Impulse to activate action potential in skeletal muscle,

My notes....
VIII.Muscle Fibers
a.specific structures in muscle cells (muscle fibers)(skeletal) allow the cells to contract and relax
i.myofibrils (fills nearly all cytoplasm)
1.cylindrical structures that make up bulk of cytoplasm
2.consist of a chain of small, contractile units (sarcomeres)
a.give muscle fiber striped appearance
b.composed of myosin-II and actin
i.myosin-II = thick center
ii.actin = thin filaments that overlap mysoin-II
1.attach plus end to Z disc
a.intersection of 2 sarcomeres
b.Contraction
i.when sarcomeres shorten, muscle fibers contract
1.heads of myosin filaments start walking along respective actin filaments
a.pulls actin and myosin past each other
b.occurs very quickly (less than 1/10 of a second)
i.each myosin filament has 300 heads pulling on the actin
ii.muscle fibers relax when myosin heads release the actin filaments
iii.steps
1.attached (myosin attached to actin)(post stroke)
a.rigor state
b.ATP binds causes release
2.released
a.ATP hydrolysis to ADP + Pi allows cocked state
3.cocked
a.Pi release is coupled with binding of myosin head to actin
4.force-generating(POWER STROKE)
a.actin moves
b.then ADP is released
c.ATP binds
5.detached
a.caused by ATP binding
6.ATP is hydrolysed to ADP +Pi
a.Cause cocked state
iv.Steps (alternate)
1.Myosin starts attached to actin (just after the last stroke)("rigor" state).
2.ATP binds to myosin head causing a release from actin.
3.Then ATP hydrolysis to ADP + Pi allows cocked state.
4.Then Pi release is coupled with binding of myosin head to actin? (yes?)
5.Then (magically) the force-generating (power stroke) happens, actin moves.
6.Then ADP is released.
(starting over)
7.Then ATP binds to relase myosin from actin.
8.ATP is hydrolysed to ADP + Pi to form cocked state
v.Signaling contraction
1.muscle fibers contract upon receiving signals from the nervous system
a.action potential spreads to muscle cells
b.electrical signal spreads into transverse tubules that extend around muscle fibers
c.electrical signal spreads to sarcoplasmic reticulum
i.specialized region of endoplasmic reticulum
ii.contains high concentration of Ca2+
d.sarcoplasmic reticulum releases Ca2+ into cytoplasm
e.Ca2+ binds to troponin and induces change in shape
f.troponin change induces change in tropomyosin, allowing myosin heads to bind actin filaments and initiate contraction
g.Electrical signal passes, Ca2+ is pumped back into sarcoplasmic reticulum
h.muscle fiber relaxes (less info)