Muscle

• Sliding filament theory

  1. Action potential arrives at the neuromuscular junction

  2. Depolarises the sarcolema and spreads down the T-tubules to the sarcoplasmic reticulum

  3. Calcium ions are released from the sarcoplasmic reticulum and they bind to troponin causing it to change shape

  4. Troponin and tropomyosin change position in the actin filament as troponin displaces and exposes the myosin binding site on the actin molecule

  5. Bulbous/globular heads of the myosin molecule attach to the binding site and form cross-bridges between actin and myosin

  6. Myosin heads bound to the actin filament undergo a configurational change in shape and this bending causes ADP + Pi to be released

  7. Myosin head performs a power stroke and causes actin filaments to slide past the stationary myosin (towards the centre)

  8. ATP binds to myosin heads, displaces ADP and breaks the crossbridge, changing the shape of myosin head

  9. Calcium ions activate calcium ATPase which hydrolyses the ATP, this provides energy to 're-cock' the myosin heads

  10. Myosin head re-attaches to the binding steps on the actin filament so cross-bridges form further along the muscle fibre

  11. Process continues and many cross-bridges form and break, shortening the sarcomere and contracting the muscle

  12. Once muscle in no longer stimulated, calcium ions are re absorbed by active transport to the sarcoplasmic reticulum

  13. Troponin reverts to original shape, tropomyosin moves and blocks the binding site

  14. Sarcomere lengthens as actin filaments slide back
• what four proteins are involved in the sliding filament theory?

  troponin, tropomyosin, myosin and actin
• in the sliding filament theory thin filaments slide past stationary thick filaments
• fast-twitch muscle fibres


  -contract rapidly and work under high intensity, short term work

  -thicker more numerous myosin heads

  -myosin heads bind and unbind quicker so the muscle contracts quicker over a shorter period of time

  -large amounts of calcium ions are required

  -rely on anaerobic respiration so fatigue easily due to lactate production

  -glycogen stores release energy quickly

  -no/few myoglobin present

  -few mitochondria and capillaries
• why do fast-twitch muscles have few mitochondria and capillaries?

  -no need for an oxygen supply and anaerobic respiration occurs in the cytoplasm
• why are fast-twitch muscle fibres paler in colour?

  -no/few myoglobin present as there is no need for an oxygen storage molecule
• where are fast-twitch muscles used and needed for

  -high intensity such as sprinting

  -high proportion in human eyelids

  -limbs of animas that hunt or flee predators at high speed
• Slow twitch muscle fibres


  -contract slowly and work in moderate intensity/endurance

  -contract sower over a longer period of time

  -rely on aerobic respiration so fatigue less quickly due to no lactate production

  -little glycogen stores as aerobic respiration provides plenty of ATP (38 molecules)

  -large myoglobin stores to increase the rate of oxygen supply

  -dense network of capillaries and lots of mitochondria
• why do slow twitch fibres contain a dense network of capillaries and lots of mitochondria?

  -mitochondria is the site of aerobic respiration

  -capillaries produces a rich supply of oxygen with a short diffusion distance
• why do slow twitch muscle fibres appear red in colour?

  -due to many myoglobin stores
• Where and when are slow twitch muscles are used?

  -high proportion in human back muscles

  -limbs of animals that migrate and stalk prey over long distances

  -moderate intensity such as walking
• what are muscles?

  effector organs that respond to nervous stimulation by contracting and so they bring about movement
• what are the three types of muscles?

  cardiac, smooth and skeletal
• where are cardiac muscles found?

  exclusively in the heart
• where are smooth muscles found?

  Internal organs, blood vessels, and the digestive tract.

  (walls of blood vessels and the gut)
• what are skeletal muscles?


  -muscle that make up the bulk of body muscles in vertebrates and are attached to bones

  -act under voluntary, conscious control
• Skeletal muscle


  -under voluntary control and concerned with skeletal movement

  -effector in response pathways

  -activated by motor neurones

  -acetylcholine is the neurotransmitter at the end of all muscle activity
• smooth muscle


  -under involuntary control

  e.g arterioles with vasoconstriction and vasodilation
• Skeletal muscle force


  -exert force by contracting and shortening

  -anchored to bone by tendons which leads to pulling action

  -when a muscle relaxes the other is contracting and pulling in a direction

  -ANTAGONISTIC PAIRS
• what happens when both antagonistic pairs contract?

  joints lock up and you cannot move
• cytoplasm in muscle


  sarcoplasm
• myofibrils


  -millions of tiny muscle fibres

  -no force selectively but are strong collectively

  -lined parallel to each other to give max force

  -muscle is composed of smaller units bundled into progressively larger ones
• why do fast twitch muscles store phosphocreatine?


  -molecule that can rapidly generate ATP from ADP in anaerobic conditions and provide energy for muscle contraction
• cell membrane 


  sarcolemma
• endoplasmic reticulum


  sarcoplasmic reticulum
• skeletal cells are multinucleate
• muscles act in antagonistic pairs against an incompressible skeleton
• contracting muscle= agonist

  relaxing muscle= antagonist
• strong connective tissues (tendons) connect muscle to bones and ligaments attach bones to bones
• I band


  only thin actin filaments
• H zone


  only thick myosin filaments
• A band


  contains thick myosin filaments and overlapping thick and thin filaments (darkest region)
• M line


  middle of each sarcomere
• Z line


  ends of each sarcomere
• thin filaments 


  -made of actin molecules twisted into an alpha helix

  -globular protein

  -2 chains form one actin filaments
• Thick filament


  -fibrous proteins

  -globular heads
• creatine phosphate 


  -combination of Pi with creatine

  -can be stored in the cell and so when lots of ATP is required quickly for contraction it is hydrolysed to release the Pi which is combined with ADP

  ADP + Phosphocreatine —> Creatine + ATP

  allows contraction of muscles to continue continuously for a short time until mitochondria provides ATP
• some creatine is broken down into creatinine which is removed via the kidneys

  high creatinine levels are present in those who exercise regularly but can also be a show of kidney failure
• ATP made at rest is hydrolysed into ADP and Pi