15.8 Flashcards
muscle stimulation
-action potential reaches neuromuscular junctions simultaneously
-calcium ion protein channels open and calcium ions diffuse into synaptic knob
-calcium ions cause synaptic vesicles to fuse with presynaptic membrane and release acetylcholine into synaptic cleft
-acetylcholine diffuses across synaptic cleft and binds with receptors on the muscle csm, causing it to depolarise
muscle contraction
-action pt travels into fibre through T tubules
-action pt opens calcium ion protein channels in sarcoplasmic reticulum and Ca2+ diffuse into muscle cytoplasm down conc gradient
-Ca2+ cause tropomyosin molecules that were blocking the binding sites on actin filament to pull away
-ADP molecules attached to myosin heads mean they are in a state to bind to the actin filament and form a cross-bridge
-once attached, myosin heads change angle, pulling actin filament along and releasing ADP
-ATP molecules attached to myosin heads, causing it to become detached from action filament
-Ca2+ then activate enzyme ATPase. Hydrolysis of ATP to ADP provides energy for myosin heads to return to original position
-myosin reattaches further along the actin filament and process repeats as long as conc of Ca2+ in mycrofibril remain high
T tubules
extensions of csm that branch throughout cytoplasm and muscle
extra info muscle contraction
-myosin molecules are joined tail to tail in two oppositely facing sets so movement of one set of myosin heads in the opposite direction to other set
-movement of actin filaments in opposite diretions pulls them towards each other, shortening z lines
muscle relaxation
-when nervous stimulation ceases, Ca2+ actively transports back into ER using energy from hydrolysis of ATP
-reabsorption of Ca2+ ions allows tropomyosin to block actin filament again
-myosin heads unable to bind and contraction ceases
-antagonistic muscles can pull actin filaments out from between myosin
evidence for sliding filament mechanism
- I band becomes narrower
-Z lines move closer together
-H zone becomes narrower
myosin structure
-fibrous protein arranged into filament
-globular protein formed into 2 bulbous structures(head)
actin structure
globular protein whose molecules arranged into long chains that are twisted around one another to form helical strand
tropomyosin
forms long thin threads that are wound around actin filaments
ATP use in muscle contraction
- movement of myosin heads
-reabsorption of Ca2+ into ER by active transport
phosphocreatine
stored in muscle
rapidly generates ATP
