Physiology question - action potential and muscle contraction

[Owlie]

My A&P book simply isn't going into as much detail as I would like. It helps me to understand how things occur; for instance, the release of Ca++ from the sarcoplasmic reticulum (SR).

ACh reaches the sarcolemma at the motor end plate, causing an influx of Na+ and a leaving of K+, with a net of more a higher positive charge left on the inside of the sarcolemma, which was previous negative (most of this I had to look up elsewhere).

How does this new inner positive, and outer negative, charge extending along the sarcolemma, and t-tubules to cause Ca++ to exit the SR? Wouldn't Ca++ want to move toward the negative charge in the t-tubules through the membrane?

Thanks!

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[Freesia88]

My A&P book simply isn't going into as much detail as I would like. It helps me to understand how things occur; for instance, the release of Ca++ from the sarcoplasmic reticulum (SR).

ACh reaches the sarcolemma at the motor end plate, causing an influx of Na+ and a leaving of K+, with a net of more a higher positive charge left on the inside of the sarcolemma, which was previous negative (most of this I had to look up elsewhere).

How does this new inner positive, and outer negative, charge extending along the sarcolemma, and t-tubules to cause Ca++ to exit the SR? Wouldn't Ca++ want to move toward the negative charge in the t-tubules through the membrane?

Thanks!

This should help...

http://highered.mcgraw-hill.com/sit...action_potentials_and_muscle_contraction.html

I was super confused by it as well. I think Ca++ binds to Troponin as Troponin has receptors specifically designed for that purpose.

When the acton potential stops due to Acetylcholine being broken down by acetylcholinesterase, SR stops releasing Ca++ and starts to reabsorb it. Seems to be an active process not exactly dependent of concentration or electrical gradients but not sure...

 

[Owlie]

This should help... I was super confused by it as well. I think Ca++ binds to Troponin as troponin has receptors specifically designed for that purpose

http://highered.mcgraw-hill.com/sit...action_potentials_and_muscle_contraction.html

Thanks! I hadn't thought of voltage-gated channels.

How does the change in charge open them exactly? Wouldn't it also only open those channels adjacent the t-tubule, rather than opening to the sarcomere?

 

[Freesia88]

Thanks! I hadn't thought of voltage-gated channels.

How does the change in charge open them exactly? Wouldn't it also only open those channels adjacent the t-tubule, rather than opening to the sarcomere?

here is a quote from a website to help you...this for me was one of those things I did not try too much to understand...just get the basics down...

" The primary function of the SARCOPLASMIC RETICULUM is to STORE CALCIUM IONS. Sarcoplasmic reticulum is very abundant in skeletal muscle cells and is closely associated with the MYOFIBRILS (and, therefore, the MYOFILAMENTS). The membrane of the SR is well-equipped to handle calcium: there are "pumps" (active transport) for calcium so that calcium is constantly being "pumped" into the SR from the cytoplasm of the muscle cell (called the SARCOPLASM). As a result, in a relaxed muscle, there is a very high concentration of calcium in the SR and a very low concentration in the sarcoplasm (and, therefore, among the myofibrils & myofilaments). In addition, the membrane has special openings, or "gates", for calcium. In a relaxed muscle, these gates are closed and calcium cannot pass through the membrane. So, the calcium remains in the SR. However, if an impulse travels along the membrane of the SR, the calcium "gates" open &, therefore, calcium diffuses rapidly out of the SR & into the sarcoplasm where the myofibrils & myofilaments are located. "


Source : http://people.eku.edu/ritchisong/301notes3.htm

 

[Owlie]

here is a quote from a website to help you...this for me was one of those things I did not try too much to understand...just get the basics down...

" The primary function of the SARCOPLASMIC RETICULUM is to STORE CALCIUM IONS. Sarcoplasmic reticulum is very abundant in skeletal muscle cells and is closely associated with the MYOFIBRILS (and, therefore, the MYOFILAMENTS). The membrane of the SR is well-equipped to handle calcium: there are "pumps" (active transport) for calcium so that calcium is constantly being "pumped" into the SR from the cytoplasm of the muscle cell (called the SARCOPLASM). As a result, in a relaxed muscle, there is a very high concentration of calcium in the SR and a very low concentration in the sarcoplasm (and, therefore, among the myofibrils & myofilaments). In addition, the membrane has special openings, or "gates", for calcium. In a relaxed muscle, these gates are closed and calcium cannot pass through the membrane. So, the calcium remains in the SR. However, if an impulse travels along the membrane of the SR, the calcium "gates" open &, therefore, calcium diffuses rapidly out of the SR & into the sarcoplasm where the myofibrils & myofilaments are located. "


Source : http://people.eku.edu/ritchisong/301notes3.htm

I see, so it seems the t-tubule increases the 'surface-area' and reaches near the SR in order to make sure it gets the signal close to it before it lapses. I think. Thanks!

 

[Freesia88]

I see, so it seems the t-tubule increases the 'surface-area' and reaches near the SR in order to make sure it gets the signal close to it before it lapses. I think. Thanks!

T tubules do indeed dive into the SR enabling the action potential to cover a lot more ground meaning more Ca ++ channels are open. Whether those channels are primarily on the surface of the SR or deep, I don't know and I doubt it matters much... unless it was some molecular biology class...??? And again I don't think the SR is a thick membrane such that there is a lot of layers for the action potential to work through... I should look that up... SR is the ER of muscle cells, I think, so it is a layer within a cell, which I imagine would be very very very thin...

 

[theseeker4]

T tubules do indeed dive into the SR enabling the action potential to cover a lot more ground meaning more Ca ++ channels are open. Whether those channels are primarily on the surface of the SR or deep, I don't know and I doubt it matters much... unless it was some molecular biology class...??? And again I don't think the SR is a thick membrane such that there is a lot of layers for the action potential to work through... I should look that up... SR is the ER of muscle cells, I think, so it is a layer within a cell, which I imagine would be very very very thin...

What do you mean by "very very very thin"? The membrane of the SR, ER, cell membrane, etc. is basically a phosphlipid bilayer, regardless of its location. Associated proteins, sugars, cholesterol content, etc. can modify the membranes in various ways, but the "thickness" of the membrane is basically constant among the various types of membranes, unless you are talking about something else..

 

[Owlie]

T tubules do indeed dive into the SR enabling the action potential to cover a lot more ground meaning more Ca ++ channels are open. Whether those channels are primarily on the surface of the SR or deep, I don't know and I doubt it matters much... unless it was some molecular biology class...??? And again I don't think the SR is a thick membrane such that there is a lot of layers for the action potential to work through... I should look that up... SR is the ER of muscle cells, I think, so it is a layer within a cell, which I imagine would be very very very thin...

I believe the SR is just phospholipid bilayer, because it acts like a Smooth ER in a "normal" cell. It's for A&P, and it helps me to get how this occurs in order to more fully assimilate the concept.

I spoke with my professor, and she said we're right. The T Tubules are continuous with the SR, and have channels to allow the negative charge to directly effect the voltage-gated ion channels in the SR that open at a lowered charge, allowing Ca++ to flow into the sarcomere area. They don't uptake into the t-tubules (except for maybe a few), because there are so many Ca++ in the SR, and so few in the sarcomere cytoplasmic area, that most instead diffuse toward the sarcomere, as opposed to everything diffusing toward the negative charge in the t-tubules.