Skeletal Muscles

• Muscles act in antagonistic pairs against an incompressible skeleton.
• A skeletal muscle is made up of a bundle of muscle fibres.
• Each muscle fibre is a single cell enclosed by a sarcolemma.
• Each muscle fibre contains many microfibrils which are bundles of protein filaments.
• The microfibrils in a muscle fibre are arranged in sarcomeres.
• Each fibre is a single cell enclosed by a sarcolemma.
• The sarcolemma folds in forming T-tubules.
• There are nuclei and many mitochondria in the sarcoplasm.
• There are folded membranes similar to ER called sarcoplasmic reticulum.
• Each fibre is made up of many myofibrils organised into sarcomeres (contracting unit) myofibrils are organelles.
• Sarcomeres contain:

  • thick myosin filaments
  • thin actin filaments
  • structural proteins
• A muscle is made up of many muscle fibres. Each is a single cell enclosed in a specialised plasma membrane called a sarcolemma, which is folded forming T-tubules. The sarcoplasm contains a nucleus and many mitochondria to produce ATP needed for muscle contraction. There are also many organelles called myofibrils organised into contracting units or sarcomeres are made up of two types of protein filaments thick myosin and thin actin filaments.
• How Muscles Contract:

  • Action potential from motor neurone depolarises sarcolemma, spreads down T-tubules.
  • Causes sarcoplasmic reticulum to release Ca2+ ions.
  • Ca2+ ions bind to receptors and moves tropomyosin from the actin binding site.
  • Myosin head binds with actin to form actinomyosin bridges.
• • Myosin bridge changes its angle and delivers the 'powerstroke' thus sliding the actin further along the myosin filament.
  • ATP attaches to the head which detaches it from the binding site.
  • Ca2+ activates ATP hydrolase, which hydrolyses ATP to release energy for the 'recocking' of the myosin head back to its original position.
• and to relax...

  • Ca2+ ions actively transported back to sarcoplasmic reticulum, tropomyosin moves back to blocking the binding site.
  • actin filaments slide back to original position.
• The I band, is made up of only actin and decreases in size with contraction.
• The A band, this is made up of myosin, sometimes overlapped by actin. It's length doesn't decrease.
• The H zone, this is the part of the myosin filament not overlapped with actin. It's length decreases with contraction.
• The Z lines, this is where actin fibres meet of adjacent sarcomeres. They demark the ends of the sarcomeres.
• The M line is in the centre of the myosin fibres. It anchors them.
• The proportions of slow to fast muscle vary between everyone.
• Slow twitch fibres contract more slowly for longer but with less power. Adapted for endurance work such as marathon running.
• Slow twitch muscles are adapted for aerobic respiration with:

  • a large amount of myoglobin.
  • a large number of mitochondria found at the edge of the muscle fibre.
  • short diffusion pathway.
• Fast twitch fibres contract quickly for a short period of time with more power. Adapted for intense work such as weight lifting.
• Fast twitch muscle is adapted for anaerobic respiration by having a large amount of glycogen and a store of phosphocreatine.