Skeletal and Smooth Muscles

Here are a few questions I have gotten regarding Skeletal and Smooth Muscles:  As always, if there are more please feel free to email me and/or leave a comment here on the blog!

1)  9. During excitation-contraction coupling in a skeletal muscle fiber which of the following occurs?

1. The Ca2+ ATPase pumps calcium into the T-tubule
2. Action potentials propagate along the membrane of the sarcoplasmic reticulum
3. Calcium floods the cytosol through the dihydropyridine (DHP) receptors
4. DHP receptors trigger the opening of lateral sac ryanodine receptor calcium channels
5. Acetylcholine opens the DHP receptor channel

Why not B?

The sarcoplasmic reticulum is an intracellular membraneous system that is equivalent to the endoplasmic reticulum.  Therefore, the action potential propagates along the sarcolemma and into the T-tubules, but not along the sarcoplasmic reticulum membrane.

2)  11.  What prevents a drop in muscle fiber ATP concentration during the first few seconds of intense contraction?

1. Because cross-bridges are pre-energized, ATP is not needed until several cross-bridge cycles have been completed.
2. ADP is rapidly converted back to ATP by creatine phosphate
3. Glucose is metabolized by glycolysis, producing large quanities of ATP
4. The mitochondria immediately begin oxidative phosphorylation
5. Fatty acids are rapidly converted to ATP by oxidative glycolysis

I don't understand the concept. 

This is not a concept that we covered in great detail as I stated I am not a biochemist.  However, in the notes section of slide 51 you can see that the fastest form of ATP to be formed is from the breakdown of creatine phosphate and therefore, the initial source of ATP that is utilized in a skeletal muscle fiber is that source of ATP.  That source maintains the concentration of ATP in the skeletal muscles initially, but is quickly used up and the muscle fiber then needs to turn to the ATP being created by glycolysis and oxidation phosphorylation.

3)  17.  In skeletal muscle, which of the following events occurs before depolarization of the T tubules in the mechanism of excitation-contraction coupling?

            A.  Depolarization of the sarcolemmal membrane

            B.  Opening of Ca2+ release channels on the sarcoplasmic reticulum

            C.  Uptake of Ca2+ into the SR by Ca2+ ATPase

            D.  Binding of Ca2+ to troponin C

            E.  Binding of actin and myosin

Why is it A?

The T-tubules are a membraneous system that is a continuation of the sarcolemmal membrane (otherwise called the sarcolemma).  Therefore, in order for the T-tubules to become depolarized, the action potential (depolarization) must first travel along the plasma (or sarcolemmal) membrane.  After the action potential propogates into the T-tubules, it will cause the DHP receptors to open, thereby causing the RyR to open on the membrane of the sarcoplasmic reticulum, releasing Ca2+ that then binds to troponin C, thereby initiating the interaction between actin and myosin.  This whole event is then terminated by the uptake of Ca2+ into the SR by the Ca2+ATPase.

4) I have question regarding tetanic contractions and temporal summation. Are tetanic contractions the result of temporal summation? I understand that tetanic contractions result from multiple stimuli being sent from the motor neuron to the motor unit, but would this be considered an example of temporal summation?

 

Tetanic contractions are the result of the summation of Ca2+ concentration within the intracellular space of the muscle cell, not the result of temporal summation of action potentials.  The action potentials themselves cannot summate, but the Ca2+ being released can be released faster then it is reuptaken back into the sarcoplasmic reticulum and can therefore cause a summation of the amount of tension being produced by the muscle cell.

5) I had a question regarding #10 in your practice problems for skeletal muscle. I didn't like answer choice C because eccentric contractions are also considered isotonic, and they would not require the extra time for the muscle to develop a greater tension than what is created by the load.

10.  Why is the latent period longer during an isotonic twitch of a skeletal muscle fiber than it is during an isometric twitch?

A.Excitation-contraction coupling is slower during an isotonic twitch

B.Action potentials propagate more slowly when the fiber is shortening, so extra time is required to activate the entire fiber

C.In addition to the time for EC coupling, it takes extra time for enough cross-bridges to attach to make the tension in the muscle fiber greater than the load

D.Fatigue sets in much more quickly during isotonic contractions, and when muscles are fatigued the cross-bridges move much more slowly

E.The latent period is longer because isotonic twitches only occur in slow (type I) muscle fibers.   

An isometric contraction starts the MOMENT there is an increase in tension in the muscle fiber.  This contraction continues to occur as that tension increases until the tension is equal to a load that is to be moved.  Once the tension is equal to the load, then the isotonic contraction can begin.  Therefore, the latent period for the isometric contraction is allows for the intracellular calcium  concentration to increase and cross-bridge cycling to begin.  The latent period for the isotonic contraction allows for those steps to occur, but ALSO allows for the isometric contraction to occur before the isotonic contraction is allowed to occur.  The latent period for the isotonic contraction is, therefore, longer than the latent period for the isometric contraction.

6) Why isnt there a dip in the total tension line on the graph for smooth muscles? What causes the dip in the skeletal muscles?  

Total tension is the combination of both active and passive tension.  In the skeletal muscles, the passive tension begins at a longer length (past the muscles 'rest length').  In smooth muscles, there IS some passive tension at the rest length of the muscles because there is some pull on the dense bodies at that length.  In addition, there is a broader active-tension curve in smooth muscles due to the shape of the myosin molecules with all head-groups facing an actin molecule directly.  The combination of these two differences creates a total tension curve (remember again, the sum of the two tensions) that does not have a dip, but rather is added together to only create a plateau and increase at longer lengths beyond that point.