You saw in the last web page (on the sliding filament theory) that actin and myosin slide past one another (as myosin pulls along) to shorten a sarcomere.  Sarcomere shortening should therefore be something that you can imagine (and it is shown on the web page I linked you to in the sliding filament theory page).

    I have drawn my own version of a sarcomere shortening.  Take a look:

Now, imagine that there are more of these sarcomeres, in a long chain to make a myofibril...  I have made that as well, but to fit them on the page I have shrunken the size of the individual sarcomere by 1/3.  Here are three sarcomeres in a chain:

And here are 6 sarcomeres in a chain:

    Have you noticed how when the individual sarcomeres all shorten, the entire myofibril shortens?  The left and right edges of the myofibril have moved much farther in... I have tried to show you this here with still images:

     The top drawing is the long version of the 6-sarcomere-long-myofibril.  The Z-lines at its outside edge go all the way to the ends of the image.  The bottom drawing is the shortest version of the 6-sarcomere-long-myofibril.   The purple arrows were added to point out just how much the entire myofibril shortened.  You see, as each sarcomere gets shorter, the entire myofibril shortens by that much; so the longer the myofibril in sarcomere numbers, the more it can shorten.   Now consider our own muscle fibers that contain myofibrils with hundreds of sarcomeres... it's incredible just how much they can shorten!

    We are still no where near an entire myofibril or an entire muscle fiber, let alone a muscle, in my drawing with only 6 sarcomeres!  We're going to have to add many, many more sarcomeres to start to create a real muscle!

    So, now you have to use your imagination to picture how even larger sets of sarcomeres might shorten because I just can't really make an animation of an entire muscle fiber shortening-- remember, my Ph.D. is in biology, not in computer animations!!!

    Let's build a larger, static, muscle... Here are four myofibrils that are each 12 sarcomeres long:

Note that I had to shrink the size of each sarcomere so that the drawing still fit on the page.  I shrank it by 50%.

    How about an even larger set of myofibrils and sarcomeres?  Here's a drawing of 8 myofibrils, each with 24 sarcomeres (again, I had to shrink the size of the individual sarcomeres by 50%):

    Let's keep going... here's a drawing of 16 myofibrils, each with 48 sarcomeres (again, shrunken 50%):

    It starts to get pretty overwhelming, huh?  Well, just don't lose sight of the basics... the myofibrils are each running from left to right.  They line up at their Z-lines, so all the Z-lines are in columns from top to bottom.  I think that this image is pretty neat, since now you are starting to see the striations that you can see under the microscope.  A view of muscle like this is similar to what you would see when you look with your oil immersion lens (100X objective for a total magnification of 1000X) at skeletal muscle.

    Back to shortening, though... Whenever a muscle fiber is activated, every single one of its sarcomeres in every single one of its myofibrils will shorten.  Can you think for a second about why it is an all-or-none situation for sarcomere shortening within a muscle fiber?

    Hint:  in order for the filaments to slide, calcium needs to be released from the SR...

    Have you figured it out yet?

    OK.  Now I'll tell you, but I hope you thought about it first.

    When an action potential runs along the sarcolemma, calcium ions are released from the SR.  The SR is found throughout the entire muscle fiber, right next to the t-tubules of the sarcolemma.   So, as the entire sarcolemma is electrically active (because of the action potential occurring on it), all of the SR is releasing calcium.  Once the calcium is out, it is in the sarcoplasm.  The sarcoplasm is continuous throughout the entire muscle fiber.  So all the sarcomeres of all the myofibrils are exposed to it.   That's what they need to contract, so they start doing it.  Only when the calcium is taken back up into the SR do the sarcomeres stop shortening.

    The only piece of information that I haven't explained here is why the action potential on the sarcolemma causes calcium ions to be released.  That would require a very detailed explanation, beyond the scope of our course.  The entire phenomenon is called excitation-contraction coupling, since the excitation of the sarcolemma causes the contraction (due to calcium release) of the sarcomeres.  But the precise mechanism is too detailed (and not thoroughly understood even by scientists), so don't worry about it.

    Once a single muscle fiber contracts (due to all of its myofibrils shortening), that affects the length of the entire muscle.  Why?  Because all the muscle fibers within the muscle are surrounded by endomysium (connective tissue coating, remember?).  The connective tissue of the endomysium is continuous with the connective tissue of the perimysium which is continuous with the connective tissue of the epimysium which is continuous with the connective tissue of the tendons.  So, any shortening of the muscle fibers within the muscle will have an effect on the entire muscle.  Got it?

© 2006 STCC Foundation Press
written by Dawn A. Tamarkin, Ph.D.