Muscles are categorized into smooth, cardiac, and skeletal. This categorization is based upon their structural and functional properties. Cardiac, as the name suggests, are the muscles of the heart. Skeletal ones are in conjunction with the bone structures and are also known as voluntary. The cardiac and skeletal types can be termed as striated muscles, as they contain alternate light and dark bands, whereas the smooth ones are non striated. They are present in the walls of hollow organs due to which these organs can contract. One such example is the intestine. This article tells you about the smooth muscle function.
The major fact that these muscles are classified as smooth, is the absence of striations in these as compared to the others of the muscular system. They are further classified as single unit and multi unit fibers. Their arrangement is such that the fibers form dense sheets and the autonomic nervous system innervates just one cell in the bundle and the action potential generated is conveyed to the surrounding cells via gap junctions. In multi-unit fibers, the nerves innervate every cell in the bundle. Most of the muscles constitute of the single-unit fibers. However, the organs, like the trachea, large arteries, and the iris consist of the multi-unit type.
- Since smooth muscles are present in the walls off hollow organs, they help in contraction, which allows movement of substances within or from the organ to another. The contraction and dilation thus facilitates fluid movement as well.
- Cells are connected to one another by adherens junctions to resist the change in shape of the organs when force is applied. Thus, when one cell in an organ or tissue contracts, contraction sets in the adjoining cells as well.
- Single unit ones have gap junctions within the bundle through which the signal for contraction is mediated. Their contraction is spontaneous via ionic channels or is induced by chemical transmitters, such as hormones, neurotransmitters, drugs.
- The single unit ones line the blood vessels, digestive system, and the urinary tract. They form valve-like structures called sphincters that regulate the flow of fluids or blood within these systems.
- They also show different patterns of contraction. One is the rhythmic contraction with rapid contraction and relaxation, and the other is tonic movement where the contraction is slow.
- The reproductive system, digestive system, urinary tracts, show this tonic movement.
- The movement of these muscles in the arteries maintains the diameter of the arteries, and thus, plays a vital role in the maintenance of blood pressure.
- Within the digestive tract, the peristaltic movement is due to these muscles. This function in the intestine helps in digestion of food and also forces the food from one organ to the other in the digestive system.
- Most of the contraction in systems, such as the lungs, kidneys, and reproductive system is stimulated by hormones and transmitters such as norepinephrine and epinephrine.
- The smooth muscle movement within the vasculature is also stimulated by these catecholamines (epinephrine and norepinephrine), due to which vasoconstriction and vasodilation is possible.
- In the iris of the eye, they are known as the pupillary sphincter muscle, which is responsible for the changing size of the pupil in light and dark.
- Within the urinary tract, they help to expel urine from the body by contraction.
Thus, they play an important role in involuntary movement of the different organ of the human body.
Disclaimer: This Bodytomy article is for informative purposes only, and should not be used as a replacement for expert medical advice.