Mechanism of Muscle Contraction (DP IB Biology)

Sliding Filament Model

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.

Muscle Relaxation

• 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