Thanks Again Worms – You Inspire Me*

Muscle cells come in 3 main varieties – skeletal, smooth and cardiac.  As you might expect skeletal muscles are those that attach to the skeletal system and provide the contractile forces that let us (and other animals) move around. Not all animal locomotion is generated by skeletal muscles. Smooth muscles in worms generate a peristaltic contraction that results in the characteristic worm squirms. Peristaltic contraction is cool to watch. 

The basic principle is that the ring-like muscles squeeze one after another so that the contraction propagates down the length of the worm and it moves forward.  This would be like moving a sausage by squeezing it at one end then sliding your hand down the length of it.  With a sausage you might make a mess as the increase pressure of meaty goodness accumulates against the end of the casing and eventually bursts. 

With peristaltic contraction the action doesn’t work on moving a gooey meaty center but rather works to push the adjacent muscle forward a little bit.  It rained throughout the day today in the Minneapolis area and there are quite a few worms out.  I definitely recommend watching them move, it is pretty awesome. 

Worms aren’t the only place you can observe smooth muscles in action, in fact all you have to do is look in your own gastrointestinal system. I suppose this is easier said than done but if you pay attention you can at least feel the effects a little bit.  Smooth muscles line the intestines and their characteristic peristaltic contractions are responsible for squeezing the digested food further and further down the GI pipe.  You can’t feel this in action but you can feel it secondarily a little bit earlier on in the digestive system if you pay attention.

If you’ve ever taken a really big bite of food, upon swallowing you may have experience the awkward sensation of that food being “stuck in your throat”.  A common reaction to this is to drink something in order to force the food down or make a few hard swallows.  If you didn’t take that big swig or do some forced swallows in the still of your post bite moment you may have  start to feel the food inch its way down further and further. That is peristalsis. 

Swallowing is, for the most part, an involuntary reaction and once the food is just a few inches beyond your mouth it is out of your control.  Imagine a conveyer belt that is wrapped around to make a cylinder. Once the food is on the conveyer belt it is on a one way trip to its next GI destination.  The conveyer belt example falls apart a bit because the conveyer is ultimately arranged as a circuit the moves along with the transported item until it reaches its end then is returned, usually below and out of site to the beginning of the transport area. The smooth muscles don’t move along with the food, they stay in place and just push it farther along. 

A better example is a pitching machine.  Imagine a pitching machine with its two large rubber wheels spinning in opposite directions.  After you manually feed the ball into the hopper it is committed and it gets shot out the other end while the rubber wheels remain in place.  If you lined up a bunch of pitching machines in series so the ball was continually fed into the adjoining set of spinning wheels that would be more like peristaltic contraction.  It would also be awesome to see. Perhaps not as awesome as watching a worm move across the ground, but pretty close.

*In fairness I must also thank my lovely wife for pointing out the great congregation of worms we had in our yard tonight. Thanks for knowing what I like!