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Physiology 3120 Lecture Notes - Endoplasmic Reticulum, Dihydropyridine, Ryanodine

Course Code
Tom Stavraky

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Human Physiology
Wednesday, October 7, 2009
Sliding Filament Theory
T-tubule is continuous with extracellular fluid
Thin & thick filament lengths didn’t change
Sarcomere length shortens
Myosin (thick) pulls on the actin filaments, such that the filaments slide past each other
Length-Tension Curve
Maximal length of sarcomere is 3.65 microns
Tension at this point is very low
When the filaments overlap, cross-bridges can form
Maximal tension occurs at maximal overlap
Length of sarcomere is about 2.25 – 2.00 microns
When thin filaments overlap with themselves, friction prevents tension from reaching maximum
When either filaments come in contact with the Z-lines, there is no tension
Excitation-Contraction Coupling
The process by which an AP in muscle leads to release of calcium from sarcoplasmic reticulum,
crossbridge activity, and contraction
T-tubule is continuation of cell membrane
Thus, APs generated on end plate can travel deep into the muscle itself
Wraps around the myofibrils
Comes in contact (or very close to it) with the lateral sacs/terminal cisternae of sarcoplasmic
Sequence of events
1. AP on muscle cell travels down the T-tubules
2. Dihydropyridine (DHP) receptor/Ca channel (voltage sensor) on T-tubule changes conformation
due to the AP
3. DHP receptor opens ryanodine receptor/Ca channel on sarcoplasmic reticulum
4. Ca released from terminal cisternae of SR
Simple diffusion 100 million ions/sec/channel
5. Ca binds to troponin C on troponin complex
6. Tropomyosin rolls off myosin binding sites
7. Energized myosin attaches to actin
8. Cross bridge undergoes power stroke – actin slides over myosin
9. Cross bridge movement continues as long as Ca is present
10.Ca [concurrently] actively pumped back into SR
Tropomyosin re-covers myosin binding sites on actin
Ca ATPase – 20molecules/sec
90% of SR membrane covered by ATPase
Pumping Ca back into SR takes time so the muscle contraction outlasts action potential (100ms vs. 1-
One AP releases enough Ca to completely saturate all troponin molecules
Allows cross bridge formation & muscle contraction
This response to one AP is called a twitch, which is NOT a full contraction
Actin-Myosin ATP Cycle
At rest, myosin is attached to ATP, but it has not be hydrolyzed
Next step, hydrolyze ATP, which forms energized myosin
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