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Smooth muscle is one of the two muscles over which you have no control. Smooth muscle forms sheets of spindle-shaped cellsThe basic structural and functional units of life.. These cells are aligned in the same direction. They contract in a coordinated fashion. Your tube-like, hollow organs of the gastrointestinal (GI) tract have two sheets of smooth muscle around them. This includes the esophagusThe muscular tube that transports food from the pharynx to the stomach via peristalsis. and the small intestine. This arrangement creates a coordinated contraction called peristalsis Rhythmic contractions of smooth muscle that move food through the digestive tract..
Much like the cylinder shaped cells of skeletal muscle, the spindle-shape of smooth muscle can be hard to see. This is especially true in a 2D impression. Most pictures of smooth muscle will be cut as in a cross section A cut or slice of the body or an organ for study. of the spindle shaped cells. This reveals a circular 2D impression.
The darkly-stained nucleiClusters of neurons in the CNS responsible for processing information. of smooth muscle fibers are almost dead center. This is different from the nuclei that are pushed to the side of skeletal and cardiac muscle fibers. This resembles squamous epithelial cells. However, there would always be a lumenThe inside space of a hollow organ or structure., which is an open space on a slide showing epithelial tissue.
I have never established the ability to recognize this tissue without difficulty. When I absolutely can’t tell what a tissue is, I usually default to smooth muscle. I guess that’s like choosing C for every multiple choice answer you don’t know.
Sheets of Smooth Muscle
Smooth muscle is usually found in a sheet, but there are some single unit smooth muscle cells in your body. Each hair in your skinThe body’s largest organ, providing protection and regulation. is attached to a single smooth muscle fiber. These single units are all controlled together; if one contracts, they all contract. When the muscle fibers contract, your hairs “stand on end.” This is like the reaction your neckNarrow region just below the head; common fracture site. hairs would have if you were startled.
Other smooth muscle in your body is found in sheets of the spindle-shaped cells. These sheets contract in a coordinated fashion via the nervous systemThe organ system that controls body functions using electrical and chemical signals.. Smooth muscle doesn’t have neuromuscular junctions like skeletal muscle. It has bundles of nervous tissue called varicosities. These varicosities innervateTo invade a tissue with nervous tissue is to innervate it. and control the smooth muscle.
Smooth muscle surrounding hollow organs are usually found in two sheets of alternating orientation of the muscle fibers. An inner or deepAway from the surface of the body. layer of smooth muscle circles the tube-like organ. The orientation is much like wrapping a scarf around your neck. When these muscle fibers contract, the diameter of the tube-like hollow organ gets smaller. There is an outer or superficialNear the surface of the body. layer of smooth muscle. The spindle-shaped muscle fibers orient long-wise along the hollow tube-like organ. When these contract, the tube shortens, much like when you see an inch worm shorten while walking.
When these two sheets of smooth muscle contract in an alternating pattern, they produce a motion called peristalsis. This coordinated contraction moves the contents of the hollow organ. This process occurs in your esophagus, small intestine, and large intestine. It moves the contents to the next organ in the sequence.
Figure 1: Bladder, human, H&E, 40X Slide 212
Figure 2: Esophagus and stomach, H&E, 40X Slide 155
All by University of Michigan Histology, licensed under CC BY-NC-SA 3.0.
Explore More About Muscular Histology
List of terms
- cells
- esophagus
- peristalsis
- section
- nuclei
- lumen
- skin
- neck
- nervous system
- innervate
- deep
- superficial