Muscle contraction

• muscle contraction involves myosin and actin filaments sliding over one another
• Myosin filaments:

  • have globular heads that are hinged - so they can move back and forth
  • each myosin head has a binding site for actin and a binding site of ATP
• Actin filaments:

  • have binding sites for myosin heads - actin-myosin binding sites
  • another protein called tropomyosin is found between actin filaments - helps myofilaments move past each other
• Binding sites in resting muscles:

  • for myosin and actin filaments to slide past each other, the myosin head needs to bind to the actin-myosin binding site on the actin filament
  • in a resting (unstimulated) muscle - the actin-myosin binding site is blocked by tropomyosin
  • this means that myofilaments can't slide past each other bc myosin heads can't bind to the actin filaments
• The process of muscle contraction:

  • Arrival of an action potential
  • Movement of the actin filament
  • Breaking of the cross bridge
  • Return to resting state
• Arrival of an action potential:

  • when an action potential from a motor neurone stimulates a muscle cell, it depolarises the sarcolemma
  • depolarisation spreads down the T-tubules to the sarcoplasmic reticulum
  • this causes the sarcoplasmic reticulum to release stored calcium ions (Ca+) into the sarcoplasm
  • this influx of calcium ions into the sarcoplasm triggers muscle contraction
• • Calcium ions bind to a protein attached to tropomyosin, causing the protein to change shape
  • this pulls the attached tropomyosin out of the actin-myosin binding site on the actin filament
  • this exposes the binding site, which allows the myosin head to bind
  • the bond formed when a myosin head binds to an actin filament is called a actin-myosin cross bridge
• tropomyosin molecules actually form part of a long chain that coils round the actin filament
• • Ca2+ binds to a protein attached to to tropomyosin
  • tropomyosin moves and unblocks the binding site
  • myosin head binds to the exposed site
  • actin-myosin cross-bridge formed
• Movement of the actin filament:

  • calcium ions also activate the enzyme ATP hydrolase, which hydrolases (breaks down ATP) into ADP + Pi to provide the energy needed for muscle contraction
  • the energy released from ATP causes the myosin head to bend, which pulls the actin filament along in a kind of rowing action
  • the movement of the myosin head to the side is called a 'power stroke'
• Breaking of the cross bridge

  1. Another ATP molecule provides the energy to break the actin-myosin cross bridge

  2. Myosin head detaches from the actin filament after it's moved

  3. Myosin head returns to its starting position and reattaches to a different binding site further along the actin filament

  4. A new actin-myosin cross bridge is formed and the cycle is repeated (attach, move, detach, reattach to new binding site)

  5. Many actin-myosin cross bridges form and break very rapidly, pulling the actin filament along - which shortens the sarcomere, causing the muscle to contract
• The cycle will continue

  As long as calcium ions are present
• a good supply of ATP is essential for muscle contraction
• as the actin filaments are being moved along, the I-bands are getting shorter and the Z-lines are moving closer together
• Return to resting state:

  • when the muscle tops being stimulates, calcium ions leave their binding sites and are moved by active transport back into the sarcoplasmic reticulum (needs ATP)
  • causes tropomyosin molecules to move back, so they block the actin-myosin binding sites again
  • muscles aren't contracted bc no myosin heads are attached to actin filaments (no actin-myosin cross bridges)
  • actin filaments slide back to their relaxed position - lengthens the sarcomere
• Energy for muscle contraction:

  • aerobic respiration
  • anaerobic respiration
  • ATP-phosphocreatine PCr system
• Aerobic resp:

  • most ATP is generated via oxidative phosphorylation in the cell's mitochondria
  • aerobic resp only works when there is oxygen
  • good for long periods of low-intensity exercise e.g. a long walk
• anaerobic resp:

  • ATP is made rapidly by glycolysis
  • end product of glycolysis is pyruvate which is converted to lactate by lactate fermentation
  • lactate can quickly build up in in the muscles and cause muscle fatigue
  • anaerobic resp is good for shot periods of hard exercise e.g. a 400 m sprint
• ATP-phosphocreatine (PCr) system


  1. ATP is made from phosphorylating ADP - adding a phosphate group taken from PCr

  2. ADP + PCr = ATP + Cr (creatine)
• PCr


  Stored inside cells, the ATP-PCr system generates ATP very quickly
• PCr


  • Runs out after a few seconds, used during short bursts of vigorous exercise e.g. a tennis serve
• ATP-PCr system


  Anaerobic (doesn't need oxygen), alactic (doesn't form any lactate)
• Creatine breakdown


  1. Some of the Cr gets broken down into creatinine

  2. Creatinine is removed from the body via the kidneys
• Creatinine levels


  • Higher in people who exercise regularly and those with a high muscle mass
  • May also indicate kidney damage
• skeletal muscles are made up of 2 types of msucle fibres:

  • slow twitch
  • fast twitch
• Slow twitch:

  • contract slowly and an work for a long time without getting tired
  • good for endurance activities e.g. long distance running and maintaining posture
  • high proportions found in the muscles use for posture e.g. in the back and calves
  • energy released slowly through aerobic resp
  • lots of mitochondria and blood vessels to supply muscles with oxygen
• • mitochondria found near the edge of muscle fibres
  • so that there is a short diffusion pathway for oxygen from the blood vessels to the mitochondria
  • slow twitch muscle fibres are also rich in myoglobin, a red-coloured protein that stores oxygen, so the are reddish in colour
• Fast twitch:

  • contract very quickly
  • get tired very quickly
  • makes them good for short bursts of speed and power e.g. sprinting and eye movement
  • high proportions found in muscles used for fast movement such as the legs, arms and eyes
  • energy released quickly through anaerobic resp using glycogen in fast twitch muscle fibres
  • also have stores of PCr so energy can be generated very quickly when needed
• • FT muscle fibres have few mitochondria or blood vessels
  • don't have much myoglobin either - so can't store much oxygen
  • more of a whitish colour
• cells are able to store excess glucose as glycogen, which can be converted back into glucose when needed