Muscle

• neuromuscular junction:

  • an action potential arrives at the end of a motor neurone.
  • calcium ions are released causing vesicles containing acetylcholine to fuse with the membrane.
  • acetylcholine is released into the cleft.
  • ACh binds to receptors on the sarcolemma. Na+ channels open & Na+ depolarises the sarcolemma down T-tubules.
  • Ca2+ channels open & Ca2+ diffuse out of the sarcoplasmic reticulum.
  • Ca2+ binds to proteins in muscle fibres causing contraction.
  • ACh is rapidly broken down by AChase.
• Acetylcholine is broken down rapidly by acetylcholinesterase to ensure that the muscle contraction only occurs at impulses.
• neuromuscular junctions are from a neurone to sarcomere (muscle cell)
• neuromuscular junctions are only excitatory
• skeletal muscles:

  • can be moved voluntarily
  • is connected to bones via tendons
• muscles that move bones around joints work in antagonistic pairs.
• the contracting muscle is called the agonist and the relaxing muscle is called the antagonist
• fast twitch muscles:

  • fast contraction
  • high power contraction
  • short contraction time
  • good for short bursts
  • white in colour
  • energy from anaerobic respiration
• what colour are fast twitch muscle fibres?

  white
• do fast twitch muscles fibres use aerobic or anaerobic respiration?


  anaerobic
• slow twitch muscles:

  • slow contraction speed
  • low contraction power
  • long contraction time
  • good for long term activities
  • dark red in colour
  • energy from aerobic respiration
• what colour are slow twitch muscle fibres?

  dark red
• what kind of respiration do slow muscle fibres get energy from?

  aerobic
• slow twitch - aerobic respiration:

  • have large stores of myoglobin
  • have a rich supply of blood vessels
  • have many mitochondria
• fast twitch - anaerobic respiration:

  • have thicker fillaments
  • have a high concentration of glucose
  • have a high concentration of enzymes involved in anaerobic respiration
  • have high levels of phosphocreatine that generated ATP
• myosin = thick filaments
• actin = thin filaments
• myosin consists of long rod shaped tails with globular protein heads.

  thick filaments are made up of many myosin.
• actin consists of thin filaments made up of two strands twisted around each other forming a fibrous strand
• A bands = where actin and myosin overlap (dark bands)
• I bands = thin actin filaments only (light bands)
• H zone = thick myosin filaments only
• from one Z line to the next is a sarcomere
• the line between two I bands (light bands) are seperated by the Z line
• actin filaments have binding sites for myosin
• when muscle fibres are relaxed the myosin binding sites are covered by tropomyosin.
• skeletal muscle is made up of large bundles of long cells called muscle fibres
• the cell membrane of muscle fibre cells is called the sarcolemma
• a network of internal membranes run through the sarcoplasm called the sarcoplasmic reticulum.

  • the sarcoplasmic reticulum stores & releases calcium ions needed for muscle contraction.
• muscle fibres have lots of mitochondria to provide ATP needed for muscle contraction
• muscle fibres have many nuclei
• muscle fibres have long cylindrical organelles called myofibrils.

  They are made up of proteins & are highly specialised for contraction.
• a myofibril is made up of many shorter units called sarcomeres
• the middle of each sarcomere is an M line
• sliding filament theory:

  • myosin and actin filaments slide over one another to make the sarcomere contract.
  • when contracted the sarcomere gets shorter, the I band gets shorter & the H zone gets shorter, the A band stays the same length.
• myosin filaments have globular heads that are hinged so can move back and forth.
• each myosin head has a binding site for actin and for ATP
• actin filaments have binding sites for myosin called actin-myosin binding sites
• actin-myosin binding sites in resting / relaxed muscles are blocked by tropomyosin

  • so myofilaments cannot slide past each other because the myosin head cant bind to the actin-myosin binding site.
• Muscle contraction

  1. Action potential causes depolarisation of the sarcolemma, spreading down the T-tubules

  2. Sarcoplasmic reticulum releases Ca2+ which bind to a protein on tropomyosin causing a change shape

  3. Tropomyosin is off of the binding site

  4. Myosin head binds to actin filament forming an actin-myosin cross bridge

  5. Ca2+ activate enzyme ATP hydrolase which breaks down ATP releasing energy for muscle contraction

  6. Energy from ATP causes the myosin head to bend, pulling the actin filament along

  7. Another ATP provides energy to break the actin-myosin cross bridge, detaching the myosin

  8. The myosin head reattaches to a binding site further along the actin filament