This page serves to get you familiar with a myofibril-- the basis for the contractile ability of muscles.  By the end of this page, I hope that you are familiar with the microfilaments (cytoskeletal proteins) that make up the myofibril, and with the arrangement of those microfilaments within the myofibril.  After finishing this material, the next step is to understand how the microfilaments move in order to cause contraction... this mechanism is described as the Sliding Filament Theory.   Please keep in mind that this is the topic for next week's web pages.  You should not feel compelled to continue on to learn that information this week.   Therefore, if you link over to any other web pages on the myofilaments, please remember to stop before reading through the sliding filament theory.  OK?

General organization of this page:

Background:

    In the last page (muscle fiber composition) you read that the myofibril was a long tube of cytoskeleton.  This is true.  The myofibril contains the cytoskeletal elements that allow the muscle to contract... for that reason you will also see the cytoskeletal elements called the contractile apparatus.

    If you remember back to the review material from the beginning of the semester on the cytoskeleton in general, the cytoskeleton can be composed of microfilaments, intermediate filaments, and microtubules.   All of these things are made up of proteins.  The specific type of cytoskeletal elements involved in the contractile apparatus are microfilaments.   Microfilaments are the thinnest of the three major groups above.  However, being thin also can allow them to be flexible, which can be good for moving around.

    In order to understand the myofibril, you need to understand about microfilaments.  There are two main microfilaments:  actin and myosin.  Actin microfilaments are composed of the actin protein, while myosin microfilaments are composed of myosin proteins.  Remember that proteins have a 3-dimensional shape to them, so that they take on a certain appearance.  Let's consider each of the microfilaments independently...

Actin:

    The individual actin protein is called a "globular" protein ("g-actin") because it is globular, or ball-like, in appearance.   It takes many of these globular proteins coming together into a long chain to begin to make a microfilament.  In fact, the actin microfilament is two of these chains of g-actin twisted up together, and the filamentous form is then called "f-actin."   This is all depicted here in this schematic to the right; the two identical chains of actin are drawn in different colors only so that you can see how they come together.

    You should be able to see how the microfilament can get long, so it should make sense that it runs along the long axis of the myofilament.  You should also be able to see that the actin microfilament, although a doublet, is actually rather thin for its length (which would extend beyond the edges of your monitor)... that's why the actin microfilament has been nicknamed the thin filament.  This actin microfilament will also be associated with some other molecules, called troponin and tropomyosin, but we won't get to that in detail until next week.

Myosin:

    Myosin microfilaments, like actin microfilaments, are made up of many individual myosin protein molecules.  However, the myosin protein is not globular.  Instead, it has a head and a tail (these regions are indicated in the figure to the left).  And each complete myosin molecule in muscle is actually composed of two of these head-and-tail molecules twisted around each other.

   Note that there are three different polypeptides that contribute to this overall myosin protein... that is not important for you to memorize, but you will see it as you browse through the web, so I have it in this photo; you only need to know about the head and the tail for this class.

    Then, to make the myosin filament, you have to take these doublet myosin molecules and put them together into large bundles.  This big wad of myosin proteins is then nicknamed the thick filament.  A single thick filament typically has over 200 myosin molecules in it!  So it is really very thick.   This is shown in the figure below (the A with the circle over it in this figure stands for Angstroms, which are a unit of length measurement... 1,000,000 Angstroms fit into one micrometer, and 1,000,000 micrometers fit into a meter; you do not have to memorize the dimensions):

Again, the thick filament runs along the long axis of the myofibril.

Please note:  I found the above two drawings from educational sites, but that was a couple of years ago and I have lost the links.  I will attempt to find them again!

Are you ready to learn how to put the thin and thick filaments into the myofibril now???  Good!

Arrangement of thin and thick filaments in a myofibril:

    The thin and thick filaments are organized into neat bundles called sarcomeres.   You can read about sarcomeres in your book.  You can also check out the Muscle Page from a course at Stanford, and look at the sarcomere link for "other browsers."  I'm just going to give you the basics here.

    Thin filaments attach at a point called the "Z line" so that they are all lined up with one another.  This is shown in this schematic to the right.   The Z line is the dark line that runs perpendicular to the actin filaments.   Actually, it is simply a lot of sticky proteins that anchor the actin filaments in place.

    The thick filaments run in between the thin filaments.  I have put them in for you to see, but first I had to change the background color of the image so that both the thick and thin filaments would be readily visible.  I also had to shrink the components down a bit more.

    Here is the image of the thick and thin filaments together:

  In order to fit the thick filaments between the thin, I needed 2 sets of the thin filaments.  Also, it looks like the thick filaments are just floating in the middle... however, they are anchored by proteins (of the "M line"); I just didn't show that.  As you look at this image, the following items should become easier to understand:

1. A sarcomere runs from Z-line to Z-line.
2. Sarcomeres run along the longitudinal axis of the muscle fiber.
3. One sarcomere connects directly to the next... so I extended the drawing above just a little bit to help you imagine this:  You can also look at Figure 9.4 in your book to see this.
4. If you look at the sarcomere from the side , it would look darker in the area where the myosin runs than in the area where the actin runs.  Because of this, we can talk about light and dark bands.  The dark band, where the myosin runs, is called the A band.  The light band, where the actin runs, is called the I band.
5. Now for the culmination of the entire page... how does this relate to the myofibril?   A chain of sarcomeres, like the one I drew above in point #3, that runs from one end of the muscle fiber to the other end of the muscle fiber is a myofibril.  It is these chains of sarcomeres, or myofibrils, that are going to allow for contraction, since they are made up of cytoskeletal machinery.  Therefore, our muscle fiber is going to need many of these myofibrils in order to be good at contraction.

Putting the myofibrils back into the muscle fiber...

    First of all, there are hundreds of myofibrils in each muscle fiber.   To see that, take a look at the muscle section of the JayDoc HistoWeb, slides 6 & 7 in their expanded views.  These are images of cross sections through muscle fibers... you'll see many dots on the cut edges; each of those dots is a myofibril.

    Adjacent myofibrils line up evenly with each other.  That means that the Z-lines of every sarcomere in one myofibril lines up with the Z-lines of every sarcomere in the adjacent myofibrils.  Because of that, the I bands and the A bands in all the myofibrils within a muscle fiber are lined up.  This is a difficult point to be able to understand with mental imagery.  Take a look at the image here (stolen from the muscle section of the JayDoc HistoWeb, slide #2... you can go there and see the expanded view).

<-----The muscle fibers & the myofibrils run this way----->

t
|
|
But the A and I bands
run up and down
|
|
u

    If you look up again at the drawing above of the sarcomere, you'll see that the sarcomere runs left and right, but the bands run up and down.  Just like in this photo.  This causes the muscle fibers to look striped, and this appearance is called striated.  We can say that the muscle fibers are striated because they have striations (stripes).

    Take a look at your textbook Figure 9.5 to see how adjacent myofibrils line up within a muscle fiber.  For the sake of clarity, this figure only shows 9 myofibrils within the muscle fiber-- but there would be hundreds!

    Now if you think back to the last web page, you'll remember that there are cell membrane invaginations called t-tubules.  These t-tubules run into the muscle fiber at the Z-lines (although your book's Figure 9.5 doesn't really show it like that).

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