6.3 skeletal muscles

• how muscles work

  work in antagonistic pairs → pull in opposite directions eg. biceps / triceps

  one muscle contracts (agonist), pulling on bone / producing force

  one muscle relaxes (antagonist)

  skeleton is incompressible so muscle can transmit force to bone
• advantage of muscles

  the second muscle required to reverse movement caused by the first (muscles can only pull) and contraction of both muscles helps maintain posture
• describe the gross and microscopic structure of skeletal muscle

  made of many bundles of muscle fibres (cells) packaged together

  attached to bones by tendons

  muscle fibres contain ->

  sarcolemma (cell membrane) which folds inwards (invagination) to form transverse (T) tubules

  sarcoplasm (cytoplasm)

  multiple nuclei

  many myofibrils

  sarcoplasmic reticulum (endoplasmic reticulum)

  many mitochondria
• describe the ultrastructure of a myofibril

  made of two types of long protein filaments, arranged in parallel

  myosin - thick filament

  actin - thin filament

  arranged in functional units called sarcomeres

  ends – Z-line / disc

  middle – M-line

  H zone – contains only myosin
• explain the banding pattern to be seen in myofibrils

  I-bands - light bands containing only thin actin filaments

  A-bands - dark bands containing thick myosin filaments (and some actin filaments)

  H zone contains only myosin

  darkest region contains overlapping actin and myosin
• give an overview of muscle contraction

  myosin heads slide actin along myosin causing the sarcomere to contract

  simultaneous contraction of many sarcomeres causes myofibrils and muscle fibres to contract

  when sarcomeres contract (shorten) ->

  H zones get shorter

  I band get shorter

  A band stays the same

  Z lines get closer
• myofibril contraction (1) - role of calcium

  depolarisation spreads down sarcolemma via T tubules causing Ca2+ release from sarcoplasmic reticulum, which diffuse to myofibrils
• myofibril contraction (2) - role of calcium, actin and tropomyosin

  calcium ions bind to tropomyosin, causing it to move → exposing binding sites on actin
• myofibril contraction (3) - role of calcium, actin, tropomyosin and myosin

  allowing myosin head, with ADP attached, to bind to binding sites on actin → forming an actinomyosin crossbridge
• myofibril contraction (4) - role of actin, myosin and ATP

  myosin heads change angle, pulling actin along myosin, (ADP released), using energy from ATP hydrolysis
• myofibril contraction (5) - role of actin, myosin and ATP

  new ATP binds to myosin head causing it to detach from binding site
• myofibril contraction (6) - role of calcium, myosin and ATP

  hydrolysis of ATP by ATP hydrolase (activated by Ca2+) releases energy for myosin heads to return to original position
• myofibril contraction (7) - role of actin and myosin

  myosin reattaches to a different binding site further along actin

  process is repeated as long as calcium ion conconcentration is high
• during muscle relaxation

  Ca2+ actively transported back into the endoplasmic reticulum using energy from ATP

  tropomyosin moves back to block myosin binding site on actin again → no actinomyosin cross bridges
• describe the role of phosphocreatine in muscle contraction

  a source of inorganic phosphate (Pi) → rapidly phosphorylates ADP to regenerate ATP

  ADP + phosphocreatine → ATP + creatine

  runs out after a few seconds → used in short bursts of vigorous exercise

  anaerobic and alactic
• general properties of slow twitch muscle fibres

  specialised for slow, sustained contractions (eg. posture, long distance running)

  obtain ATP mostly from aerobic respiration → release energy slowly

  fatigues slowly
• location of slow twitch muscle fibres

  high proportion in muscles used for posture eg back or calves

  legs of long distance runners
• structure of slow muscle fibres

  high concentration of myoglobin → stores oxygen for aerobic respiration

  many mitochondria → high rate of aerobic respiration

  many capillaries → supply high concentration of oxygen/glucose for aerobic respiration and to prevent build up of lactic acid causing muscle fatigue
• general properties of fast twitch muscle fibres

  specialised for brief, intensive contractions (eg sprinting)

  obtain ATP mostly from anaerobic respiration → release energy quickly

  fatigues quickly due to high lactate concentration
• location of fast twitch muscle fibres

  high proportion in muscles used for fast movement eg biceps or eyelids

  legs of sprinters
• structure of fast twitch muscle fibres

  low levels of myoglobin

  lots of glycogen → hydrolysed to provide

  glucose for glycolysis/anaerobic respiration which is inefficient so large quantities of glucose required

  high concentration of enzymes involved in anaerobic respiration (in cytoplasm)

  store phosphocreatine