Muscle structure

• muscles are effectors - they contract in response to nervous impulses
• there are three types of muscle in the body:

  • smooth muscle
  • cardiac muscle
  • skeletal muscle
• smooth muscle - contracts without conscious control - found in walls of internal organs (apart from the heart) e.g. stomach, intenstine and blood vessels
• cardiac muscle: - contracts without conscious control (like smooth muscle) but it is only found in the heart
• skeletal muscle: - (also called striated, striped or voluntary muscle) is the type of muscle used to move e.g. biceps and triceps move the lower arm
• Role of skeletal muscles:

  • attached to bones by tendons
  • ligaments attach bones to other bones to hold them together
  • pairs of skeletal muscles contract and relax to move bones at a joint
  • the bones of the skeleton are incompressible (rigid) so they act as levers giving the muscles something to pull against
• Antagonistic pairs:

  • muscles that work together to move a bone
  • contracting muscle = agonist
  • relaxing muscle = antagonist
• Biceps and triceps:

  • bones of the lower arm are attached to a biceps muscle and a triceps muscle by tendons
  • the biceps and triceps work together to move the arm - as one contracts the other relaxes
  • when biceps contracts the triceps relax - pulls the bone so the arm bends (flexes) at the elbow - biceps = agonist triceps = antagonist
  • when triceps contract biceps relax - pulls the bone so arm straightens (extends) at the elbow - triceps = agonist biceps = antagonist
• ligaments are bands of strong connective tissue
• muscles work in pairs bc they can only pull - they can't push
• Structure of skeletal muscle:

  • made up of large bundles of long cells, called muscle fibres
  • cell membrane of muscle fibre cells is called the sarcolemma
  • bits of the sarcolemma fold inwards across the muscle fibre and stick into the sarcoplasm (muscle cell's cytoplam)
  • these folds are called transverse (T) tubules and they help to spread electrical impulses throughout the sarcoplasm so they reach all parts of the muscle fibre
• • a network of internal membranes called the sarcoplasmic reticulum runs through the sarcoplasm
  • this stores and releases calcium ions that are needed for muscle contraction
• • muscle fibres have lots of mitochondria to provide the ATP needed for muscle contraction
  • they are multinucleate (contain many nuclei)
  • have lots of long cylindrical organelles called myofibrils
  • myofibrils are made up of protens and are highly specialised for contraction
• Examination under an optical microscope:

  • can look at skeletal muscle under an optical microscope
  • what you see will depend on how the sample has been stained an whether looking at a longitudinal or transverse cross-section
  • blue parts - nuclei
  • cross-striations (alternating darker and lighter pink stripes) are A-bands and I-bands of myofibrils
• • longitudinal cross-sections are taken along the length of a structure
  • transverse cross-sections cut through the structure at a right angle to its length
• Myofibrils:

  • contain bundles of thick and thin myofilaments that move past each other to make muscles contract
  • thick myofilaments are made of the protein myosin
  • thin myofilaments are made of the protein actin
• If looking at a myofibril under an electron microscope:

  • see patterns of alternating dark and light bands
  • dark bands contain the thick myosin filaments and some overlapping thin actin filaments - A bands
  • light bands contain thin actin filaments only - I bands
• A myofibril is made up of many short units called sarcomeres:

  • the ends of each sarcomere are marked with a Z-line
  • in the middle of each sarcomere is an M-line - the middle of the myosin filaments
  • around the M-line is the H-zone - only contains myosin filaments
• The sliding filament theory:

  • muscle contraction is explained by the sliding filament theory
  • where myosin and actin filaments slide over one another to make the sarcomeres contract
  • the myofilaments themselves don't contract
  • the simultaneous contraction of lots of sarcomeres means the myofibrils and muscle fibres contract
  • sarcomeres return to their original length as the muscle relaxes
• • A-band stays the same length
  • I-band gets shorter
  • H-zone gets shorter
  • Z-lines get closer together
  • sarcomeres get shorter