skeletal muscle

• Muscles: effector organs respond to nervous stimulation by contracting and so bring abot movement
• • There are three type of muscles:
  • Cardiac muscle: found exclusively in the heart is under unconscious control
  • Smooth muscle: found in walls of blood vessels and the gut is under unconscious control
  • Skeletal muscle: make up bulk of body muscle in vertebrates, attached to bone and acts under voluntary, conscious control
• Individual  muscle made up of millions of tiny muscle fibres, called myofibrils

  • Myofibrils are arranged parallel to each other to give max force
  • Separate cells of muscles are fused together into muscle fibres
  • They share nuclei and also cytoplasm, called sarcoplasm-  found mostly around circumference of the fibre
  • Sarcoplasm: large concs of mitochondria and endoplasmic reticulum
• • Myofibrils mainly made up of two types of filament:
  • Actin: thinner and consist of two strands twisted around one another
  • Myosin: thicker and consist of long rod-shaped tails with bulbous head that projects to the side

  A) nucleus

  B) blood capillary

  C) nerve

  D) I band

  E) z-line

  F) H-zone

  G) a myofibril

  H) sarcomere

  I) actin

  J) myosin

  
• Myofibrils


  Striped due to alternating light-coloured and dark-coloured bands
• I bands (isotropic bands)


  Light bands where actin and myosin do not overlap
• A bands (anisotropic bands)


  Dark bands where myosin and actin overlap
• H-zone


  Lighter-coloured region at the centre of each A band
• Z-line


  At the centre of each I band
• Sarcomere


  Distance between adjacent Z-lines
• Muscle contraction

  1. Sarcomere shorten

  2. Pattern of light and dark bands changed
• Tropomyosin


  Another protein found in muscle, forms a fibrous strand around the actin filament
• Types of muscle fibre

  • Slow-twitch fibres
  • Fast-twitch fibres
• Slow-twitch fibres

  • Contract more slowly than fast-twitch fibres
  • Provide less powerful contraction but over long period of time
  • Adapted to endurance work, e.g. running a marathon
  • More common in muscle: like calf muscle, must contract constantly to maintain body in an upright position
  • Suited to role by being adapted for aerobic respiration to avoid build-up of lactic acid, which would cause them to function less effectively and prevent long duration contraction
• Adaptations of slow-twitch fibres

  • Large store of myoglobin (red molecule carrying oxygen)
  • Rich supply of blood vessels to deliver oxygen and glucose for aerobic respiration
  • Lots of mitochondria produce ATP
• Fast-twitch fibres


  • Contract more rapidly
  • Produce powerful contraction but only for short period of time
  • Adapted for intense exercise, e.g. weight-lifting
  • More common in muscles: which need to do short bursts of intense activity, like biceps muscle of the upper arm
• Adaptations of fast-twitch fibres

  • Thicker and more numerous myosin filaments
  • High concentration of glycogen
  • High concentration of enzymes involved in anaerobic respiration which provides ATP rapidly
• Types of muscle fibre

  • Slow-twitch
  • Fast-twitch
• Slow-twitch fibres

  • Contract more slowly than fast-twitch fibres
  • Provide less powerful contraction but over long period of time
  • Adapted to endurance work, e.g. running a marathon
  • More common in muscle: like calf muscle, must contract constantly to maintain body in an upright position
  • Suited to role by being adapted for aerobic respiration to avoid build-up of lactic acid, which would cause them to function less effectively and prevent long duration contraction
• Adaptations of slow-twitch fibres

  • Large store of myoglobin (red molecule carrying oxygen)
  • Rich supply of blood vessels to deliver oxygen and glucose for aerobic respiration
  • Lots of mitochondria produce ATP
• Fast-twitch fibres


  • Contract more rapidly
  • Produce powerful contraction but only for short period of time
  • Adapted for intense exercise, e.g. weight-lifting
  • More common in muscles: which need to do short bursts of intense activity, like biceps muscle of the upper arm
• Adaptations of fast-twitch fibres

  • Thicker and more numerous myosin filaments
  • High concentration of glycogen
  • High concentration of enzymes involved in anaerobic respiration which provides ATP rapidly
• Neuromuscular junction


  The point where a motor neurone meets skeletal muscle fibre
• Neuromuscular junctions

  • Many along the muscle, ensuring contraction of a muscle is rapid and powerful when simultaneously stimulated by action potentials
• Motor unit


  All muscle fibres supplied by single motor neurone act together as single functional unit
• Arrangement of neuromuscular junctions


  • Gives control over force that muscle exerts
  • If only slight force needed, few units stimulated
  • Greater force needed, larger number of units stimulated
• Neuromuscular junction activation

  1. Nerve impulse received

  2. Synaptic vesicles fuse with presynaptic membrane and release acetyl choline

  3. Acetylcholine diffuses to postsynaptic membrane (membrane of muscle fibre), alters permeability to Na ions, enters rapidly, depolarising membrane

  4. Acetylcholine broken down by acetylcholinesterase ensure muscle not over-stimulated

  5. Resulting choline and ethanoic acid (acetyl) diffusing back into neurone, where it recombines to form acetylcholine using energy provided by mitochondria found there
• Skeletal muscle


  Muscle attached to skeleton
• Contraction of skeletal muscle

  1. Different parts of skeleton moved by relative to one another around series of points called joints

  2. Muscle cannot push they only pull

  3. Moving limbs in opposite direction require second muscle that works antagonistically to first one- which stretches partner muscle (relaxed) returning to original state ready to contract again
• Skeletal muscle


  • Occur and act in antagonistic pairs
  • Pairs pull in opposite direction and when one contracted other is relaxed
• Contraction of muscle fibre

  1. Arrangement of proteins

  2. Involves actin and myosin filaments sliding past one another

  3. Called sliding filament mechanism
• sliding filament summary

  A) actin

  B) myosin

  C) H-zone

  D) z-line

  E) A-band

  F) I band

  G) A-band

  
• Evidence for sliding filament mechanism:

  • Myofibrils appear darer in colour where actin and myosin filaments overlap and lighter where they do not
  • If sliding filament is correct: more overlap of actin and myosin in contracted muscle than in relaxed one. When muscle contracts, following changes occur:
  • I-band becomes narrower
  • Z-lines move closer together (sarcomere shortens)
  • H-zone becomes narrower
  • A-band remains same width, since the width of this band determined by length of myosin filaments, so myosin filaments not shortening
• • Myosin made up of 2 proteins:
  • Fibrous protein, arranged into filament made up of hundreds of molecules (tail)
  • Globular protein, formed into two bulbous structures at one end (head)
  • Actin is a globular protein; molecules arranged into long chains twisted around one another to form a helical strand
  • Tropomyosin forms long thin threads wound around actin filaments

  
• Sliding filament mechanism of muscle contraction


  Theory that actin and myosin filaments slide past one another during muscle contraction
• Sliding filament mechanism

  • Supported by changes seen in the band pattern on myofibrils
• Muscle contraction

  1. Bulbous heads of myosin filaments form cross-bridges with actin filaments

  2. Attach to binding sites on actin filaments

  3. Flex in unison, pulling actin filaments along myosin filaments

  4. Become detached

  5. Use ATP as source of energy

  6. Return to original angle and reattach further along actin filaments
• This process is continuous
• Stages of muscle contraction process

  • Stimulation
  • Contraction
  • Relaxation
• Muscle contraction

  The process by which muscles shorten and exert force