Re: What is the 'sliding filament hypothesis of muscle contraction'?

Stephen,

Although you call (as many textbooks do) it the 'sliding filament 
hypothesis', I would like to start off by emphasizing that it's no longer 
really a hypothesis, but rather fact.  The filaments have been seen by 
various types of microscopy (I've looked at them myself, in fact), and 
the 'pull' of each individual molecule has even been measured.  This is 
the way muscle works.  No doubt about it anymore.

There is a lot of information available that explains the sliding filament 
model.  You could search (as I did) at www.google.com (my favorite search 
engine) for 'sliding filament muscle' and you'll find lots of links, many 
with very in depth information.  Almost any physiology textbook will have 
a good description of the sliding filaments also.

Here is a web site which explains the organization of muscle: 
http://web.indstate.edu/thcme/mwking/muscle.html

Even better, you can sit through this whole movie on muscle (this is 
pretty cool, I recommend it highly):  
http://npbsn41.nimr.mrc.ac.uk/m-stewar/movie.html

(if that link doesn't work, try getting to it through here:  
http://npbsn41.nimr.mrc.ac.uk/m-stewar/introduc.html#play
(click on the orange 'play' button)

Basically, you see that the main things to think about are 'actin' 
and 'myosin' (also sometimes called thin and thick filaments), which are 
the 'filaments' that slide in the 'sliding filament' model.  Basically, 
actin and myosin are long stringy things lying in rows parallel to each 
other (the arrangement of the actin, myosin, and a few other things is 
called a 'sarcomere').  Myosin has these little bits that stick out, 
called 'heads', which can attach to actin.  After the heads attach to 
actin, the heads sort of 'nod', and that pulls on the actin and shortens 
the whole muscle.  It takes energy (from ATP) to release the myosin heads 
from the actin, so this process can start over again.  If you have no 
energy (perhaps because you're dead, your myosin heads can't release, and 
your muscles 'lock up'.  That's what rigor mortis is (when dead people get 
really stiff). (Eventually this stiffness wears off because the muscle 
starts to... uh.... rot.  Kind of gross.  Sorry)

So, you see, the filaments really do slide past each other, and the 'pull' 
of all the myosin heads is what generates the force in muscle.  A good 
movie of the shortening process is shown here:  
http://bio.winona.msus.edu/berg/ANIMTNS/SlidFila.htm
Here's another one: 
http://www.phymac.med.wayne.edu/CAI/sarc.htm

Don't hesitate to ask if you have more questions, or want more in-depth 
information.

-Dave