6.3 Muscles

• Muscles act as antagonistic pairs against an incompressible skeleton

  Muscles work in antagonistic pairs

  One muscle contracts (agonist) → pulls on bone / produces force

  One muscle relaxes (antagonist)

  Examples: biceps and triceps in the arm

  Attached to bones by tendons

  Ligaments attached from one bone to the other

  Skeletal muscle is incompressible so muscle can transmit force to bone Advantages of skeletal muscles being arranged in antagonistic pairs

  Muscles can only contract / pull

  2nd muscle required to reverse the movement caused by 1st

  Help maintain posture → contraction of both muscles
• muscle made up of bundles of muscle fibres

  Cell membrane- sarcolemma

  cytoplasm- sarcoplasm

  endoplasmic reticulum- sarcoplasmic reticulum

  
• Ultrastructure of a myofibril

  Myofibril made up of many sarcomeres which are made up of partly overlapping myosin and actin filaments (proteins)

  
• A sarcomere consists of

  • Ends - Z-line - M-lineH zone - around M line which contains only myosin
  • Myosin filaments are thicker than thinner actin filamentsThis causes a banding pattern to be seen (in a relaxed myofibril) under an electron microscope:
  • I-bands → light bands containing only thick actin filaments
  • A-bands → dark bands containing thick myosin filaments and some overlapping actin

  
• A sarcomere consists of

  • Ends - Z-line Middle - M-line. H zone - around M line which contains only myosin
  • Myosin filaments are thicker than thinner actin filaments. This causes a banding pattern to be seen (in a relaxed myofibril) under an electron microscope:
  • I-bands → light bands containing only thick actin filaments
  • A-bands → dark bands containing thick myosin filaments and some overlapping actin

  
• Muscle contraction

  • Myosin heads slide actin past/along myosin causing the sarcomere to contractSimultaneous contraction of lots of sarcomeres causes myofibrils and muscle fibres to contract
  • When sarcomeres contract (shorten)...
  • H zones shorter
  • I band shorter
  • A band sameZ lines closer

  
• describe structure of Actin filament

  Two actin wrapped around each other making an actin filament

  actin molecule has a myosin binding site

  tropomyosin cover the binding site on the actin + double helix shape

  • thin compared to myosin

  
• actin

  Tropomyosin blocks myosin binding sites on actin molecules, preventing cross-bridge formation. troponin bind to:

  • troponin C- ca2+
  • troponin T- tropomyosin
  • troponin I- actin- inhibits contraction
• myosin

  biding site for actin and atp

  head points upwards towards acting

  tail.

  2 myosin wrapped around each other

  
• Slow twitch fibres are specialised for slow sustained contractions.
• Endurance activities such as maintaining posture and long distance running use slow twitch fibres.
• Aerobic respiration produces ATP (oxidative phosphorylation) to release energy slowly.
• Slow twitch fibres are located in muscles that give posture and in leg muscles of long distance runners.
• High levels of myoglobin, a red coloured protein that stores oxygen, makes slow twitch fibres a reddish colour, indicating a large storage of oxygen in the muscle for aerobic respiration.
• Slow twitch fibres have many capillaries, providing a short diffusion pathway and a large surface area for supplying high concentrations of oxygen and glucose for aerobic respiration and to prevent the build-up of lactic acid causing muscle fatigue.
• Fast twitch muscles are specialised for producing rapid, intense contractions of short duration.
• Fast twitch muscles are used for short bursts of speed and power, such as sprinting.
• Fast twitch muscles are located in the legs of sprinters.
• Anaerobic respiration produces ATP to release energy quickly.
• Low levels of myoglobin make fast twitch muscles a whitish colour, indicating that anaerobic respiration doesn't need oxygen.
• Fast twitch muscles have a high concentration of enzymes involved in anaerobic respiration, which is why they have a high rate of anaerobic respiration.
• Fast twitch muscles store phosphocreatine which rapidly generates ATP from ADP by providing phosphate.
• Muscles can get fatigued quickly because of high amounts of lactate.
• Role of phosphocreatine in muscle contraction

  • Phosphocreatine stored inside cellsRapidly makes ATP by phosphorylating ADP (adding phosphate group from PCr)
  • PCr runs out after a few seconds so it's used in short bursts of vigorous exercise
  • Anaerobic and alactic
• The sliding filament theory explains that action potential/ depolarisation spreads down T-tubules causing the release of calcium ions from the sarcoplasmic reticulum, which diffuses through the sarcoplasm to the myofibril.
• Calcium ions bind to Tropomyosin, causing it to move as it changes shape, exposing the myosin binding site on actin.
• Myosin heads, with ADP attached, attach to binding sites forming an actinomyosin cross bridge, a process that requires ATP.
• Myosin heads move along actin, causing it to slide, a process that releases energy and requires ATP.
• ATP binds to myosin head, causing it to detach from the actin binding site, breaking the crossbridge.
• The hydrolysis of ATP by ATPase, activated by Ca2+, releases energy for myosin head to move back to the original position, also known as the recovery stroke.
• Myosin attaches to a different binding site further along the actin filament, a process that is repeated as long as Ca2+ is present.
• during muscle relaxation:

  when never impulse ceases, calcium ions are actively transported back into the sarcoplasmic reticulum

  2. in the absence of calcium ions, the tropomyosin molecule changes shape and blocks binding site on actin again

  3. this means myosin heads are unable to attaxh to actin

  4. muscle contraction stops, meaning the sarcomeres on the muscle’s myofibrils expand again

  5. this leads to muscle becoming relaxed