SKELETAL MUSCLES

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Cards (48)

  • Skeletal muscles are stimulated to contract by nerves and act as effectors 
    • Skeletal muscles act in antagonistic pairs against an incompressible skeleton. 
  • There are three types of muscle:
    smooth
    cardiac
    skeletal
  • Skeletal muscles act in antagonistic pairs against an incompressible skeleton: they pull in opposite directions.
    • Gross structure: seen by naked eye
    • Microscopic structure: seen using a microscope
    • Ultrastructure: seen only using an electron microscope
  • Actin and myosin are proteins.
  • Muscle fibres are the cells that make up skeletal muscle.
  • Muscle fibres contain myofibrils made of protein.
  • Myofibrils are NOT cells.
  • The cytoplasm of a muscle fibre is called the sarcoplasm.
  • The endoplasmic reticulum of a muscle fibre is called sarcoplasmic reticulum.
  • The cell-surface membrane of a muscle fibre is called the sarcolemma.
  • Muscle fibre cell
    A) myofibril
    B) Nucleus
    C) light (I) band
    D) Dark (a) band
    E) Cell surface membrane of muscle fibre
  • The neuromuscular junction is a special type of synapse that has the same basic mechanism as a cholinergic synapse.
  • imilarities between a normal cholinergic synapse and neuromuscular junction:
    • Use acetylcholine;
    • Have neurotransmitter that diffuses across the synaptic cleft;
    • Have receptors on a post-synaptic membrane, that on binding with neurotransmitter, cause an influx of sodium ions into the postsynaptic cell, depolarising it;
    • Use a sodium ion-potassium ion pump protein to restore resting potential;
    • Use acetylcholinesterase to break down the acetylcholine.
  • Differences between a normal cholinergic synapse and a neuromuscular junction
    • cholinergic synapse - neurone to neurone
    • neuromuscular junction - neurone to muscle fibre
    • cholinergic synapse - can be inhibitory or excitatory
    • neuromuscular junction - always excitatory
  • Motor unit: all the muscle fibres under control of one motor neurone.
    • What happens at the molecular level when a muscle relaxes?
    • Impulses to neuromuscular junction stop and acetylcholine hydrolysed by acetylcholinesterase.
    • Calcium ions actively pumped from the cytoplasm back into the sarcoplasmic reticulum.
    • Tropomyosin goes back to its original shape and blocks the myosin binding sites on actin.
    • Myosin heads can no longer bind to actin so no actinomyosin cross-bridges form.
    • Muscle now relaxed and can be freely lengthened by the pull from the other muscle in the antagonistic pair.
  • Suggest why rigor mortis occurs after death.
    • Sarcoplasmic reticulum degrades and calcium ions released into sarcoplasm;
    • Tropomyosin changes shape and exposes myosin binding sites on actin;
    • This triggers muscle contraction;
    • ATP needed to bind to and detach myosin heads from actin;
    • Death 🡪 no respiration, so no ATP production.
    • Myosin heads remain attached to actin, locking the muscle in the contracted state.
    • Organophosphate: inhibits acetylcholinesterase so it cannot break down acetylcholine; acetylcholine continues to bind to its receptors on the sarcolemma, triggering the entry of sodium ions into the muscle fibres,  depolarising the membrane of each muscle fibre; muscle stays contracted.#
    • Curare: similar shape to acetylcholine; binds to and blocks acetylcholine receptors on the membrane of each muscle fibre; acetylcholine therefore unable to cause the opening of sodium ion channels so no depolarisation of the muscle fibre membrane (curare itself does not cause depolarisation); muscles cannot contract.
    • Botulinum toxin: inhibits release of acetylcholine from synaptic vesicles into synaptic cleft; this means acetylcholine cannot lead to the depolarisation of the membrane of each muscle fibre since no sodium ion channels open; muscles cannot contract. 
  •  electron micrograph of myofibrils.
    A) A-band
    B) M-line
    C) I-band
    D) Z-line
    E) Sarcomere
    F) Z-line
  • Lactate is a major source of muscle soreness and fatigue.
  • Only 2 ATP produced (net) per glucose
    • Anaerobic respiration can be caused by oxygen supply not meeting demand but muscles can also use anaerobic respiration simply because it is a quicker way of making ATP than the longer process of aerobic respiration so intense exercise can rapidly fatigue your muscles. After exercise lactate can be oxidised back to pyruvate to be respired.
  • Anaerobic respiration in muscles produces lactate
    • Lactate lowers pH
  • The lowering of pH from lactate  alters hydrogen bonds and ionic bonds in proteins, changing tertiary structure/shape
  • Phosphocreatine is found in muscle fibres and can buffer (maintain) levels of ATP to a small extent.
  • During exercise, phosphocreatine phosphorylates ADP to ATP, so more ATP is available.
    At rest, ATP phosphorylates creatine back to phosphocreatine.
  • There are two types of muscle fibres:
    slow (long distance)
    fast (sprinting)
    • Fast fibres use mainly anaerobic respiration, slow fibres use mainly aerobic respiration.
    • Anaerobic respiration is a quicker process for producing ATP than aerobic respiration…
    • Therefore fast fibres are better for short bursts of intense exercise.
    • …but aerobic respiration yields much more ATP per glucose molecule.
    • Therefore slow fibres are better for endurance.
    • The different fibre types have adaptations to assist the type of respiration they perform.
  • Fast fibres contain high concentrations of glycogen.
    Anaerobic respiration only produces 2 ATP net per glucose molecule.
    Fast fibres compensate for this by respiring a lot of glucose.
    This glucose is stored in extensive glycogen deposits in the the fast fibre.
  • Myoglobin is an oxygen storage protein in muscle fibres
  • differences
    A) ONE
    B) FOUR
    C) FOUR
    D) ONE
    E) Muscle fibres
    F) red blood
  • Slow fibres have more myoglobin, since they need oxygen for aerobic respiration.
  • Myoglobin is found in muscle fibres.
    It is not normally found in the blood.
    #