Question 12

[M.C.]

Dr. Ferdinand,

What is cross-bridge formation in muscles and how does it improves contractile power of the muscles?

I was wondering if it is the same as musle contraction?

Thanks

[Matthew]

Cross bridges are where the actin filaments attach to the myosin heads. So when you are trying to understand how cross bridges generates force, think of a strong arm (connection) that can grab hold tightly to a muscle (myofibril).

[Matthew]

http://en.wikipedia.org/wiki/Muscle_contraction

Found it to be very conclusive. Especially the brain dealing with the voluntary and spinal cord dealing with the involuntary.

[admin]

To understand the levels of organization of muscle, pay special attention to the diagram http://upload.wikimedia.org/wikipedia/commons/c/c0/Skeletal_muscle.jpg or if you have the Gold Standard, BIO 5.2 has a full page schematic on muscle from the molecular level to the gross level.

[mvenus9292913]

Also, though I'm not sure if it matters for the MCAT or not, but when you first move a muscle, only some of the cross bridges are used. To increase force, you increase the number of active cross bridges. This can be done by increasing the frequency or voltage of stimulation, or by increasing the number of neurons sending messages.

[anon3543]

From what I understood, the stretching of the heart actually decreases overlap between myosin and actin fibers (thus decreasing the amount of myosin head to actin bonds initially), giving it more room to slide back in resting position and thus exerting a greater contraction. In other words, there is some overlap between actin fibers in the middle of sarcomeres of the resting cell of the heart, which makes contracting anymore difficult. Stretching the sarcomere at first will increase the number of cross bridges by eliminating this actin overlap, but subsequent stretching (most of it) will start decreasing the number of cross bridges, but allowing more room to decrease back in length. Obviously even more stretching beyond some optimal overlap makes the contraction weaker, but I still think the bulk of the strength gathered from the increased heart contraction is from the actually less actin/myosin overlap (less cross bridges) because it allow more space to become small in length again. Ok, that was redundant...


12. Ventricles are able to increase their output only up to a certain point upon exposure to increased volume. Beyond that the output will drop. This is most likely explained by which of the following?

1. Increased volume is associated with the separation of actin and myosin molecules from each other up to a certain point. [x]
2. Increased volume causes formation of cross bridges between muscular myosin and actin molecules up to a certain point. [x]
3. Increased volume causes generation of new cardiac muscle fibers up to a certain point. [x]
4. Increased volume causes formation of lactic acid up to a certain point. [x]

INCORRECT:
Your Answer: A
Correct Answer: B

[admin]

[quadalpha]

So what explain the subsequent drop in output?

[addiaz2714087]

Like anon3543, I also thought the answer was A.
First paragraph says
"....one of the major factors contributing to the force of contraction is the INITIAL LENGTH OF THE CARDIAC MUSCLE. The increased length (preload) is related to the volume of the cardiac ventricles right before contraction".
The sarcomere has an optimal operating length- once overstreched, the thin and thick filaments wont be able to overlap, and therefore sliding will not be able to occur.
The stretching of the muscle then is more relevant here than the formation of cross bridges. Actually I remember reading somewhere that only a fraction of potential cross bridges are formed at any given time.

[admin]

In the human heart, maximal force is generated with an initial sarcomere length of 2.2 micrometers, a length which is rarely exceeded in the normal heart. Initial lengths larger or smaller than this optimal value will decrease the force the muscle can achieve. For larger sarcomere lengths, this is the result of less overlap of the thin and thick filaments; for smaller sarcomere lengths, the cause is the decreased sensitivity for calcium by the myofilaments.


As you said, “The sarcomere has an optimal operating length”.

Initial lengths larger or smaller than this optimal value will decrease the force the muscle can achieve. For smaller sarcomere lengths, the cause is the decreased sensitivity for calcium by the myofilaments. For larger sarcomere lengths, which is the case here, this is the result of less overlap of the thin and thick filaments; answer
choice B