Smooth muscle shows multiple kinds of plasticity

A definition first - in a physiological situation, plasticity is used to describe long-term changes in how a cell/tissue functions, although it may also include long-term changes in the appearance of the cell/tissue.   Some changes come under the heading of "mechanical plasticity" - meaning that the muscle can change the mechanics of how it works, even though the basic mechanism of contraction remains the same.  Skeletal muscle shows several kinds of plasticity that will be discussed in a later physiology lecture.  

Smooth muscle's response to stretch is one form of plasticity:  Let's think about the stomach for a moment - every Thanksgiving Day, most of us eat enough food to stock a small pantry for at least a month...   This food goes to our stomach and stretches it.  The muscle of the stomach has a job to do though:  it is supposed to contract and mix the ingested food with the secretions of the stomach and then propel them into the intestine.  If we used skeletal muscle in the stomach, we would have a serious problem - skeletal muscle is not able to generate much tension if we stretch it (or make it too short).  We acted this out in class - either the filaments are spread too far apart or they run into one another.  

The length-tension (usually called force-length) relationship of smooth muscle is very different.  If you'll recall back to skeletal muscle, when we stretch the skeletal muscle, we get a decrease in the active tension (what the cross-bridges are doing) and an increase in passive tension (the non-contractile elements like the membrane behave like stretched rubber bands, cause passive tension).  If we stretch smooth muscle, we see a small, temporary increase in passive tension, but that rapidly returns to normal.  The ability of the smooth muscle cell to generate active tension remains normal over a very wide range of changes in length. 

Smooth muscle can do this because it has a way around this problem - and it ties back to its histology.  Remember that smooth muscle is called smooth because it shows no striations.  The reason it has no striations is that the thick and thin filaments are not lined up in the rigid way that we see in skeletal muscle.  Well, this lack of a rigid structure means that I can stretch the muscle fiber and there will still be some myosin head able to bind to the active site.  It may not be the same one that  usually binds to the site, but that doesn't matter - all we need to do is get some binding and the rest of the crossbridge cycle continues normally, allowing for force generation. 

Other Forms of Plasticity

Smooth muscle can also a show a far more drastic form of plasticity, often called "phenotypic plasticity" Although the term "phenotypic plasticity" sounds rather complicated (and the process is probably complex - we don't understand it to any great degree), what it means to you (and to Scott) is that smooth muscle can change its appearance and its function in response to a wide array of stimuli.  These changes can be quite drastic - in response to the right stimuli, smooth muscle can lose it's contractile apparatus and become a secretory cell.  These secretory smooth muscle cells can begin to secrete collagen and other components of the extracellular matrix.  The process of losing the contractile apparatus and gaining the secretory function is called modulation.  The process can be reversed - in maturation, a secretory smooth muscle cell loses its secretory functions and synthesis the contractile machinery necessary to become contractile.  (For test taking purposes, a "smooth muscle cell" is contractile and has all the contractile apparatus necessary.  Only with the specfication of "secretory smooth muscle cell" does the question refer to  a smooth muscle cell that has undergone modulation and begun to secrete collagen and other extracellular proteins.)  

This form of plasticity is believed to be very important in the development of atherosclerosis and primary pulmonary hypertension.  Although we don't understand the full scope of the biochemical cues (many are inflammatory in origin) that produce the modulation of smooth muscle cells into secretory smooth muscle cells, histological observations of the vascular lesions seen in these diseases show that the smooth muscle has undergone dramatic changes.  Needless to say (have you ever noticed how people say that right before they say the thing it isn't necessary to say???), a great deal of research related to this topic is being pursued in attempt to prevent/cure these diseases.  

Note:  Some books refer to the possibility that smooth muscle cells can de-differentiate and become fibroblasts, thus becoming secretory in nature.  Whether you name them "secretory smooth muscle cells" or "fibroblasts", the end-result is still the same - what was once a contractile cell has become a secretory cell. The issue of exactly what is happening has not been resolved - Dr. Karius will use the term "secretory smooth muscle" to indicate a smooth muscle cell that has lost its contractile apparatus and become secretory in nature.      

Smooth muscle cells can move from one place to another:  Unlike skeletal muscle (which is strictly constrained by the structure of the muscle), smooth muscle cells can migrate (move) over some distance. 

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