Single-unit muscle has its muscle fibers joined by gap junctions so that the muscle contracts as a single unit. This type of smooth muscle is found in the walls of all visceral organs except the heart which has cardiac muscle in its walls , and so it is commonly called visceral muscle. Because the muscle fibers are not constrained by the organization and stretchability limits of sarcomeres, visceral smooth muscle has a stress-relaxation response.
Vascular smooth muscle
This means that as the muscle of a hollow organ is stretched when it fills, the mechanical stress of the stretching will trigger contraction, but this is immediately followed by relaxation so that the organ does not empty its contents prematurely. This is important for hollow organs, such as the stomach or urinary bladder, which continuously expand as they fill.
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In general, visceral smooth muscle produces slow, steady contractions that allow substances, such as food in the digestive tract, to move through the body. Multiunit smooth muscle cells rarely possess gap junctions, and thus are not electrically coupled. As a result, contraction does not spread from one cell to the next, but is instead confined to the cell that was originally stimulated.
Stimuli for multiunit smooth muscles come from autonomic nerves or hormones but not from stretching. This type of tissue is found around large blood vessels, in the respiratory airways, and in the eyes. Similar to skeletal and cardiac muscle cells, smooth muscle can undergo hypertrophy to increase in size.
Unlike other muscle, smooth muscle can also divide to produce more cells, a process called hyperplasia. This can most evidently be observed in the uterus at puberty, which responds to increased estrogen levels by producing more uterine smooth muscle fibers, and greatly increases the size of the myometrium. Smooth muscle is found throughout the body around various organs and tracts. Smooth muscle cells have a single nucleus, and are spindle-shaped. Smooth muscle cells can undergo hyperplasia, mitotically dividing to produce new cells.
Smooth muscle can be stimulated by pacesetter cells, by the autonomic nervous system, by hormones, spontaneously, or by stretching. The fibers in some smooth muscle have latch-bridges, cross-bridges that cycle slowly without the need for ATP; these muscles can maintain low-level contractions for long periods.
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Single-unit smooth muscle tissue contains gap junctions to synchronize membrane depolarization and contractions so that the muscle contracts as a single unit. Single-unit smooth muscle in the walls of the viscera, called visceral muscle, has a stress-relaxation response that permits muscle to stretch, contract, and relax as the organ expands. Multiunit smooth muscle cells do not possess gap junctions, and contraction does not spread from one cell to the next.
Why can smooth muscles contract over a wider range of resting lengths than skeletal and cardiac muscle? Skip to content Increase Font Size.
Smooth Muscle – Anatomy and Physiology
We say there is less resistance in the circulatory system and the blood pressure drops. To counteract this, we can increase the resistance to blood flow in other parts of the circulatory system by narrowing arteries vasoconstriction. This will help to stabilise blood pressure. Conversely, if the pressure within arteries becomes too high, there is an increased strain on the heart and a risk that blood vessels may burst. By dilating blood vessels, the resistance to blood flow in the circulatory system decreases, helping to lower blood pressure. By causing vasodilatation and vasoconstriction and adjusting the resistance to blood flow in this way, vascular smooth muscle helps us to regulate our blood pressure.
If you looked at vascular smooth muscle tissue under a microscope, you will notice it is composed of several individual spindle-shaped fibers. There are two types of filaments — thick filaments which are predominantly made of a protein called myosin and thin filaments which are made from a protein called actin. The filaments are arranged so that they can slide over one another. When the thick filaments slide over the thin filaments, tension is produced. This is essentially the basis of vascular smooth muscle contraction. Calcium is required to trigger the process of filaments sliding over one another.
For this reason, levels of calcium, both in the blood and in tissues, can influence vascular smooth muscle contraction and relaxation. Furthermore, factors that affect calcium levels, such as your Vitamin D intake or the levels of certain hormones particularly parathyroid hormone [PTH] , will have an indirect effect on your vascular smooth muscle function. Owing to the key role of calcium on vascular smooth muscle, your Vascular Smooth Muscle Contraction trait is heavily influenced by your Serum Calcium Level Trait. One of the elegant things about the human body is that it has evolved to automatically match blood flow to the energy demands of a tissue.
For example, when we exercise a skeletal muscle, it produces carbon dioxide, lactate, potassium ions, adenosine and other waste products which accumulate in the bloodstream. These molecules act on vascular smooth muscle nearby, causing it to relax, thereby promoting vasodilatation and greater blood flow to the exercising muscle. This rather neat process of matching blood flow to energy demands is called autoregulation. Molecules produced as a result of inflammation can also cause local changes in blood flow by either causing vascular smooth muscle relaxation or contraction.
Vascular smooth muscle also receives input from nerves. Vascular smooth muscle in arteries and arterioles typically receives input from sympathetic nerves. When these nerves are stimulated, they produce a chemical called noradrenaline or norepinephrine. This binds to special receptors called alpha receptors on vascular smooth muscle, causing it to contract and thereby giving rise to vasoconstriction. It is this sympathetic stimulation, for example, which limits blood flow to your digestive system during exercise.
It is released from the adrenal glands, which are located just above each kidney. In addition to receptors for sympathetic nerves, vascular smooth muscle also contains receptors for adrenaline circulating in the blood. When adrenaline binds to one type of receptor beta-2 receptors , it causes vascular smooth muscle to relax, thereby causing vasodilatation.
This is the process by which blood flow to skeletal muscles is increased whenever we exercise. There are also various hormones involved in the regulation of the blood pressure that cause vasoconstriction. Both of these hormones cause contraction of vascular smooth muscle, vasoconstriction and a resultant increase in blood pressure.
Author Dr. Haran Sivapalan. What is vascular smooth muscle? R; Tzvetkov, N. G Part 1: Molecular targets and pathways". Biotechnology Advances. Muscle tissue. Calmodulin Vascular smooth muscle.
Sarcospan Laminin, alpha 2. NOS1 Caveolin 3. Epimysium Fascicle Perimysium Endomysium Connective tissue in skeletal muscle. Neuromuscular junction Motor unit Muscle spindle Excitation—contraction coupling Sliding filament mechanism. Myocardium Intercalated disc Nebulette.
Desmin Sarcoplasm Sarcolemma T-tubule Sarcoplasmic reticulum.