Muscles

• Types of muscle:

  • Smooth (muscle contracts without conscious control)
  • Cardiac (muscle contracts without conscious control - in heart)
  • Skeletal (muscle used in movement)
• Skeletal muscle fibers are highly specialised
• Skeletal muscles are attached to bone by tendons
• Muscles can have antagonistic pairs e.g. biceps and triceps
• Myofibrils contain:

  • Sarcoplasmic reticulum
  • Mitochondria
  • Myosin (thick)
  • Actin (thin)
• Sarcomere - The distance between two z disks
• A band - Area of thick and thin filaments that overlap
• H zone - Centre of the myofilament where there is no overlap with actin
• Z disk - Attached to actin filaments, holds sarcomeres together
• I band - Light area where only thin filaments are visible
• Muscle contraction:

  • Sarcomere shortens
  • Brings Z lines closer together
  • Actin pulled over myosin so increases amount they overlap
• Muscle contraction - components:

  • A band stays the same
  • I band shortens
  • H zone becomes smaller
  • Z discs shorten by coming closer together
  • Sarcomere shortens
• If a muscle fibre is excited by a motor neurone then the sliding filament theory occurs
• Sliding filament theory:

  1. Ca2+ released from sarcoplasmic reticulum into sarcoplasm
  2. Ca2+ diffuses and causes tropomyosin to move exposing myosin head binding sites on the actin filament
  3. Myosin heads bind to actin binding sites to form cross bridges
  4. Myosin head bends pulling the actin filament over the myosin (POWER STROKE)
  5. ADP and Pi are released from the myosin head
  6. New ATP binds to myosin head so breaks cross bridge
  7. This ATP is hydrolysed by ATP hydrolase to form ADP and Pi
  8. Energy released retracts the myosin head (RECOVERY STROKE)
• Role of Ca2+ ions in the sliding filament theory:

  • Ca2+ ions actively transported back into the sarcoplasmic reticulum
  • Ca2+ activate ATP hydrolase
• ATP releases energy and changes the shape of the myosin head
• ATP is needed to break cross bridges
• ATP is needed for:

  • Sliding of the filaments during contraction
  • Active transport of Ca2+ ions into the sarcoplasmic reticulum
• Fast twitch muscles can store phosphocreatine (donates phosphate to ADP to form ATP in the short-term)
• Why phosphocreatine is needed:

  • Muscle fibres store enough ATP for 3-4 seconds of muscle contraction
  • Anaerobic respiration takes 10 seconds to produce ATP
  • Aerobic respiration takes longer than anaerobic respiration
• ADP + phosphocreatine = ATP + creatine
• Phosphocreatine:

  • Energy released from the hydrolysis of phosphocreatine is not used in muscle contraction directly
  • Energy released is used in the phosphorylation of ATP so then used in muscle contraction#
• Slow twitch muscle fibres contract over long periods of time and have a slower rate of contraction - they are also slow to fatigue
• Large numbers of slow twitch fibres are in the legs and involved in maintaining posture
• Slow twitch muscle fibres:

  • Specialised to use aerobic respiration for ATP
  • Have lots of mitochondria
  • High concentrations of myoglobin (oxygen store)
  • Very close to a large number of capillaries so good oxygen supply
  • Less extensive sarcoplasmic reticulum as less calcium ions needed at one time
  • Less glycogen as glucose broken down fully via aerobic respiration
• Fast twitch muscle fibres provide strong contractions and have a faster rate of contraction - fast to fatigue due to faster build-up of lactate
• Fast twitch muscle fibres:

  • Specialised to use anaerobic respiration for ATP
  • Less and small mitchondria
  • Low concentration of myoglobin
  • Fewer capillaries
  • Extensive sarcoplasmic reticulum as more calcium ions needed
  • More glycogen for glucose in anaerobic respiration (glycolysis)
• Role of tropomyosin:

  • Moves out of the way when calcium ions bind
  • This allows myosin to bind to actin by forming a cross bridge
• Role of myosin:

  • Myosin head binds to actin and pulls actin (power stroke)
  • Detaches from actin and re-sets (recovery stroke)
  • Using ATP
• Why mitochondria in muscles contain may cristae:

  • Larger surface area for oxidative phosphorylation
  • Provides ATP for muscle contraction
• Why increased cardiac output is an advantage during exercise:

  • More respiration for respiring muscles
  • Higher cardiac output so more oxygen intake
  • Increases glucose supply to muscles
  • Increases carbon dioxide removal
  • Increases heat removal from muscles
• Importance of ATP hydrolase:

  • Hydrolyses ATP so releases energy
  • Breaks cross bridges (actomyosin bridges)
• Advantages of using aerobic rather than anaerobic respiration for a long-distance race:

  • Aerobic respiration releases more ATP so more energy
  • Less lactate produced
  • Less muscle fatigue
  • CO2 is easily removed from the body
• Explain what causes cross bridges to remain firmly bound after death:

  • Respiration stops
  • No more ATP produced
  • ATP required for separation of actin and myosin (cross bridges)
• Role of calcium ions in the contraction of a sarcomere:

  • Ca2+ interact with tropomyosin
  • Reveals actin binding sites
  • Allows myosin heads to bind to actin
  • Activates ATP hydrolase
• Describe fast twitch muscle fibres:

  • Used for strong contractions
  • Phosphocreatine used rapidly during contraction
  • Anaerobic respiration involved
  • ATP used to reform phosphocreatine
  • Lots of phosphocreatine in fast twitch muscle fibres
• Role of phosphocreatine:

  • Provides phosphate/phosphorylation
  • To synthesise ATP