Chapter 13 - Muscles

• What is skeletal muscle?

  • Make up the bulk of body muscle tissue
  • Responsible for movement e.g. biceps and triceps. Conscious control
  • Rapid contraction speed and short contraction length
  • Fibre appearance is striated and are mutlinucleated
  • Regular arrangement so muscle contracts in one direction

  
• What is cardiac muscle?

  • Muscle cells found in the heart. Myogenic (contract without the need for nervous stimulus); allow the heart to beat in a regular rhythm
  • Involuntary control
  • Intermediate contraction speed and intermediate contraction length
  • Fibre appearance is specialised striated, branched and uninucleated
  • Cells branch and interconnect resulting in simultaneous contraction

  
• What is involuntary (smooth) muscle?

  • Found in organs like the stomach, bladder and also found in walls of blood vessels and digestive tract
  • Involuntary control; slow contraction speed but can remain contracted for a long time
  • Fibre appearance is non-striated, spindle shaped and uninucleated
  • no regular arrangement - different cells can contract in different directions

  
• What are muscle fibres?

  • Skeletal muscles that consist of bunches of long, rod-shaped cells
  • These bundles are enclosed within a plasma membrane known as a sarcolemma
• What is the structure of muscle fibres?

  • Contain a number of nuclei
  • Longer than normal due to them fusing together, leads to increased strength
  • Cells share a cytoplasm called the sarcoplasm
  • Parts of the sarcolemma fold inwards (called T tubules) to help spread impulses through the sarcoplasm ensuring the whole fibre receives the impulse to contract at the same time
  • Have a lot of mitochondria for ATP needed for contraction
  • Contain sarcoplasmic reticulum which contain calcium ions needed for contraction
• What are myofibrils?

  • Each muscle fibre contains many myofibrils
  • Long cylindrical organelles made of protein and specialised for contraction
  • Collectively they provide a lot of force
  • Arranged parallel to provide maximum force when contracted together
• What are the 2 types of protein filaments that make up myofibrils?

  Actin = thinner filament that consist of 2 strands twisted around each other

  Myosin = thicker filament that consist of long rod-shaped fibres with bulbous heads that project to one side
• What is the structure of myofibrils?

  • Light bands = region where actin and myosin do not overlap (I band)
  • Dark bands = appear dark due to thick myosin filament edges. Overlap with actin (A band)
  • Z-line = found at the centre of each light band. Distance between adjacent Z lines is called the sarcomere which decreases when muscle contracts
  • H-zone = found in the centre of each dark band where only myosin filaments are present. Decreases when muscle contracts

  
• What happens during muscle contraction?

  • Myosin filaments pull the actin filaments inwards towards centre of sarcomere
  • Light band becomes narrower, Z-lines move closer together shortening sarcomere, H-zone becomes narrower. Dark band remains same width

  
• What is the structure of myosin?

  • Globular hinged heads; on head is binding site for actin and ATP
  • Tails align to form myosin filament

  
• What is the structure of actin?

  • Have binding sites for myosin heads but often blocked by protein tropomyosin which is held in place by troponin
  • When contracted, myosin heads from cross bridges with actin

  
• What is the neuromuscular junction?

  • Point of contact between a motor neurone and a skeletal muscle
  • Advantage of having many neuromuscular junctions is it ensures all fibres contract together producing more force; makes it faster
• What happens when an action potential reaches a neuromuscular juction?

  • Calcium ion channels open
  • Calcium ions diffuse from the synapse into synaptic knob where they cause synaptic vesicles to fuse with the presynaptic membrane
  • Acetylcholine is released into synaptic cleft by exocytosis and diffuse across synapse
  • Binds to receptors on postsynaptic membrane (sarcolemma) opening sodium ion channels and resulting in depolarisation
• How is acetylcholine broken down in muscles?

  • Acetylcholinesterase breaks it down into choline and ethanoic acid preventing overstimulation of muscle
  • Choline and ethanoic acid diffuse back into neurone where recombined back into acetylcholine using ATP.
• What happens in the sarcoplasm during an action potential?

  • Depolarisation of sarcolemma travels deep into muscle fibre by spreading through T-tubuls. In contact with sarcoplasmic reticulum which contains stored calcium ions
  • Action potential stimulates calcium ion channels to open and calcium diffuses down gradient into sarcoplasm
  • Bind to troponin causing changed shape. Pulls on tropomyosin moving it away from actin-myosin binding sites. Myosin head forms actin-myosin cross bridge
• What happens after the actin-myosin cross bridge is formed?

  • Myosin head changes angle pulling actin filament along. Molecule of ADP bound to head is released and an ATP can now bind to myosin head detaching it from actin filament
  • Calcium ions in sarcoplasm also activate the ATPase activity of myosin, hydrolyses ADP restoring myosin to original position
  • Myosin head can now attach to another actin-myosin binding site further along actin filament. Cycle continues as long as muscle is stimulated. Many bridges are formed and broken rapidly shortening the sarcomere and causing muscle contraction

  
• How does creatine phosphate work?

  • A way that the body regenerates ATP
  • Stored in the muscle and acts as a reserve supply of phosphate which is available immediately to combine with ADP reformign ATP rapidly
  • Used for intense short bursts of activity
  • When muscle relaxes creatine phosphate store is replenished using phosphate from ATP