CERTAIN CONCEPTS IN MUSCLE PHYSIOLOGY

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# Excitation-Contraction Coupling; This refers to the series of events by which the transmission of action potential along the sarcolemma leads to the sliding of myofilaments. Steps involved in E-C coupling include;
1 Propagation of the AP along the sarcolemma  and down the T-tubules
2 Release of Ca ion; As the the AP is transmitted along the T-tubule, it causes a change in the proteins of the tubule (which are obviously voltage sensitive). This shape change in the tubule cuases the opening of the Ca ion release  channels in the terminal cisterns of the sarcoplasmic recticulum and so an influx of this ion into the cytosol ( sarcoplasm)
3 Calcium binds to troponin; When the Ca ion is released, it binds to the troponin thereby removing the blocking action of the tropomyosin  and exposing binding sites for myosin on the thin filaments
4 Contraction begins; At this point, myosin binds to actin and cross bridge is formed and contraction begins. ( at this point E-C coupling has ended)
This is the summary of the steps involved in E-C coupling but note that when talking about this concept, we are concerned about the roles of certain important factors; ATP ( be sure to mention the role of ATP before and after E-C has been concluded), sarcoplasmic recticulum, how calcium stimulates excitation. Note that once the cross bridge has been established, E-C stops ( by the myosin breaking off its attachment using ATP) but if there is nothing stopping the reattachment of the myosin, a new crossbridge cycle would be established.
# Cross Bridge Cycle;  This refers to the series of events DURING which the myosin heads pull the thin filaments towards the middle of the sarcomeres. Steps include;
1 Creation of the cross bridge; the energized myosin head attaches to the actin myofilament and croos bridge is established
2 The power stroke; here ATP is broken down and ADP and Pi is released and the myosin enters it low-energy state. The myosin then pulls the actin filament toward the Mline.
NOTE; At this stage in the absence of ATP, the myosin will not detach and rigor mortis will occur
3 Detachment of cross bridge; When ATP attaches to the myosin head. The myosin and actin attachment weakens and the cross bridge breaks off
4 Cocking of the myosin head; When the cross bridge has been broken, then the myosin returns to its prestrike high-energy state ( cocked position)
This cycle will continue as long as there is presence of ATP and Ca ion is attached to troponin, if not it stops at the power stroke level.

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