Muscle Contraction

• What type of heads do myosin filaments have?

  Globular heads that are hinged, so can move back and forth. Each myosin head has binding site for actin, and binding site for ATP.
• What do actin filaments contain?

  Binding sites for myosin heads (actin-myosin binding sites). Protein tropomyosin found between actin filaments- helps myofilaments move past each other.
• Why can't myofilaments slide past each other in resting muscles?

  For myosin & actin filaments to slide past each other, myosin head needs to bind to actin-myosin binding site on actin filament. In resting muscle, this site is blocked by tropomyosin.
• What happens when an action potential stimulates a muscle cell?

  Depolarises sarcolemma. Depolarisation spreads down T-tubules to sarcoplasmic reticulum. Causes sarcoplasmic reticulum to release stored Ca2+ into sarcoplasm. This influx of Ca2+ into sarcoplasm triggers muscles contraction. Ca2+ bind to protein attached to tropomyosin, causing it to change shape. Pulls tropomyosin out of actin-myosin binding site, allowing myosin head to bind to site. Actin-myosin cross bridge formed.
• How does the actin filament move?

  Ca2+ activates ATP hydrolase- hydrolyses ATP to provide energy for contraction. Energy released causes myosin head to bend, pulling actin filament along in a rowing action. Movement of myosin head to side= power stroke.
• What happens during the breaking of the cross-bridge?

  ATP provides energy to break cross-bridge, so myosin head detaches from actin filament after it's moved. Head returns to starting position, & reattaches to different binding site further along actin filament. Cycle repeated. Many bridges form & break rapidly, pulling actin filament along- shortens sarcomere, causing muscle to contract.
• What happens when the muscle stops being stimulated?

  Ca2+ leave their binding sites & are moved by active transport back into sarcoplasmic reticulum. Causes tropomyosin to move back & block binding sites again. Actin filaments slide back to relaxed position, which lengthens sarcomere.
• How does the ATP-phosphocreatine (PCr) system work?


  ATP made by phosphorylating ADP- adding phosphate from PCr. Makes ATP & creatine (Cr). System anaerobic and alactic. Some of Cr broken down into creatinine, which is removed via kidneys. Creatinine levels can be higher in those with high muscle mass & who exercise regularly. May also indicate kidney damage.
• What are slow-twitch muscle fibres?

  Contract slowly & can work for long time. Good for endurance activities. High proportions found in muscles used for posture. Energy released slowly through aerobic respiration. Many mitochondria & blood vessels to supply muscles with oxygen. Mitochondria mainly near edge of muscle fibres (short diffusion pathway). Rich in myoglobin, so fibre red in colour.
• What are fast-twitch muscle fibres?

  Contract quickly & get tired quickly. Good for short bursts of speed & power. High proportions in muscles used for fast movements. Energy released quickly via anaerobic respiration using glycogen. Stores of PCr. Few mitochondria or blood vessels. Not much myoglobin, so can't store much oxygen- gives fibre white colour.