muscular system

• Types of muscles


  Smooth muscle - unstriated and involuntary

  Cardiac muscle - striated and involuntary

  Skeletal muscle - striated and voluntary
• Skeletal muscle


  Muscle -> bundles of muscle fibres = fascicle -> muscle fibre (single muscle cell) -> myofibrils -> myofilaments
• Myofibril


  Has a thin (actin) filament and a thick (myosin) filament.
• Sarcolemma


  Cell membrane is composed of the sarcolemma and basement membrane. It fuses with tendons which connects the muscle to bone. Assists with the transmission of action potentials along the muscle and transports metabolites in and out of the cell.
• Sarcoplasmic reticulum

  Specialised smooth endoplasmic reticulum in skeletal muscle, network of tubules tun along and around myofibrils. Storage site of calcium. Network of tubules that run along and around myofibrils.
• Transverse tubules (T-tubules)


  Extensions of the sarcolemma that pass laterally through the cell. It allows action potentials to be transmitted into the myofibrils.
• Sliding filament theory


  The interaction of myofilaments causes muscles to contract/shorten. Myofilaments slide past one another = the sarcomere shortens = the myofibril shortens. When a sarcomere is shortens, myofibril, muscle fibre, muscle fascicles and entire muscle shorten to produce muscular contraction.
• Myosin


  Thick filaments are bundles of myosin molecules. Each globular 'head' of the mysoin has:

  • Myosin ATPase site - breaks down ATP
  • Actin binding site - actin molecule can bind to myosin
• Actin


  Thin filaments are paired chains of actin molecules each with a myosin binding site.
• Sliding filament theory

  At rest, the binding sites are covered by the regulatory proteins - troponin and tropomysoin.
• Role of calcium

  At rest, myosin and actin are unable to bind due to troponin and tropomyosin. Tropomyosin covers the binding sites on actin and troponin holds tropomyosin in place. Troponin is the 'lock' that keeps binding sites inaccessible and calcium is the 'key'.
• Sliding filament theory


  Exposure of active sites - calcium binds to troponin causing it to pull tropomyosin of the myosin binding site on the actin, opening up the active site. The myosin head has ADP and iP. Energy is stored in the myosin head.

  Cross bridge formation - attachment of myosin and actin. The iP leaves the myosin head.

  Power stroke - energy stored in the myosin head is used to cause the power stroke. The myosin head move inwards and pulls actin to the m-line of the sarcolemma.
• Sliding filament theory


  Cross bridge release - an ATP molecule binds to each head and causes detachment of myosin from actin.

  ATP to ADP and P - hydrolysis of ATP. The myosin ATPase portion of the myosin heads split ATP into ADP and P, which remain attached to the myosin head.

  Recovery stroke - the myosin head returns to resting position and energy is stored in the head. If calcium is still attached to troponin, the cycle starts again. This cycle occurs as many time during a muscle contraction. Not all cross bridges form and release simultaneously.
• Sliding filament theory


  Calcium binds to troponin. It changes shape, moves tropomyosin aside and exposes the myosin binding site. Cross bridging occur resulting in attachment of myosin and actin.
• Role of ATP


  Muscle contraction is an active process, therefore it requires energy in the form of ATP. Myosin contains a binding site for ATP. In a relaxed state, the energised myosin heads are cocked back with ADP and iP attached.

  Power stroke - iP and energy is release causing myosin head to pivot, pulling actin towards the centre of sarcomere, ADP is released from myosin.

  Cross bridge release - a new ATP binds to myosin head to release it from actin.

  Recovery stroke - ATP is broken down by myosin ATPase to form ADP and iP and energy to re-energise and re=position the head of myosin.
• Role of ATP


  Without ATP:

  • Myosin and actin remain bound
  • In rigor mortis, the muscles stiffen because there is no ATP to release the bond

  When neural muscle stimulation ceases:

  • Calcium actively transported back into the sarcoplasmic reticulum using ATP
  • Tropomyosin will cover binding sites on actin

  Muscle relaxation requires energy supply by ATP.
• Role of ATP


  Functions that require ATP within the muscles:

  • Power stroke
  • Release of myosin from actin after power stroke
  • Transporting calcium back to sarcoplasmic reticulum
• Motor unit


  Alpha-motor neuron and all the muscle fibres it innervates.
• Excitation-contraction coupling mechanism


  Includes all of the sequences of events that trigger a muscle to contract:

  • Excitation of a motor nerve
  • Propagation of an action potential
  • Events at the neuromuscular junction and transfer of action potential
  • Calcium release from sarcoplasmic reticulum
  • Sliding of filaments
  • Muscular contraction