This allows for the maintaining of muscle “tone” in smooth muscle that lines arterioles and other visceral organs with very little energy expenditure. Similar to skeletal muscle cells, smooth muscle can undergo hypertrophy to increase in size.

Smooth muscle is found throughout the body around various organs and tracts.

This remaining calcium keeps the muscle slightly contracted, which is important in  certain functions, such as maintaining pressure in blood vessels.

Unitary Smooth Muscle. Smooth Muscle Tissue. A dense body is analogous to the Z-discs of skeletal muscle, anchoring the thin filaments in position. Although smooth muscle cells do not have striations, smooth muscle fibers do have actin and myosin contractile proteins which interact to generate tension.

Sarcoplasmic reticulum is present in the fibers but is less developed than that observed in skeletal muscle. Single-unit smooth muscle produces slow, steady contractions that allow substances, such as food in the digestive tract, to move through the body. Some examples of multi-unit smooth muscle are the ciliary muscle of the eye, the iris muscle of the eye, and the pilo-erector muscles that cause erection of the hairs when stimulated by the sympathetic nervous system. This article was most recently revised and updated by,, University of Illinois at Chicago - Smooth muscle. Smooth muscle fibers are often found forming sheets of tissue and function in a coordinated fashion due to the presence of gap junctions between the cells. Smooth muscle contraction is initiated when the Ca++ binds to intracellular calmodulin, which then activates an enzyme called myosin kinase that phosphorylates myosin heads so they can form the cross-bridges with actin and then pull on the thin filaments. For smooth muscle stimulated by neurons, the axons from autonomic nervous system neurons do not form the highly organized neuromuscular junctions as observed in skeletal muscle. By the end of this section, you will be able to: Smooth muscle, so-named because the cells do not have visible striations, is present in the walls of hollow organs (e.g., urinary bladder), lining the blood vessels, and in the eye (e.g., iris) and skin (e.g., erector pili muscle). Updates? This can most evidently be observed in the uterus at puberty, which responds to increased estrogen levels by producing more uterine smooth muscle fibers. Smooth muscle, muscle that shows no cross stripes under microscopic magnification. Multiunit smooth cells lack gap junctions, and their contractions are not synchronous. The same neurotransmitter can even cause opposite effects depending partly on the tissue where it acts.

Some smooth muscle can also maintain contractions even as Ca++ is removed and myosin kinase is inactivated/dephosphorylated. Multi-unit smooth muscle, the second type of smooth muscle observed, are composed of cells that rarely possess gap junctions, and thus are not electrically coupled. Smooth muscle fibers are spindle-shaped and, unlike skeletal muscle fibers, have a single nucleus; individual cells range in size from  30 to 200 μm. Multiunit smooth muscle cells do not possess gap junctions, and contraction does not spread from one cell to the next.

Unlike other muscle, smooth muscle will also divide quite readily to produce more cells, a process called hyperplasia. Smooth muscle is also present in the eyes, where it functions to change the size of the iris and alter the shape of the lens; and in the skin where it causes hair to stand erect in response to cold temperature or fear.

Instead, it means a mass of hundreds to thousands of smooth muscle … Smooth muscles can contract over a wider range of resting lengths because the actin and myosin filaments in smooth muscle are not as rigidly organized as those in skeletal and cardiac muscle. They generally contract voluntarily (via somatic nerve stimulation), although they can contract involuntarily through reflexes. It constitutes much of the musculature of internal organs and the digestive system. Our editors will review what you’ve submitted and determine whether to revise the article. Because the smooth muscle work with reflex as the stimulation, the brain, and the nervous system can’t control the movement of the smooth muscle.

A network of intermediate fibers run between the dense bodies providing an internal framework for contractile proteins to work against. Muscle contraction continues until ATP-dependent calcium pumps actively transport Ca++ ions out of the cell or back into the sarcoplasmic reticulum. The smooth muscle of the GI tract is a good example of single-unit smooth muscle. Smooth muscle tissue, unlike striated muscle, contracts slowly and automatically.

SmartDraw includes 1000s of professional healthcare and anatomy chart templates that you can modify and make your own. Describe the differences between single-unit smooth muscle and multiunit smooth muscle. Because most smooth muscles must function for long periods without rest, their power output is relatively low to minimize energy needs. The Cardiovascular System: Blood, Chapter 19. Single-unit smooth muscle is found in the walls of hollow organs; multiunit smooth muscle is found in airways to the lungs and large arteries.

This type of smooth muscle is observed in the large airways to the lungs, in the large arteries, the arrector pili muscles associated with hair follicles, and the internal eye muscles which regulate light entry and lens shape.

Also, visceral muscle in the walls of the hollow organs (except the heart) contains pacesetter cells.

Termed unitary smooth muscle or visceral muscle, this type of smooth muscle is the most common observed in the human body, forming the walls of hollow organs. View the University of Michigan WebScope at to explore the tissue sample in greater detail. Anatomy & Physiology by Lindsay M. Biga, Sierra Dawson, Amy Harwell, Robin Hopkins, Joel Kaufmann, Mike LeMaster, Philip Matern, Katie Morrison-Graham, Devon Quick & Jon Runyeon is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License, except where otherwise noted.

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