Smooth Muscle

Smooth Muscle Locations

Smooth muscle is found in locations usually associated with your viscera or your internal organs.  Smooth muscle is usually found in sheets. These sheets surround organs that need to change shape. This shape change allows things like gases and food to move through them.  Smooth muscle is usually found in two sheets. As pictured on the right side of this slide, these sheets surround a squishy organ.  One sheet is oriented so that the smooth muscle cellsThe basic structural and functional units of life. contract, reducing the diameter of the tubular organ.  The other, more superficialNear the surface of the body. sheet of smooth muscle has the muscle fiber oriented along the longitudinal axisSecond cervical vertebra; has the odontoid process (dens) for pivoting head (“no” motion)..  When this sheet contracts, the tube shortens in length, much like an inchworm.  The smooth muscle surrounding the bladderA muscular organ that stores urine before excretion. is slightly different, and we’ll discuss how that muscle works in a moment.

Smooth Muscle Anatomy

The anatomyThe study of the structure of the human body. of a smooth muscle fiber is very different than that of those cylinder-shaped skeletal muscle fibers.  Recall that these fibers are spindle shaped.  What’s a spindle?  That thing that Sleeping Beauty cut her finger on.  These are spindles up here.  In factA statement based on direct observation that is repeatedly confirmed., that tool up there is how you take a bag of wool and make it into yarn.  I’ve done that.  It’s an annoying process and I’d rather pay someone else to do it. 

There are no myofibrils, the cylinder-shaped contractile proteinsProteins (actin and myosin) responsible for muscle contraction., in smooth muscle. The arrangement of actin and myosin does not form sarcomeres.  This also means that there are no striations in smooth muscle as there are in cardiac and skeletal.  There are no alternating I and A bandsThe part of the sarcomere containing thick filaments..  Whereas skeletal muscle fibers were found in fasciclesBundles of nerve fibers within a nerve.
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Bundles of nerve fibers within a muscle., smooth muscle fibers are found in sheets.  The sheets are innervated by a nerve, but that nerve form varicosities, these bulbs here, instead of forming neuromuscular junctions. 

Smooth Muscle Contraction

Inside the muscle fiber, actin and myosin are kind of like twisted around each other like a twisty tie.  The actin fibers are attached to these dense bodiesStructures in smooth muscle that anchor actin filaments..  These are like anchors attached to the sarcolemma.  Think of typing your sneakers.  You have the shoelace threaded through all those holes and criss-crossed across the tongue of the shoe.  When you pull the shoelace, it tightens the entire structure. The actin shortens pulling those dense bodies closer to each other.  As each individual spindle-shaped muscle fiber shortens, the entire sheet shortens. 

Skeletal muscle had an optimum length-tension relationship The relationship between sarcomere length and contraction force., smooth muscle does not.  Smooth muscle fibers can get to maximum tension pretty much whenever it wants.  With skeletal muscle we were limited by the orientation of the sarcomere.  Remember my analogy of lifting the dryer?  That kind of thing doesn’t happen with smooth muscle. 

This, in part, is based on their ability to use calcium from the fluids around them.  Recall that skeletal muscle takes the calcium from its sarcoplasmic reticulum.  But, smooth muscle can supplement that, it takes calcium from the SR AND from the ECF. This is what enables it to have a slow and long protracted contraction. 

Smooth muscle is capable of expanding and then resting at the new length. This is exemplified by how the detrusor muscleThe smooth muscle layer of the bladder that contracts to expel urine. of the bladder works. As your bladder fills, the muzzle around it stretches to accommodate the increased volume. When your bladder remains at that volume, the detrusor muscle will relax. As more urineThe liquid waste excreted by the kidneys. enters it will stretch and relax again stretch and relax again.  Skeletal muscle, on the other hand, had an optimum length-tension relationship. Its range is respectable but falls between 70 and 130 percent of the original length. Smooth muscle, such as the detrusor muscle, can contract to 30 percent of its regular length. It can also contract up to 150 percent of its regular length.

Smooth Muscle Comparison

Smooth muscle is most commonly found in sheets of those spindles shaped cells. These sheets that surround almost all of your visceral organs. We indicated that there are two sheets. One is longitudinally arranged to shorten the organ upon contraction. The other is circularly arranged to decrease the diameter upon contraction. Most of these visceral sheets are controlled by your autonomic nervous systemThe part of the peripheral nervous system that controls involuntary functions such as heart rate, di which makes sense because smooth muscle is involuntary.

Most of these sheets can respond to hormones. This response is simply another form of control over organs, as opposed to nervous systemThe organ system that controls body functions using electrical and chemical signals. control. Individual smooth muscle fibers are scattered throughout your body. They all act together, despite being separated by location and having a large distance between them. These types of multi-unit smooth muscle fibers can only be stimulated by the nervous system. They do not respond to hormones.

Two examples of these are eye muscles. You have separate muscles on each eye. Both are controlled by the optic nerves and have exactly the same movements at the same time. The bulb of a hair follicle connects to smooth muscle fibers. This provides a great example of multi unit smooth muscle. These fibers make your hair stand up straight when contracted. This is the phenomenon we discussed back in the integumentary systemThe body system that includes the skin, hair, nails, and associated glands. topic. It happens when all of the hairs on the back of your neckNarrow region just below the head; common fracture site. stand up as you become frightened. Those hairs are controlled by individual smooth muscle fibers. These muscles are scattered and separated by location. However, they have the same control.