sliding filament theory and excitation contraction coupling

week 4 unit 3

sliding filament theory

• shortening of sarcomere during contraction

• increase of overlap due to thin myofilament silding over thick myofilament

• thus moving towards the m-line

• sliding event occurs due to cross bridge

what is cross bridge

• formed between actin in the thin myofilament and myosin in thick myofilament

what is the power stroke and what happens after

• head of the myosin molecule binds to the myosin binding site on actin

• upon binding, head changes shape, propelling the thin filament towards the m-line

• following a power stroke, thin filament slides past myosin, moving closer to the m-line and pulling the z-lines/discs closer, shortening the sarcomere

order of sliding filaments using crowd surfing analolgy

1. contraction is triggered

2. myosin head binds to actin- cross bridge is formed (hand of crowd catch the board)

3. myosin head changes shape - power stroke occurs (hand propel the board + surfer forward thru wrist motion)

4. thin myofilament slides past thick myofilament, moving towards the m-line (surfer+board move only in one direction)

5. z-discs/lines come closer together

excitation-contraction coupling 

occurs when an action potential on the sarcolemma leads release of calcium ions from the sarcoplasmic reticulum (SR) leading to cross bridge formation, power-stroke and muscle contraction

explain 1. energized state (atp hydrolysis)

• ATP binds to ATPase binding site on myosin head

• ATPase enzyme breaks down (or hydrolyzes) ATP into ADP and inorganic phosphate (Pi)

• this process releases energy that moves the myosin head into position to bind to actin (it activates/energizes hyosin head)

• ADP and Pi remain in the ATPase site during the energized state of the myosin head

presence of calcium

• if an AP was generated in muscle cell, calcium is release from the sr

• which binds to troponin c

• troponin c moves to tromyosin, exposing the myosin binding site on actin

• if the myosin head is activated, myosin binding site on actin is exposed, then cross bridge forms and contraction occurs

absence of calcium

• if no AP generated in muscle cell, calcium wont be released from the sr

• the myosin binding site on actin wont be exposed and cross bridge wont form

• therefore no contraction 

power stroke process

• involves change in conformation of the myosin head

• that change in conformation occurs when the Pi is released from ATPase binding site in myosin

• As Pi moves out, the myosin head pulls on actin

• directs the thin myofilament towards the m-line and shortening of the sarcomere

• after the power stroke, ADP is also released from ATPase site on myosin

ATP binding (DETACHMENT)

• when the ATPase site on myosin is empty, a new ATP molecule binds

• the binding of atp leads to another change in the myosin head

• it detaches from actin and resumes low energy conformation

• as soon as ATPase breaks apart the ATP (it hydrolyzes it), the cycle resumes starting at energized state

rigor mortis

• stiffening of muscles after death (3-4 hours after)

• no oxygen =no ATP - in the cell, mitochondria need oxygen to produce ATP

• no atp=no detachment - since atp causes the actin-myosin cross bridge to break by binding to ATPase site on myosin, no ATP means actin and myosin cannot detach

• no detachment=actin and myosin remain fused

• no atp=no calcium back in sr - if theres no atp, calcium atpase on sr cant function bc it need substrate

• no calcium back in sr=more cross bridges form - since now calcium is available in the cytoplasm, it binds to troponin c, exposing the myosin binding site and allowing more cross bridges to form

• more cross bridges form + actin-myosin remaining fused = muscle is fused or in rigor

can the muscle remain permanently in rigor mortis?

no bc muscle begins to decompose over time meaning cross bridges begin to fall apart as the contractile proteins are denatured during the decomposition process. at higher temps, rigor mortis occurs faster. humidity, bacteria age, body fat, etc will impact rigor mortis