23) Muscle structure and Contraction

• There are two different types of muscle:

  1. Skeletal
  2. Cardiac
• Skeletal muscles:

  • Long, Cylindrical, Multinucleate Cells with Striations
  • No gap junctions and units must be stimulated individually
  • Function: Voluntary movement
  • Attached to bones
• Cardiac Muscles:

  • Branched, Striated, Uniucleate cells
  • Has gap Junctions between cells so they contract as a unit
  • Has pacemaker cells
  • Function: Involuntary movement - Contracts to propel blood
  • Location: Walls of the heart
• Smooth muscle:

  • Elongate cells with central nuclei, no striations
  • Function: Propels substances along internal passages (eg large intestines)

  
• The endomysium is a thin layer of connective tissue that covers each muscle/fibre/cell
• The perimysium is connective tissue that surrounds and contains each bundle of muscle fibres (fasicles) together

  
• The epimysium is a connective tissue layer that covers the surface of the entire muscle

  
• Organisation level of muscles:

  Muscle (organ) ⟶ Fascicle (portion of the muscle)⟶ Muscle fibre (cell)
• Motor neurones carry impulses from the CNS to muscle fibres which are stimulated as independent units:

  Spinal cord ⟶ axon ⟶ muscle fibres ⟶ muscle contracts

  
• Muscle fibres are striated (stripy) and made up of contractile proteins

  ⤷ Actin = I band (thin)

      Myosin = A band (Thick)
• When muscles contract, components of the sarcomere slide between each other

  • This is why it appears darker when viewed under microscopes.  (rather than shorten) 
• Actin is made up of Globlar G actin subunits and form Fibrous F actin when polymerised.

  ⤷ Tropomyosin and Troponin act as regulatory proteins. 
• Myosin is a dimer (made up of the tail and head)

  ⤷ it has an actin-binding site at their tail, and a ATPase site at their head
• Muscles contract when an action potential arrives:

  1. Action potential (AP) reaches the axon terminal motor neurone. ⤷  ACh is released, releasing Na+ from channels

  2. Influx of Na+ leads to depolarisation of membrane

  3. AP travels down t-tuble which opens Ca2+ channels, stimulating a conformational change

  4. Ca2+ binds to troponin and cross bridge cycle takes place

  5. Sarcomere “shortens”, muscle contracts

  
• When AP stops, membrane is repolarised (returns to resting pot)

  ⤷ No more Ca2+ is released and any Ca2+ left is actively transported back into SR by a Ca2+ pump