|MadSci Network: Cell Biology|
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
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