11.2 Movement

• Skeletons function to provide support and protection for body organs

  • Skeletons can be internal (endoskeletons) or external (exoskeletons)
  • Skeletons provide a surface for muscle attachment and thus facilitate the movement of an organism
  • Bones act as levers, moving in response to muscular contraction
  • Bones are connected to other bones by ligaments, and bones are connected to muscles by tendons 
• Skeletal muscles exist in antagonistic pairs (when one contracts, the other relaxes) to enable opposing movements 

  • eg, flexion vs extension, abduction vs adduction, protraction vs retraction
  • Many insects have hind legs for jumping so that when the flexor muscle contracts (extensor relaxes) the tibia and femur are brought together (prepare for jump) and when flexor muscle relaxes, extensor muscle contracts and tibia and femur are straightened (jump)
  • In humans, biceps contract and tricep relax when arm is pulled up but bicep relax and tricep contract when arm is straightened
• • Synovial joints are capsules that surround where the bones connect and are made of:
  • Joint capsule – Seals the joint space and provides stability by restricting the range of movement
  • Cartilage – Lines the bone surface to facilitate smoother movement, shock absorbent
  • Synovial fluid – Provides oxygen and nutrition to the cartilage, as well as reducing friction
  • There are six main types of synovial joints that allow for different ranges of movement, which are (in order of mobility):
  • Plane joints, hinge joints, pivot joints, condyloid joints, saddle joints, ball and socket joints
• Synovial Joints

  
• Humerus - anchors muscle

  Radius - lever for bicep

  Ulna - lever for tricep

  Bicep - bends arm

  Tricep - straightens arm

  
• • Skeletal muscles consist of tightly packaged muscular bundles surrounded by connective tissue
  • Each bundle contains multiple muscle fibres, which are formed when individual muscle cells fuse together
  • Muscle fibres contain tubular myofibrils that are responsible for muscular contraction
  • The myofibrils can be divided into repeating sections called sarcomeres, each of which represents a single contractile unit 

  
• Striated muscle fibre:

  • They are multinuclear (formed from the fusion of individual muscle cells)
  • They have a large number of mitochondria (muscle contraction requires ATP hydrolysis)
  • They have a specialised endoplasmic reticulum (sarcoplasmic reticulum, stores calcium ions)
  • They contain tubular myofibrils made up of two different myofilaments – thin filament (actin) and thick filament (myosin)
  • The continuous membrane surrounding the muscle fibre is called the sarcolemma and contains invaginations called T tubules

  
• Sarcomeres:

  • The thick myosin filament contains small protruding heads which bind to regions of the thin actin filament
  • Each individual sarcomere is bordered by dense protein discs called Z lines, which hold the myofilaments in place
  • The centre of the sarcomere appears darker due to the overlap of both actin and myosin filaments (A band)
  • The dark A band may also contain a slightly lighter central region where only the myosin is present (H zone)
  • The edges of the sarcomere appear lighter as only actin is present in this region (I band)

  
• 1. Action potential triggers release of acetylcholine, depolarisation in sarcolemma, sarcoplasmic reticulum releases calcium ions

  2.  On actin, binding sites for myosin heads are covered by troponin and tropomyosin, calcium ions bind to troponin and reconfigure complex, binding site is exposed for myosin heads

  3. ATP binds to the myosin head, ATP hydrolysis causes the myosin heads to bind to actin, myosin attaches to new actin (row boat)

  4. As actin filaments are anchored to Z lines, dragging of actin pulls Z lines closer together, shortening sarcomere and muscle fibre contracts