6.3 Skeletal muscles are stimulated

• Describe 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 - 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
• myofibril structure

  A) sarcolemma

  B) mitochondria

  C) myofibrils

  D) nucleus

  E) t tubule

  F) sarcoplasmic reticulum

  
• 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
• sarcomere structure

  A) H zone

  B) Z disc

  C) I band

  D) I band

  E) A band

  F) M line

  
• 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
• Describe myofibril contraction

  • Depolarisation spreads down sarcolemma via T tubules - Ca2+ release from sarcoplasmic reticulum - diffuse to myofibrils
  • Ca2+ bind to tropomyosin, moves - exposing actin binding sites
  • myosin head, ADP attached, binds to actin - forming actinomyosin crossbridge
  • Myosin heads change angle - pull actin along myosin, release ADP - energy from ATP hydrolysis
  • New ATP binds to myosin head - detaches from binding site
  • Hydrolysis of ATP by ATPase releases energy - myosin heads return to original position
  • Myosin reattaches to different binding site further along actin
• what happens during muscle relaxation

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

  2. 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
• Compare the general properties of slow and fast skeletal muscle fibres

  Slow twitch

  ● Specialised for slow, sustained contractions (eg. posture, long

  distance running)

  ● Produce more ATP slowly (mostly) from aerobic respiration

  ● Fatigues slowly

  Fast twitch

  ● Specialised for brief, intensive contractions (eg. sprinting)

  ● Produce less ATP rapidly (mostly) from anaerobic respiration

  ● Fatigues quickly due to high lactate concentration
• Compare the location of slow and fast skeletal muscle fibres

  Slow twitch

  ● High proportion in muscles used for posture eg. back, calves

  ● Legs of long distance runners

  Fast twitch

  ● High proportion in muscles used for fast movement eg. biceps, eyelids

  ● Legs of sprinters
• Compare the structure of slow and fast skeletal muscle fibres

  Slow twitch

  ● High conc. of myoglobin → stores oxygen - aerobic respiration

  ● Many mitochondria → high aerobic respiration

  ● Many capillaries → supply high conc. of oxygen/glucose for aerobic respiration prevents build-up of lactic acid causing muscle fatigue

  Fast twitch

  ● Low levels of myoglobin

  ● Lots of glycogen → hydrolysed to provide glucose for glycolysis/anaerobic respiration - inefficient so large quantities of glucose required

  ● High conc. of enzymes involved in anaerobic respiration

  ● Store phosphocreatine
• explain how a decrease in the concentration of calcium ions within muscle tissues could cause a decrease in the force of muscle contraction

  • less tropomyosin moved from binding site
  • less actinomyosin bridges formed
  • myosin head does not move
• describe the roles of calcium ions and ATP in the contraction of a myofibril

  • Calcium ions diffuse into myofibrils
  • cause movement of tropomyosin
  • exposure of the binding sites on the actin
  • Myosin heads attach to binding sites on actin
  • Hydrolysis of ATP causes myosin heads to bend pulling actin molecules
  • Attachment of a new ATP molecule to each myosin head causes myosin heads to detach