Gross structure to fine structure

What’s in a muscle?

  • Muscle cells are called muscle fibres

Muscle → Muscle fibre → Myofibrils → Sarcomere → Actin and myosin

Skeletal muscle

  • Looks stripy due to protein filaments

  • Contain two protein called actin and myosin

  • Each muscle is made of muscle fibres

  • The muscle fibres are the same as most cells containing cytoplasm, mitochondria and endoplasmic reticulum

  • The muscle fibres contain many myofibrils, which are made up of sarcomeres

  • Very long, sometimes called acellular as they don’t seem to have distinct cell membranes and may have many nuclei (multinucleate)

  • Each cell is quite complex

    • Each cell has a surface membrane is called sarcolemma and cytoplasm is called sarcoplasm 

    • Large number of mitochondria = lots of ATP

    • Inside each fibre cell there are many longitudinally running fibres called myofibrils and it’s these that give the characteristic striations (stripes)

  • The sarcoplasm is interwoven with special sarcoplasmic reticulum (modified endoplasmic reticulum) and also infoldings of the sarcolemma called the T system or T tubules

  • These are important because they wrap around the myofibrils and are important in muscle contraction

Fine structure of muscles

A sarcomere

Sarcomeres

  • The markings of the myofibrils correspond to regular repeating units called sarcomeres

  • The sarcomere consists of different protein filaments, thin ones (actin) and thick ones (myosin)

  • The sarcomeres are joined end to end by thin plates called Z lines or Z plates

  • The filament form bands of differing shades

  • During contraction the filaments slide together to form a shorter sarcomere

  • Cross bridges are formed

  • Z line (Mark the end of sarcomere and are regions where actin fibres cross link)

  • A band or Dark band represents the length of the thicker myosin strands. (but the actin also overlaps here)

  • The I band or light band contains only actin

  • The H zone inside the A band is myosin only

  • The M line is a protein that holds the myosin together (like the Z line for actin)

Actin:

  • Thin filaments (8nm diameter)

  • Has myosin binding sites

  • Individual molecules are helical (2 of them)

  • 2 regulatory proteins called Tropomyosin and Troponin are associated with it

  • Tropomyosin normally blocks the myosin binding site

  • Troponin can bind to calcium ions

Myosin:

  • Thich filaments (16nm)

  • Contain myosin heads that can bind to actin

  • Individual molecules have a common shaft and a protruding bulbous head

  • The heads can make cross bridges and contain ATP binding sites and an enzyme called ATPase.

Troponin and tropomyosin

  • Tropomyosin molecules coil around the actin

  • Troponin complex is attached to each tropomyosin