Unit 3-- Revised Steps of a Muscle Contraction

step 1

the muscle fiber is at rest. the myosin is not bound to the actin, and the sarcomere is at its longest possible length. no impulse has occurred yet

step 2

an electrical impulse is generated from the brain and is carried to the muscle fiber by a motor neuron

step 3

impulse causes the release of the neurotransmitter acetylcholine (ACH) form synaptic vesicles

step 4

ACH diffuses across the synaptic cleft to receptor proteins on the sarcolemma

step 5

calcium ions are released from the sarcoplasmic reticulum

step 6

calcium ions diffuse over to the actin and the myosin filaments to initiate contraction, the binding sites on the actin filament become activated and the myosin crossbridges look for an active site to make a temproary linkage

step 7

after the crossbridge is made, the myosin begins to slide the actin toward the center of the sarcomere. ATP then allows cross bridges to release actin and then reset for the next powerstroke. cycle continues

step 8

the entire sarcomere begins to shorten in length

step 9

the sarcomere is at its shortest possible length, representing maximal contraction. the I-bands have almost disappeared and the actin filaments have been moved to as close together as they can get

step 10

the nerve impulse ends

step 11

calcium ions are reabsorbed back into the sarcoplasmic reticulum storage area, the myosin cross bridges are no longer able to make new linkages

step 12

ATP is needed to unjoin any linked crossbridges and actin filaments begin to slide away from myosine. relaxation is initiated

step 13

as relaxation continues, the sarcomere lengthens, the I-band enlarges, and the muscle fiber settles back to its original length