muscle structure and contraction

• what are muscles?

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

  cardiac muscle- found exclusively in the heart. involuntary

  smooth muscle- walls of blood vessels and the gut. involuntary

  skeletal muscle- makes up the bulk of the body muscle in vertebrates. its attached to bone and is under voluntary control
• what are muscles made of?

  millions of tiny muscle fibres called myofibrils. they are arranged parallel to each other to give maximum strength. muscle is composed of smaller units bundled into progressively larger ones. seperate cells are fused to form these muscle fibres, which share nuclei and sarcoplasm
• what is found in sarcoplasm?

  a large concentration of mitochondria and endoplasmic reticulum
• what two types of protein filament are myofibrils mainly made of?

  actin - which is thinner and consists of two strings twisted around one another

  myosin- which is thicker and consists of long rod - shaped tails with bulbous heads that project to the side
• myofibril structure

  light bands - I bands [isotropic bands]

  they appear to be lighter because the thick and thin filaments do not overlap in this region

  dark bands- A bands [anisotropic bands]

  they appear darker because the thick ans thin filaments overlap in this region
• what is at the centre of each A band

  a lighter coloured region called the H zone
• what is at the centre of each I band?

  a line called the Z - line
• what is the distance between adjacent Z - lines called?

  a sarcomere, when a muscler contracts, the sarcomeres shorrten and the pattern of light and dark bands change
• what is tropomyosin

  a protein found in muscle that forms a fibrous strand around the actin filament
• what are slow twitch muscle fibres?

  they contract more slowly and provide less powerful contractions but over a longer period

  they're adapted to endurance work

  they are more common in muscles that must contract more constantly

  they are suited to their role by being adapted for aerobic respiration in order to avoid a build up of lactic acid
• what are the adaptations of slow twitch muscle fibres?

  1. a large store of myoglobin [a bright red molecule that stores oxygen, which is why slow twitch fibres are red]
  2. a rich supply of blood vessels to deliver oxygen and glucose for aerobic respiration
  3. numerous mitochondria to produce ATP
• what are fast twitch muscle fibres?

  these contract more rapidly and produce powerful contractions but only for a short time period.

  they are adapted to intense exercise

  they are more common in muscles that need to do short bursts of intense activity
• how are fast twitch muscle fibres adapated to their role?

  1. thicker and more numerous myosin fibres
  2. a high concentration of glycogen
  3. a high concentration of enzymes involved in anaerobic respiration which provides ATP rapidly
  4. a store of phosphocreatine, a molecule that can rapidly generate ATP from ADP in anaerobic conditions to provide energy for muscle contraction
• what is a neuromuscular junction?

  the point where a motor neurone meets a skeletal muscle fibre
• all muscle fibres supported by a single motor neurone act together as a single functional unit and are known as a motor unit
• why is a motor unit beneficial?

  it gives control over the force that the muscle exerts
• what are the similarities between a neuromuscular junction and a synapse?

  • they both have neurotransmitters that are transported by diffusion
  • they both have receptors, that on binding with the neurotransmitter, cause an influx of sodium ions
  • they both use a sodium - potassium pump to repolarise the axon
  • they both use enzymes to breakdown the neurotransmitter
• what are the differences between neuromuscular junctions and synapses?

  1. neuromuscular is only excitatory, synapses can be excitory or inhibitory
  2. neuromuscular only links neurones to muscles, synapses link neurones to neurones or to effector organs
  3. only motor neurones are involved in neuromuscular junctions but motor sensory and intermediate may be involved in synapses
  4. action potential ends at a neuromuscular junction
  5. acetylcholine binds to receptors on the membrane of muscle fibres on a neuromuscular junction, post synaptic membrane in synapse
• skeletal muscles occur in antagonistic pairs that pull in opposite directions and when one is contracted the other is relaxed
• what is the process that involves actin and myosin filaments sliding past one another?

  sliding filament mechanism
• what changes occur when muscles contract?

  • The I - band becomes narrower
  • the Z - lines move closer together [the sarcomere shortens]
  • the H zone becomes narrower
  • the A band stays the same
• outline the process of muscle stimulation:

  1. an action potential reaches many neuromuscular junctions simultaneously, causing calcium ion protein channels to open and calcium ions to diffuse into the synaptic knob
  2. this causes synaptic vesicles to fuse the presynaptic membrane and release their acetylcholine into the synaptic cleft
  3. acetylcholine diffuses across the synaptic cleft and binds with receptors on the muscle cell - surface membrane, depolarising it
• describe muscle contraction

  • action potential travels deep into muscle fibre via T tubules that branch through the sarcoplasm
  • they're in contact with sarcoplasmic reticulum which actively transported calcium ions from sarcoplasm
  • the action potential causes calcium ions to diffuse into sarcoplasm
  • tropomyosin molecules blocking binding sites on actin pull away due to calcium
  • ADP molecules on myosin heads bind to actin filament
  • cross bridge formed, power stroke, ADP released, ATP attaches to myosin head, detaches from actin, ATPase hydrolyses ATP to ADP and returns to original position
• How does muscle relaxation occur?

  • when nervous stimulation stops, calcium ions are actively transported back into the endoplasmic reticulum using energy from the hydrolysis of ATP
  • The reabsorbtion of calcium allows tropomyosin to block the actin filmane tagain
  • myosin heads can't bind and contraction ceases to the muscle relaxes
  • in this state, force from antagonistic muscles can pull actin filaments out from between myosin to a point