Skeletal Muscles

• Describe how muscles work:

  1)Work in antagonistic pairs -> pull in opposite directions e.g. biceps and triceps

  • One muscle contracts (agonist) pulling on the bone / producing force
  • One muscle relaxes (antagonistic)

  2)Skeleton is incompressible so muscle can transmit force to bone
• Describe the gross and microscopic structure of skeletal muscle:

  • Made of many bundles of muscle fibres packaged together
  • Attached to the bone by tendons

  Muscle fibres contain:

  • Sarcolemma (cell membrane) which folds inwards (invagination) to form transverse (T) tubules
  • Sarcoplasm
  • Multiple Nuclei
  • Many Myofibrils
  • Sarcoplasmic 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 = only contains myosin
• Explain the banding pattern to be seen in Myofibrils:

  • l-bands - light bands containing only thin actin filaments
  • A-bands - dark bands containing thick myosin filaments (and some actin filaments)
  • H zone only contains myosin
  • Darkest region contains overlapping actin and myosin
• Give an overview of muscle contraction:

  • Myosin heads slide actin along myosin causing sarcomere to contract
  • Simultaneous contraction of many sarcomeres causes myofibrils to contract When sarcomeres contract:
  • H zones get shorter
  • l band gets shorter
  • A band stays the same
  • Z lines get closer
• What initiates the power stroke mechanism in muscle contraction?

  Depolarisation spreads down sarcolemma
• How does depolarisation affect the sarcolemma?

  It causes Ca$^{2+}$ release from the sarcoplasmic reticulum
• Where does Ca$^{2+}$ diffuse after its release?


  To myofibrils
• What happens when calcium ions bind to tropomyosin?

  Tropomyosin moves, exposing binding sites on actin
• What is the role of myosin heads in muscle contraction?

  They bind to actin binding sites
• What happens to myosin heads after they bind to actin?

  They change angle, pulling actin along
• What is released when myosin heads change angle?

  ADP
• What provides energy for myosin heads to pull actin?

  Energy from ATP hydrolysis
• What happens when new ATP binds to myosin heads?

  Myosin heads detach from binding sites
• What is the role of ATP hydrolase in muscle contraction?

  It hydrolyzes ATP, releasing energy
• What happens to myosin heads after ATP hydrolysis?

  They return to their original position
• What happens during muscle relaxation?

  • Ca2+ is 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 if vigorous exercise
  • Anaerobic and aerobic
• Describe the general properties of a Slow Twitch:

  • Specialised for slow, sustained contractions (e.g. posture, long distance running)
  • Produce more ATP slowly from aerobic respiration
  • Fatigues slowly
• Describe the location of a Slow Twitch:

  • High proportion in muscles used for posture
  • Legs of long distance runners
• Describe the structure of Slow Twitch:

  • High conc. of myoglobin -> stores oxygen for aerobic respiration
  • Many mitochondria -> high rate of aerobic respiration
  • Many capillaries -> supply high conc. of oxygen / glucose for aerobic respiration and to prevent build up of lactic acid causing muscle fatigue
• Describe the general properties of a Fast Twitch:

  • Specialised for brief, intensive contractions (e.g. sprinting)
  • Produce less ATP rapidly from anaerobic respiration
  • Fatigues quickly due to high lactate concentration
• Describe the location of Fast Twitches:

  • High proportion in muscles used for fast movement e.g. biceps, eyelids
  • Legs of sprinters
• Describe the structure of Fast Twitches:

  • 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 conc. of enzymes involved in anaerobic respiration (cytoplasm)
  • Store phosphocreatine