twitch (skeletal) muscle fibres are used during long term exercise.
• 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.
• 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.
• Actin attaches to the Z line at the end of the sarcomere.
• ATPase breaks down ATP.
• Tropomyosin covers the binding site on actin in a relaxed myofibril.
• Phosphocreatine combines with ADP to form ATP.
• Skeletal muscles are attached to bones and are under voluntary control.
• Sequence of events at the Neuromuscular Junction (NMJ):

  1. Action potential arrives at the motor neuron end.
  2. Voltage-gated calcium channels open, Ca2+ enters.
  3. Vesicles fuse with presynaptic membrane and release acetylcholine (ACh).
  4. ACh diffuses across synapse and binds to receptors on the sarcolemma.
  5. Sodium ion channels open, Na+ enter the muscle cell → depolarisation.
  6. This spreads across the sarcolemma and down T-tubules to the sarcoplasmic reticulum (SR).
  7. SR releases Ca2+ ions, which trigger contraction.
• Sliding Filament Theory:

  1. Calcium ions bind to troponin, causing tropomyosin to move, exposing binding sites on actin.
  2. Myosin heads bind to actin = cross-bridge formed.
  3. Power stroke: Myosin head pulls actin → ADP + Pi released.
  4. New ATP attaches, myosin head detaches from actin.
  5. ATP is hydrolysed → myosin head returns to original position.
• Muscle Relaxation:

  • Ca2+ actively transported back into sarcoplasmic reticulum
  • Tropomyosin moves back, blocking actin binding sites
  • Cross-bridges no longer form