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The human intestine is a crucial part of the digestive system. It is divided into the small intestine and the large intestine. The small intestine is further divided into the duodenum, jejunum, and ileum. The duodenum receives a mix of gastric acid and food from the stomach, and this is where nutrient absorption begins. The large intestine, on the other hand, is responsible for the absorption of water and electrolytes, as well as the elimination of feces. The intestinal wall is made up of smooth muscle, which is a type of muscle tissue that is found in the walls of hollow organs, and it contracts involuntarily. This smooth muscle tissue is responsible for the wave-like contractions that move food through the digestive tract, a process known as peristalsis.
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What You'll Learn
The small intestine has two layers of smooth muscle
The small intestine is a crucial part of the gastrointestinal system, facilitating nutrient digestion and absorption, endocrine functions, and immune protection. It is made up of three components: the duodenum, jejunum, and ileum. The small intestine follows the general structure of the digestive tract, with a mucosa of simple columnar epithelium, submucosa, smooth muscle, and serosa.
Smooth muscle is found in the wall of hollow organs, passageways, tracts, the eye, and the skin. It is also present in sphincters, the uterus, and the ducts of exocrine glands. Smooth muscle is made up of bundles that do not run strictly parallel, allowing for stronger contractions than those of striated musculature. The innervation of smooth muscle is complex, lying under the influence of the visceral nervous system while also working autonomously. Smooth muscle contracts involuntarily, and is functionally differentiated into single-unit and multi-unit types.
The small intestine's smooth muscle layers are important for the forward movement of intestinal contents. The duodenojejunal junction, for example, is widened by smooth muscle contraction, aiding in the forward movement of intestinal contents. Smooth muscle is also important for the continued peristalsis and catastalsis along the gut.
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Smooth muscle is found in the wall of hollow organs
Smooth muscle is a type of tissue found in the walls of hollow organs, such as the intestines, stomach, bladder, and uterus. It is also present in the walls of passageways and tracts, including arteries and veins of the cardiovascular system. This type of involuntary non-striated muscle is found in the tracts of the urinary, respiratory, and reproductive systems. Smooth muscle is further identified in the eyes, where it is responsible for changing the size of the pupil and the shape of the lens. The skin also contains smooth muscle, which allows hair to stand up in response to cold temperatures or fear.
Smooth muscle consists of two types: single-unit and multi-unit. Single-unit smooth muscle, also known as visceral muscle, is found in the walls of hollow organs, with the exception of the heart. It is characterized by multiple cells connected through connexins, allowing for synchronous contraction. This type of smooth muscle exhibits a stress-relaxation response, enabling the muscle to stretch, contract, and relax as the organ expands. Single-unit smooth muscle is also found in the walls of internal organs and blood vessels, including arteries of the gastrointestinal tract.
Multi-unit smooth muscle, on the other hand, is composed of independent cells that require individual innervation, allowing for more precise muscle control. It is found in the airways of the lungs, large arteries, ciliary muscles of the eye, and the iris and hair erector muscles.
The innervation of smooth muscle is complex, influenced by the visceral nervous system while also functioning autonomously. Smooth muscle contraction is initiated by the binding of Ca++ to intracellular calmodulin, activating an enzyme that facilitates the formation of cross-bridges between myosin and actin filaments. Smooth muscle cells are spindle-shaped and range in size from 3-10 µm thick and 20-200 µm long. They have a single nucleus and group into branching bundles, enabling stronger contractions compared to striated musculature.
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Smooth muscle contracts involuntarily
Smooth muscle is one of the three major types of vertebrate muscle tissue, the others being skeletal and cardiac muscle. Smooth muscle is found in the walls of hollow organs, including the stomach, intestines, bladder, and uterus. It is also found in the walls of blood vessels and lymph vessels, as well as in the tracts of the respiratory, urinary, and reproductive systems. In the intestines, smooth muscle helps with digestion and nutrient collection.
Smooth muscle differs from skeletal muscle in structure and function. Unlike skeletal muscle, smooth muscle is non-striated, meaning it has no sarcomeres and therefore no bands or stripes. It consists of thick and thin filaments that do not arrange into sarcomeres, resulting in a non-striated pattern. On microscopic examination, it appears homogeneous. Smooth muscle can also maintain its tone for extended periods and often contracts involuntarily.
The primary function of smooth muscle is contraction. Smooth muscle cells are electrically connected by gap junctions and contract uniformly. This type of cell is found in the wall of internal organs and blood vessels. Smooth muscle contraction depends on calcium influx. Calcium increases within the smooth muscle cell through depolarization, hormones, or neurotransmitters, which cause calcium to enter the cell through L-type channels. Intracellular calcium then stimulates the release of calcium from the sarcoplasmic reticulum by activating ryanodine receptors and IP3. Once calcium has entered the cell, it binds to calmodulin, which activates the enzyme myosin light chain kinase (MLCK). MLCK then phosphorylates a regulatory light chain on myosin, leading to muscle contraction.
Smooth muscle can be divided into two subgroups: single-unit and multi-unit smooth muscle. Single-unit smooth muscle consists of multiple cells connected through connexins that can become stimulated in a synchronous pattern from a single synaptic input. This allows for cell-to-cell communication between groups of single-unit smooth muscle cells, enabling ions and molecules to diffuse between cells and creating calcium waves. As a result, synchronous contraction occurs. Single-unit smooth muscle is usually active, even when it is not receiving neural stimulation. In contrast, multi-unit smooth muscle cells are independent and must be innervated individually, allowing for more precise muscle control. They are found in the iris and hair erector muscles.
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Smooth muscle is stimulated or relaxed by local changes
Smooth muscle, also known as non-striated muscle, is found in the walls of hollow organs, such as the urinary bladder, and in the eyes and skin. It is also present in the intestinal walls. Smooth muscle is stimulated or relaxed by local changes, such as stretching, which can trigger its contraction. This is in contrast to skeletal muscle, which is not stimulated by local changes. Smooth muscle is also stimulated by external factors, such as hormones, neural stimulation, and neurotransmitters.
Smooth muscle can contract involuntarily and is often controlled by the autonomic nervous system. It can be divided into two types: single-unit and multi-unit. Single-unit smooth muscle consists of multiple cells connected through connexins that can be stimulated in a synchronous pattern from a single synaptic input. This type of smooth muscle is found in the wall of internal organs and blood vessels. Multi-unit smooth muscle cells, on the other hand, are independent and need to be innervated individually, allowing for more precise muscle control. They are found in the iris and hair erector muscles.
The contraction of smooth muscle is dependent on calcium influx. Calcium enters the smooth muscle cell through two processes: depolarization and the action of hormones or neurotransmitters. Once inside the cell, calcium stimulates the release of more calcium from the sarcoplasmic reticulum through a process called calcium-induced calcium release. The calcium then binds to calmodulin, which activates the enzyme myosin kinase. This enzyme phosphorylates the myosin heads, allowing them to form cross-bridges with actin and pull on the thin filaments, resulting in muscle contraction.
Smooth muscle can contract phasically, with rapid contraction and relaxation, or tonically, with slow and sustained contraction. Tonic smooth muscle can maintain force for a prolonged time with little energy utilization. The relaxation of smooth muscle is mediated by cell-signalling pathways that increase myosin phosphatase activity, decrease intracellular calcium levels, and regulate actin and myosin dynamics.
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Smooth muscle is controlled by the autonomic nervous system
Smooth muscle, found in the intestinal wall, is under the control of the autonomic nervous system. This type of muscle is found in the walls of hollow organs, including the stomach and bladder, as well as in tubular structures like vessels and bile ducts, and in the eye. In the intestinal tract, smooth muscle is responsible for the transport of chyme through wavelike contractions of the intestinal tube.
The autonomic nervous system uses non-adrenergic, non-cholinergic (NANC) nerve fibres to control smooth muscle tissues. NANC neurotransmitters are involved in peripheral autonomic neuro-effector transmission, and while the exact neurotransmitters responsible for excitatory and inhibitory NANC transmission are not yet known, candidates include ATP, nitric oxide (NO), and peptides such as VIP and substance P.
The innervation of smooth muscle is complex. It is influenced by the visceral nervous system but can also work autonomously. Local changes, such as stretching, may stimulate or relax the muscle. Smooth muscle contracts involuntarily, and there are two types: single-unit and multi-unit. Single-unit smooth muscle cells are electrically connected and contract uniformly; they are found in the walls of internal organs and blood vessels. Multi-unit smooth muscle cells are independent and are found in the iris and hair erector muscles.
The smooth muscle in the intestinal tract is controlled by the autonomic nervous system, which uses NANC neurotransmitters to regulate contraction and relaxation. This process plays a crucial role in controlling blood pressure and the motility of the digestive, respiratory, and urinary tracts.
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Frequently asked questions
No, the intestinal muscle is smooth.
Smooth muscle is found in the walls of hollow organs, passageways, tracts, the eye, and the skin. Unlike skeletal muscle fibres, smooth muscle fibres do not run parallel and ordered. This means that smooth muscle cells can contract much stronger than striated muscle cells.
Smooth muscle in the intestine helps to move food through the digestive tract in a series of wave-like contractions known as peristalsis. Smooth muscle also helps to widen the duodenojejunal junction, assisting in the forward movement of intestinal contents.
Single-unit smooth muscle cells are electrically connected by gap junctions and contract uniformly. They are found in the wall of internal organs and blood vessels. Multi-unit smooth muscle cells are independent and need to be innervated individually, allowing for more precise muscle control. They are found in the iris and hair erector muscles.
