Skeletal muscles

• Name the three types of muscle in the body and where they are located

  Cardiac = exclusively found in heart

  Smooth = walls of blood vessels and intestines

  Skeletal = attached to incompressible skeleton by tendons
• What does the phrase antagonistic pair of muscles mean

  ~Muscles can only pull, so they work in pairs to move bones around in joints

  ~Pairs pull in opposite directions = agonist contracts while antagonist is relaxed
• Describe the gross structure of skeletal muscle

  ~Muscle cells are fused together to form bundles of parallel muscle fibres (myofibrils)

  ~Arrangement ensures there is no point of weakness between cells

  ~Each bundle is surrounded by endomycium = loose connective tissue with many capillaries
• Describe the microscopic structure of skeletal muscle

  Myofibrils = site of contraction

  Sarcoplasm = shared nuclei and cytoplasm with lots of mitochondria and endoplasmic reticulum

  Sarcolemma = folds inwards towards sarcoplasm to form transverse tubules
• How is muscle contraction stimulated

  ~Neuromuscular junction = actional potential = voltage gated Ca2+ channels open

  ~Vesicles move towards and fuse with presynaptic membrane

  ~Exocytosis of acetylcholine (ACh), which diffuses across synaptic cleft

  ~ACh binds to receptors on Na+ channel proteins on skeletal muscle cell membrane

  ~Influx of Na+ = depolarisation
• Explain the roe of Ca2+ ions in muscle contraction

  ~Action potential moves through T-tubules in the sarcoplasm = Ca2+ channels in sarcoplasmic reticulum open

  ~Ca2+ binds to troponin, triggering conformational change in tropomyosin

  ~Exposes binding sites on actin filaments so actinomyosin bridges can form
• Outline the sliding filament theory

  ~Myosin head with ADP attached forms cross bridges with actin

  ~Power stroke = myosin head changes shape and loses ADP, pulling actin over myosin

  ~ATP attaches to myosin head, causing it to detach from actin

  ~ATPase hydrolyses ATP - ADP +Pi so myosin head can return to original position

  ~Myosin head re-attaches to actin further along filament
• How does sliding filament action cause a myofibril to shorten

  ~Myosin heads flex in opposite directions = actin filaments are pulled towards each other

  ~Distance between adjacent sarcomere Z lines shortens

  ~Sliding filament action occurs up to 100 times per second in multiple sacromeres
• State four pieces of evidence that support the sliding filament theory

  ~H-zone narrows

  ~I-band narrows

  ~Z-lines get closer

  ~A-zone remains same width
• What happens during muscle relaxation

  ~Ca2+ is actively transported back into endoplasmic reticulum

  ~Tropomyosin once again blocks actin binding site
• Explain the role of phosphocreatine in muscle contraction

  Phosphorylates ADP directly to ATP when oxygen for aerobic respiration is limited
• How could a student calculate the length of one sarcomere

  ~View thin slice of muscle under optical microscope

  ~Calibrate eyepiece graticule

  ~Measure distance from middle of one light band to middle of another
• Where are slow and fast twitch muscle fibres found in the body

  Slow twitch = sites of sustained contraction

  Fast twitch = sites of short-term, rapid, powerful contraction
• Explain the role of slow and fast twitch muscle fibres

  Slow twitch = long-duration contraction, well-adapted to aerobic respiration to prevent lactate buildup

  Fast twitch = powerful short-term contraction, well-adapted to anaerobic respiration
• Explain the structure and properties of slow-twitch muscle fibres

  ~Glycogen store = many terminal ends can be hydrolysed to release glucose for respiration

  ~Contain myoglobin = higher affinity for oxygen than haemoglobin at lower partial pressures

  ~Many mitochondria - aerobic respiration produces more ATP

  ~Surrounded by many blood vessels = high supply of oxygen and glucose
• Explain the structure and properties of fast twitch muscle fibres

  ~Large store of phosphocreatine

  ~More myosin filaments

  ~Thicker myosin filaments

  ~High concentration of enzymes involved in anaerobic respiration

  ~Extensive sarcoplasmic reticulum = rapid uptake and release of Ca2+
• What is a motor unit

  One motor neuron supplies several muscle fibres, which act simultaneously as one functional unit