The normal characteristics of smooth muscle: 

Smooth muscle is found in many different places in the human body:  Unlike skeletal muscle, smooth muscle is generally not identifiable from gross observation of the body.  You have to know where to look where to find it. The following animation will show you some places smooth muscle exists as well as some of its characteristics. Notice how different it looks from one place in the body to another.   

Each of the pictures in the show  is an illustration of the smooth muscle found in those places of the body.  In the trachea, the smooth muscle is found on the dorsal side of the trachea, in a band of muscle that interrupts the cartilaginous rings of the trachea.  In the smaller airways, the smooth muscle is more uniformly distributed.  

Smooth muscle is used for different purposes than skeletal muscle:  If you think back to skeletal muscle physiology, we went through many aspects of skeletal muscle physiology that were quite useful for us in our voluntary activities.  However, there are a lot of places in the body where using skeletal muscle is not an ideal (or even workable) solution.  These include places where: 

    •  we need muscle contractions that last for the vast majority of the day, whether we are awake or asleep, as occurs in blood vessels or the sphincters of the GI tract.   In the case of the blood vessels, we need some sort of muscle that will maintain a contraction even if the brain has been severely damaged (obviously a problem if we were to have used skeletal muscle). 
    • we need muscle that can contract periodically to mix the contents even when the muscle has been stretched (say, by a delicious Thanksgiving dinner).   If you'll recall, skeletal muscle is very sensitive to changes in length and the tension developed decreases radically if the muscle is either too long or too short.  
    • we need muscle that will help us maintain the shape of the organ, but we don't have bone for the muscle to attach to.  Skeletal muscle has to be attached to something fairly solid or it doesn't work as well as it should.  
    • we need muscle that responds both to local influences (particularly in blood vessels - how much blood does this tissue need right now?) and to the brain (does another part of the body need blood more than this part?).   Skeletal muscle is quite good about responding to what the brain tells it to do - in fact, that's the only thing skeletal muscle responds to.  Unfortunately, there are conditions when we want muscle to respond to some other influence.  Skeletal muscle can't do that.  
    • Oh, and just for kicks, we need this muscle to be very energy efficient (using as little ATP as possible) under many circumstances, including contractions that have to be maintained for many hours at a time.  Skeletal muscle has lots of mitochondria because it needs them to supply it with ATP.  Smooth muscle doesn't have nearly as many mitochondria - so how does it get by?  

Smooth muscle is histologically very different from skeletal muscle:  As you might guess from the different demands placed on smooth muscle, the histology of the smooth muscle cell is quite different than that of the skeletal muscle cell.  

  The major differences that are important to the remainder of our discussion: 

    • Although actin and myosin both exist in smooth muscle, they are not organized in an orderly fashion so that there are no striations.
    • Smooth muscle does have a sarcoplasmic reticulum, but it is not as complex as that found in skeletal muscle.  Like the S.R. found in skeletal muscle, it stores calcium and, with the proper stimulus, releases the Ca++ into the smooth muscle.  
      • Important note:  older histology and physiology books state, often quite definitively, that smooth muscle does not have an S.R.  Better imaging techniques developed in the last 5 - 10 years have proven this assertion wrong.  Some relatively recent books may still have old information.  
    •  Unlike skeletal muscle, smooth muscle cells may be interconnected via gap junctions.  These gap junctions allow neighboring smooth muscle cells to be activated.  

  Smooth muscle has many of the same biochemical components that skeletal muscle has (but there are some differences):  At the heart of the matter, smooth muscle has to contract - and skeletal muscle has a very workable method of producing contraction.  Therefore, it shouldn't be terribly surprising that smooth muscle uses much of the same apparatus that skeletal muscle does.  For example, smooth muscle has both thick and thin filaments, composed of myosin and actin (respectively).   However, if you go back to our list of what smooth muscle has to do (click here to review the list), it should also be obvious that there have to be some significant differences in the basic biochemistry of the smooth muscle cell. 

     To name a couple important differences - the myosin found in smooth muscle has a much slower myosin ATPase and slower crossbridge cycle, while there is no troponin found in the thin filament.   As you can probably guess, the lack of troponin is an indication that excitation-contraction coupling in smooth muscle is a bit different than in skeletal muscle.  In addition, there are some additional proteins found in the smooth muscle. The calcium-binding protein calmodulin is found in smooth muscle as an independent protein, while the myosin has both a light chain kinase and phosphatase associated with it.

The Kansas City University of Medicine and Biosciences