Microfilaments have dynamic properties. They
cannot be described simply as sticks or lines or bars... instead, one has to consider the
way they behave in a muscle. Remember, microfilaments are
made of protein, and protein is the one type of organic macromolecule that can
do almost anything!
You will be considering myosin
filaments (thick filaments) and actin filaments (thin filaments) separately in these
pages. You can find more about these by going to the "sliding filament
theory" section of your Interactive Physiology: Muscular System CD (A.D.A.M.);
in this CD, these filaments are described in pages 3 - 15.
A couple of terms & ideas that will help
you (just glance through them now and come back to them when you need
them):
 | "binding
site" is a location on a protein where a particular molecule can interact. On
an enzyme, the binding site is the active site. On a receptor in a membrane, it has
a binding site where it interacts with the extracellular signal. You will see that
actin and myosin have binding sites for each other, and even other things (at least
myosin).
|
| When proteins
interact with things, they typically go through a "conformational change."
That means that the proteins undergo a slight shift in their shape. Let me try to
explain:
 | When you have to pick up
something heavy with your hands, think about how your whole body has to change position to
help you pick it up. Your knees bend, your feet take a wide stance, etc. All
because your hands have to grab onto something and move it.
|
| When proteins grab onto
something (at a binding site), they also often end up changing shape, slightly, to deal
with this thing that they have grabbed onto.
|
|
| "terminal
cisternae" is a term used throughout the CD. This is the end region of the
sarcoplasmic reticulum (SR) that faces the sarcomere. The SR travels along the Z
lines with the t-tubules making that triad. But it cannot possibly cover every
single area within the muscle fiber, so where it lies next to a sarcomere, it is
considered the edge of it, or the end of it... this is the terminal cisterna. A
cistern is a storage area, cisterna is a fancier, biological word for it, and cisternae is
the plural form for it. This implies that the SR is holding something inside of
it. You will see that it is holding calcium ions (Ca2+).
|
Head on to
the pages on the individual microfilaments to learn more. If you start to get
confused, STOP reading the microfilament pages (or go on to the actin one) and just go on
to the sliding filament page. You will understand some of that, and then come back
and figure out the details by reading through these microfilament pages.
|
