Mechanism of Muscle Contraction (HL) (HL IB Biology)

The structure of skeletal muscle

• Muscles in the body that are attached to the skeleton and aid movement are called skeletal muscles
• Skeletal muscle is striated as it has a stripy appearance when viewed under a microscope
• Striated muscle cells are bundled up into fibres
• The fibres are highly specialised cell-like units
  • Each muscle fibre contains:
    • An organised arrangement of contractile proteins in the cytoplasm
    • Many nuclei – this is why muscle fibres are not usually referred to as cells
    • Specialised endoplasmic reticulum called the sarcoplasmic reticulum (SR) which stores calcium and conveys signals to all parts of the fibre at once using protein pumps in the membranes
    • Specialised cytoplasm called the sarcoplasm contains mitochondria and myofibrils
      • The mitochondria carry out aerobic respiration to generate the ATP required for muscle contraction
      • Myofibrils are bundles of actin and myosin filaments, which slide past each other during muscle contraction

Myofibrils

• Myofibrils are located in the sarcoplasm
• Each myofibril is made up of two types of protein filament:
  • Thick filaments made of myosin
  • Thin filaments made of actin
• These two types of filament are arranged in a particular order, creating different types of bands and lines

Structure of a Myofibril Diagram

Sliding Filament Model

• The thick filaments within a myofibril are made up of myosin molecules
  • These are fibrous protein molecules with a globular head
  • The fibrous part of the myosin molecule anchors the molecule into the thick filament
  • In the thick filament, many myosin molecules lie next to each other with their globular heads all pointing away from the M line
• The thin filaments within a myofibril are made up of actin molecules
  • These are globular protein molecules
  • Many actin molecules link together to form a chain
  • Two actin chains twist together to form one thin filament
  • A fibrous protein known as tropomyosin is twisted around the two actin chains
  • Another protein known as troponin is attached to the actin chains at regular intervals
• Muscles cause movement by contracting
  • During muscle contraction, myosin heads form cross-bridges by binding with sites on the actin filaments
  • The myosin heads then change orientation which pulls the actin filaments so that they slide next to the myosin.
  • This is called a power stroke
• Sarcomeres within myofibrils shorten as the Z lines are pulled closer together

Sliding Filament Model Diagram

Sarcomere Muscle Contraction Diagram

When the muscle contracts, the sarcomere shortens due to the sliding of the actin and myosin filaments.

• Muscles are only capable of contracting or pulling, they cannot push
• As a result of this limitation muscles generally operate in pairs
• A muscle pulls in one direction at a joint and the other muscle pulls in the opposite direction
  • This is described as antagonistic muscle action
• Muscles maintain posture by antagonistic muscles both contracting at joints to keep the joint at a certain angle
  • This is known as isometric contraction - a muscle contraction without motion
• Muscle contraction and relaxation relies on a protein called titin
  • Titin is a large protein that joins the ends of the myosin filaments to the z-line
  • The many folds in the titin molecule give spring like properties which aid muscle contraction
  • In a relaxed muscle, the sarcomere lengthens and the titin is stretched out
    • Titin stores chemical energy within the structure when it is stretched
    • The presence of titin prevents overstretching
  • During muscle contraction, the sarcomeres shorten and titin proteins recoil releasing the stored chemical energy
    • Energy released from titin adds to the force of the contraction

Sarcomere Relaxed Diagram

The sarcomere is relaxed and the titin spring like properties can be seen