What’s your favorite muscle tissue? Odds are you’ll say skeletal muscle, the type used in all voluntary movements. Or, you might be partial to cardiac muscle, the main tissue component of your heart. But there’s much to appreciate in the third muscle tissue, smooth muscle. It’s a major component of your tubular organs – those of the digestive, urinary, reproductive, and respiratory systems, as well as your blood vessels – and for good reason.
The name “smooth muscle” refers to the lack of striations – the stripes visible on skeletal and cardiac muscle cells. Those stripes reflect a highly regular, organized arrangement of protein filaments that give great strength and efficiency to striated muscle tissues. But it comes at a cost – if you overstretch a skeletal or cardiac muscle cell, it becomes completely unable to contract. That’s because muscle contraction depends on the sliding of myosin and actin filaments past one another. Without any overlap to start with, the myosin molecules have nothing to grab onto.
Smooth muscle gets around this problem with a loose, net-like arrangement of myosin and actin. When the cell is stretched, this network starts to straighten out, which means each group of myosin and actin suffers little tension. The result is that much more overlap is maintained and these cells remain functional.
Why is this so important for a tubular organ? Many of your tubes undergo stretching – think of your stomach after a big meal — which in turn, stretches the muscle cells. But many other tubular organs undergo fluctuations in diameter, and smooth muscle allows them to contract under a wide variety of conditions.
Smooth muscle is also the only type of muscle cell that can divide after birth – a crucial feature in repairing a damaged wall after the passage of a chicken bone or a kidney stone. Also, blood vessels can grow and change shape in response to changing demands – made possible by the production of new smooth muscle tissue.
Let’s give smooth muscle a little respect. It may lack obvious “sex appeal” at first. But considering smooth muscle makes up a big part of your reproductive organs, maybe it’s the “sexiest” muscle tissue of all!