15 Muscles

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  • Created by: lee8444
  • Created on: 26-02-20 11:32

Structure of muscle

  • Cardiac muscle - exclusively in the heart
  • Smooth muscle - in the walls of blood vessles and in the gut
  • Skeletal muscle - conscious control, attached to bones
  • Individual muscle fibres (myofibrils) are bound together to form a muscle
  • Grouped into small ropes which are bound together to form one big rope
  • Seperate cells are fused together into muscle fibres - they share nuclei and cytoplasm (sarcoplasm)
  • Large amounts of mitochondria and endoplasmic reticulum
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Microscopic structure of skeletal muscle

  • Myofibrils are made up of actin and myosin
  • Actin - thin, thwo strands twisted around eachother
  • Myosin - think, long rods with bulbous heads
  • This causes myofibrils to appear striped to due alternating bands
  • I bands - light bands, thick and thin do not overlap
  • A bands - dark bands, thick and thin do overlap here
  • H-zone - centre of A band, light
  • Z-line - centre of I band
  • Sarcomere - distance between Z-lines
  • Sarcomeres shorten during contraction
  • Tropomyosin - fibrous stand around the actin filament
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Slow/fast twitch

  • Slow twitch fibres
    • contract slowly
    • less powerful but over longer period
    • adapted for endurance e.g. marathon
    • common in calf muscle which are needed to stand up
    • good at aerobic respiration to avoid lactic acid build-up
    • large store of myoglobin - stores oxygen
    • rich supply of blood vessels to deliver oxygen and glucose
    • numerous mitochodria to make ATP
  • Fast twitch fibres
    • contract rapidly
    • powerful but over shorter periods
    • adapted for quick bursts e.g. weight lifting
    • common in biceps
    • thicker and numerous myosin filaments
    • high glycogen content
    • high concentration of enzymes needed for anaerobic respiration proving ATp rapidly
    • store of phosphocreatine which makes ATP from ADP in anaerobic conditions
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Neuromuscular junctions

  • This is where a motor neurone meets a skeletal muscle fibre
  • Multiple neuromuscular junctions per muscle for rapid contraction at the same time
  • All muscle fibres are supplied by a motor neurone so some can fire whilst some dont if a smaller reaction is needed
  • A nerve impulse is recieved down the motor neurone
  • Synaptic vesicles fuse with the presynaptic membrane and release acetylcholine
  • Acetylcholine diffuses to the postsynaptic membrane altering its permeability to sodium ions
  • This causes sodium ions to rapidly enter the muscle fibre depolarising the membrane
  • Acetylcholine is broken down by acetylcholinesterase so that the muscle isn't over-stimulated and the product is returned by diffusion to the motor neurone
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Neuromuscular junction and synapse comparison

  • Similarities with cholinergic synapses
    • have nerotransmittors which move via diffusion
    • have receptors for the neurotransmittor causing an influx of sodium ions
    • use a sodium-potassium pump to repolarise the axon
    • use enzymes to break down the neurotransmittor
  • Differences with cholinergic synapses
    • neuromusclar junctions are only excitatory
    • only links neurones to muscles
    • only motor neurones are involved
    • action potential ends here
    • acetylcholine binds to muscle fibre instead of postsynaptic neurone
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Evidence for the sliding filament mechanism theory

  • The sliding filament theory would cause the amount of overlapping actin and myosin to increase
  • Changes to the sarcomere
    • I-band becomes narrower
    • Z-lines move closer together (sarcomere shortens)
    • H-zone becomes narrower
  • A-band stays the same width as this is determined by the length of the myosin and they don't become shorter
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Sliding filament mechanism

  • 3 main proteins
    • Myosin - made from 2 proteins - fibrous protein tail, 2 globular and bulbous heads
    • Actin - globular protein whose molecules are aranged into long chains twisted to form a helical shape
    • Tropomyosin - long, thin threads wound around actin filaments
  • The theory states that actin and myosin filaments slide past each other
  • Bulbous heads on the myosin form cross-bridges with actin filaments by binding to binding sites on the actin
  • They flex in unison pulling the actin along the myosin
  • They then detach
  • ATP returns the head back to its original angle and they re-attach
  • This is repeated up to 100 times per second
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Muscle stimulation

  • Action potential reaches neromuscular junction
  • This opens calcium ion protein channels and calcium diffuses into synaptic knob
  • Calcium ions cause the synaptic vesicles to to fuse with pre-synaptic membrane
  • Acetylcholine is released into synaptic cleft
  • Acetylcholine diffuses across the cleft
  • It binds with receptors on the muscle cell-surface membrane
  • This depolarises the membrane
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Muscle contraction

  • Action potential travels deep into fibre through T-tubules which branch through the sarcoplasm
  • Tubules are in contact with endoplasmic reticulum (sarcoplasmic reticulum) which has actively transported calcium ions from the cytoplasm of the muscle leading to low calcium concentrations in the cytoplasm
  • Action potential opens calcium ions on the sarcoplasmic reticulum and calcium ions diffuse into the cytoplasm down the concentration gradient
  • Calcium ions cause trypomyosin that blocks actin binding sites to pull away
  • ADP molecules attached to the myosin heads mean they can bind to the actin and form a cross bridge
  • Once attached, the myosin heads change their angle pulling the actin along releasing a molecule of ADP
  • An ATP attaches to a myosin head causing it to detach from the actin
  • Calcium ions activate ATPase which makes ADP from ATP providing energy for the myosin head to return to its original angle
  • Myosin head now with an ADP can reattach itself to the actin and this process is repeated multiple times
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Muscle relaxation

  • Calcium ions are actively transported into back into the sarcoplasmic reticulum using energy from the hydrolysis of ATP
  • Reabsorption of calcium ions allows trypomyosin to block the binding sites again
  • Muscle can relax
  • The force from the antagonistic muscle will pull the muscle fibres back into their longer state
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Energy supply for muscle contraction

  • ATP is used for
    • the movement of myosin heads
    • reabsorption of calcium ions into the endoplasmic reticulum
  • Large demand for ATP
  • Most ATP is regenrated from ADP durinf respiration
  • This process requires oxygen
  • Phosphocreatine helps the muscle to respire anaerobically when there isn't enough oxygen
  • Phosphocreatine doesn't supply energy directly to the muscle but instead it regenerates ATP which is used directly for energy
  • It is stored in muscle and acts as a reserve supply of phosphate
  • It is replenished by ATP when the muscle is relaxed
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