contraction

• the sliding filament theory explains how muscle contraction occurs. When muscles contract, sarcomeres shorten because actin and myosin filaments slide over one another 
• Z line is a protein disc in the middle of the thin filament. the distance between successive Z lines is a sarcomere
• A band is the length of thick filament in one sarcomere. the thin and thick filament overlap at the ends
• H zone is the space between the thin filaments of one sarcomere. it is in the middle of the A band and contains only thick filaments, making it lighter in colour
• I band is the distance between successive thick filaments. it contains only thin filament
• when contracting :

  • actin slides over myosin
  • sarcomere shortens
  • Z lines become closer
  • A band does not change length
  • H zone shortens
  • I band shortens
• Muscles are attached to bones by tendons, a fibrous, inelastic connective tissue
• muscles can only contract, they can only pull a bones together, not push them apart. therefore, muscles that move bones come in pairs, they are said to be antagonists. One muscle moves the bone in one direction, the other in the opposite
• origin is the end of the muscle that is fixed to the stationary bone

  e.g. biceps - scapula

  triceps - scapula and humerus
• Insertion is the end of the muscle fixed to the movable bone

  e.g. biceps - radius

  triceps - ulna
• Belly is the fleshy portion of the middle of a muscle
• agonist or prime mover is the muscle that causes the desired action

  e.g. when flexing arm - bicep

  when extending arm - tricep
• antagonist is the muscle that has an action opposite of the agonist

  e.g. when flexing arm - tricep

  when extending arm - bicep
• most joint movements also involve muscles that act as synergists which help the prime mover. It either produces the same movement as the agonist or steadies a joint during a particular movement so unwanted movement is prevented, allowing the agonist to function more effectively
• when synergists immobilises a joint, it is called a fixator. it acts as a stabiliser of one part of the body during the movement of another part.

  e.g. synergistic muscles immobilise the wrist so it does not flex when clenching fingers as the muscles that curl fingers also pass across the wrist

  muscles hold scapula against the chest so only the insertions are moved when arms muscle contract
• muscle tone is maintaining partial contraction of skeletal muscle
• at any time, some fibres are contracted while others are relaxed. it causes muscles to be tightened, but not enough fibres are contracted to produce movement. it is not the repeated contraction of the same fibre, different fibres take turns to contract. the fibres relieve one another smoothly so that it can be kept up for long periods of time
• muscle tone hold body parts in position :

  • head is held up by partial contraction of neck muscles
  • Posture is dependent on muscle tone
• structure of myofilament

  A) H zone

  B) Z line

  C) sarcomere

  D) thin filament

  E) thick filament

  F) M line

  G) I band

  H) A band