skeletal muscles

muscle mechanism

move in antagonistic pairs against incompressible skeleton

ultrastructure of myofibril

1. Z-line: boundary between sarcomeres

2. I-band: only actin

   • appears light under optical microscope

   • not visible when myofibril contracts

3. A-band: overlapping region between actin and myosin

   • appears dark under optical microscope

4. H-zone: only myosin

   • not visible when myofibril contracts

role of glycogen in skeletal muscle

a store of glucose

• to be hydrolysed to glucose during respiration to provide ATP

role of ATP in myofibril contraction

• ATP allows binding of myosin to actin, forming actinomyosin bridges

• provides energy to move myosin head

importance of ATPase during muscle contraction

• breaks down ATP to release energy

• energy used to form actinomyosin bridges

role of tropomyosin in myofibril contraction

moves out of the way when calcium ions bind, allowing myosin to bind to actin

role of actin in myofibril contraction

• actin are thin filaments involved in myofibril contraction

• provide myosin binding sites for myosin heads to bind, which enables the formation of actinomyosin cross bridges

role of myosin in myofibril contraction

• myosin are thick filaments with moveable heads

• myosin heads attach to binding sites on actin

• this enables the formation of actinomyosin cross bridges

• myosin heads move, pulling actin along

• detaches from binding site and moves to original position

explain how calcium ions cause myofibril to start contracting

Ca2+ binds to actin and uncovers myosin binding site on actin filament

process muscle contraction

• action potential arrives at neuromuscular junction

• calcium ions diffuse into myofibrils from sarcoplasmic reticulum

• Calcium ions bind to tropomyosin and cause the movement of tropomyosin

• this movement causes the exposure of the myosin-binding site on actin filament

  • this uncovers of binding sites on actin

• myosin heads to binds/attaches to the exposed sites on actin filament

  • myosin head binds to actin

  • actinomyosin cross bridge formed

• hydrolysis of ATP causes myosin head to bend

• bending pulls the actin filament

• attachment of a new ATP molecule to each myosin head causes the myosin head to detach from actin

how can a fall in pH lead to a reduction in the ability of calcium ions to stimulate muscle contraction

• low pH changes shape of calcium ion receptors

• fewer calcium ions bind to tropomyosin

• fewer tropomyosin molecules move away from binding site

  • so fewer binding sites on actin revealed

• so fewer myosin heads can bind and fewer actinomyosin cross bridges can form
  

• so the myosin head doesnt move and pull actin filament

4 pieces of evidence that support sliding filament theory

1. H-zone narrows

2. I-band narrows

3. Z-lines get closer

   • sarcomere shortens

4. A-zone remains same width

   • proves that myosin filaments do not shorten

muscle relaxation

1. Ca2+ is actively transported back into sarcoplasmic reticulum

2. Tropomyosin blocks binding site on actin

role of phosphocreatine in muscle contraction

phosphorylates ADP directly to ATP when oxygen for aerobic respiration is limited

how to calculate length of one sarcomere

1. view thin slice of muscle under optical microscope

2. calibrate eyepiece graticule

3. measure distance from middle on one light band to another

where are slow and fast-twitch muscle fibres found

• slow-twitch: sites of sustained contraction (e.g. calf muscle)

• fast-twitch: sites of short-term, rapid, powerful contractions (e.g. biceps)

role of slow and fast-twitch muscle fibres

slow-twitch: long-duration contraction

• well-adapted to aerobic respiration to prevent lactate build-up

fast-twitch: powerful short-term contraction

• well-adapted to anaerobic respiration

adaptations of slow-twitch muscle fibres

1. glycogen store: many terminal ends that can be hydrolysed to release glucose for respiration

2. contain myoglobin: higher affinity for oxygen than haemoglobin at lower partial pressures

3. many mitochondria: aerobic respiration produces more ATP

4. surrounded by many blood vessels: high supply of oxygen and glucose

structure and properties of fast-twitch muscle fibres

1. large stores of phosphocreatine

2. more myosin filaments

3. thicker myosin filaments

4. high concentration of enzymes involved in anaerobic respiration

5. extensive sarcoplasmic reticulum: rapid uptake and release of Ca2+

why do both slow and fast muscle fibres contain ATPase?

• ATPase causes hydrolysis of ATP

• muscle contraction requires ATP