Dominic Stone
Unstriped muscles, also known as smooth, visceral, or involuntary muscles, are found in the walls of hollow organs in the body. These include the urinary bladder, uterus, lungs, intestines, arteries, veins, and the digestive, urinary, and reproductive tracts. They are characterised by their lack of stripes or striations, hence their alternative name of smooth muscles. These muscles are not under conscious control and exhibit slow, rhythmic contractions that play a crucial role in regulating flow and movement within organs.
| Characteristics | Values |
|---|---|
| Common Name | Visceral |
| Other Names | Smooth, Involuntary |
| Location | Walls of hollow organs such as the bladder, uterus, lungs, intestines, arteries, veins, and the digestive, urinary, and reproductive tracts |
| Iris | Affects the size of the iris and the lens |
| Skin | Controls the hair's response to cold or fear |
| Cells | Spindle-shaped, unstriated with a single nucleus |
| Control | Not under conscious control |
| Innervation | ANS |
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What You'll Learn
Alimentary canal
The alimentary canal, also called the gastrointestinal (GI) tract or gut, is a one-way tube about 7.62 metres (25 feet) long. It begins at the mouth and terminates at the anus. The canal is modified along its length to suit the body's functional needs, including the pharynx, oesophagus, stomach, and small and large intestines.
The wall of the alimentary canal has four basic tissue layers: the mucosa, submucosa, muscularis, and serosa. The mucosa, or mucous membrane, is the innermost layer of the alimentary canal. It lines the lumen, or space inside the canal. The mucosa consists of epithelium, an underlying loose connective tissue layer called lamina propria, and a thin layer of smooth muscle called the muscularis mucosa. The muscularis mucosa is in a constant state of tension, pulling the mucosa of the stomach and small intestine into folds that increase the surface area available for digestion and absorption.
The submucosa is a thick layer of loose connective tissue that surrounds the mucosa. It contains blood vessels, lymphatic vessels, nerves, and mucous-secreting glands. The submucosa also joins the mucosa to the bulk of underlying smooth muscle. The muscularis propria contains smooth muscle cells organised into a tightly coiled, inner circular layer and outer longitudinal layer. These layers of smooth muscle are used for peristalsis (rhythmic waves of contraction) to move food down through the gut.
The outermost layer of the alimentary canal is the serosa, a loose layer of connective tissue within the diaphragm's cavity that protects and cushions organs. The serosa helps reduce muscle-induced friction by acting as an absorber to keep organs moving freely.
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Iris of the eye
The iris is the coloured muscular part of the eye surrounding the pupil. The word "iris" is derived from the Greek word for "rainbow", as well as its goddess and the messenger of the gods in the Iliad, due to the many colours of this eye part. The iris is located in front of the lens and behind the cornea. It is bathed in front and behind by a fluid known as the aqueous humour. The outer edge of the iris, known as the root, is attached to the sclera and the anterior ciliary body. The iris and ciliary body together are known as the anterior uvea.
The iris consists of two layers: the front pigmented fibrovascular layer, known as the stroma, and, behind the stroma, pigmented epithelial cells. The stroma is connected to a sphincter muscle (sphincter pupillae), which contracts the pupil in a circular motion, and a set of dilator muscles (dilator pupillae), which pull the iris radially to enlarge the pupil, pulling it in folds. The sphincter pupillae is the opposing muscle of the dilator pupillae. The muscle cells of the iris are smooth muscle in mammals and amphibians, but are striated muscle in reptiles (including birds). The pupil's diameter, and thus the inner border of the iris, changes size when constricting or dilating. The outer border of the iris does not change size. The constricting muscle is located on the inner border. The back surface is covered by a heavily pigmented epithelial layer that is two cells thick (the iris pigment epithelium), but the front surface has no epithelium.
The iris is usually strongly pigmented, with the colour typically ranging between brown, hazel, green, grey, and blue. Occasionally, the colour of the iris is due to a lack of pigmentation, as in the pinkish-white of oculocutaneous albinism, or to obscuration of its pigment by blood vessels, as in the red of an abnormally vascularised iris. Despite the wide range of colours, the only pigment that contributes substantially to normal iris colour is the dark pigment melanin. The quantity of melanin pigment in the iris is one factor in determining the phenotypic eye colour of an organism. Structurally, this large molecule is only slightly different from its equivalent found in skin and hair. Iris colour is due to variable amounts of eumelanin (brown/black melanins) and pheomelanin (red/yellow melanins) produced by melanocytes. More of the former is found in brown-eyed people and of the latter in blue- and green-eyed people.
The iris is lined by smooth muscle, which is also known as visceral muscle. Smooth muscle contracts without conscious control. The iris consists of two sheets of smooth muscle with contrary actions—dilation (expansion) and contraction (constriction). These muscles control the size of the pupil and thus determine how much light reaches the sensory tissue of the retina. The sphincter muscle of the iris is a circular muscle that constricts the pupil in bright light, whereas the dilator muscle of the iris expands the opening when it contracts. The amount of pigment contained in the iris determines eye colour.
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Intestinal walls
Unstriped muscle, otherwise known as smooth muscle, is found in the walls of hollow visceral organs such as the liver, pancreas, and intestines. The gastrointestinal wall of the gastrointestinal tract is made up of four layers of specialised tissue. From the inner cavity of the gut outwards, these are the mucosa, the submucosa, the muscular layer, and the serosa or adventitia.
The mucosa is the innermost layer of the gastrointestinal tract and surrounds the lumen of the tract, coming into direct contact with digested food (chyme). The mucosa itself is made up of three layers: the epithelium, where most digestive, absorptive, and secretory processes occur; the lamina propria, a layer of connective tissue; and the muscularis mucosae, a thin layer of smooth muscle. The submucosa contains nerves, blood vessels, and elastic fibres with collagen, which stretch with increased capacity but maintain the shape of the intestine.
The muscular layer surrounds the submucosa and comprises layers of smooth muscle in longitudinal and circular orientations, which help with continued bowel movements (peristalsis) and the movement of digested material out of and along the gut. The thickness of the muscular layer varies in each part of the tract; for example, in the colon, the muscular layer is much thicker because faeces are large and heavy and require more force to be pushed along. The coordinated contractions of these layers are called peristalsis and propel food through the tract.
The outermost layers of the gastrointestinal wall are the serosa or adventitia, which are made up of loose connective tissue and coated in mucus to prevent any friction damage from the intestine rubbing against other tissues.
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Uterus
The uterus is a hollow, pear-shaped organ located in the pelvis between the bladder and rectum. It is responsible for menstruation, fertility, and pregnancy. The uterus has three layers: the perimetrium, myometrium, and endometrium. The myometrium is the middle layer and is composed of smooth muscle cells. During pregnancy, the myometrium expands to accommodate the growing fetus. In labour, it contracts to push the baby out through the vagina.
The myometrium contains a high number of actin filaments, which are responsible for its contractile properties. The actin filaments are arranged in a pattern that allows for both peristaltic and anti-peristaltic activity. The uterine smooth muscle exhibits a phasic pattern, alternating between a contractile state and a resting state. This resting state is maintained by a Na+/K+ pump that creates a higher concentration of Na+ ions in the extracellular space and a higher concentration of K+ ions in the intracellular space. The excitation-contraction coupling of uterine smooth muscle is similar to that of other smooth muscles, with an increase in intracellular calcium (Ca2+) leading to contraction.
Uterine smooth muscle is also affected by the release of prostaglandins and oxytocin, which stimulate a positive feedback loop that increases the contractile strength of the uterus during labour. These contractions start in the fundus and sweep down to the rest of the uterus, causing the uterine muscles to shorten and relax, facilitating fetal descent. After birth, the myometrium continues to contract, expelling the placenta and compressing blood vessels to minimise blood loss.
Uterine smooth muscle is susceptible to various conditions, including leiomyomas or fibroids, which are benign tumours that originate from the smooth muscle cells of the uterus and are influenced by estrogen levels. They typically occur in pre-menopausal women and can grow into the uterine cavity, changing the shape of the uterus. Other conditions affecting the uterus include endometrial polyps, which are collections of endometrial tissue with smooth muscle cells that can extend into the endometrial cavity, and endometrial hyperplasia, where excess estrogen stimulation causes abnormal endometrial gland proliferation, increasing the risk of endometrial carcinoma.
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Heart
The heart is a fist-sized organ that is the main organ of the cardiovascular system, a network of blood vessels that pumps blood throughout the body. The heart has four muscular sections or chambers that briefly hold blood before moving it. The heart contains cardiac muscle tissue, which is one of three types of vertebrate muscle tissues, the others being skeletal muscle and smooth muscle. Cardiac muscle tissue can only be found in the heart.
Cardiac muscle, also called myocardium or heart muscle, forms a thick middle layer of the heart wall between the outer layer (the pericardium or epicardium) and the inner layer (the endocardium). The myocardium constitutes the bulk of the heart and is made up of individual cardiac muscle cells or cardiomyocytes joined by intercalated discs. The cardiac muscle is responsible for the contractility of the heart and, therefore, the pumping action. The cardiac muscle must contract with enough force and blood to supply the metabolic demands of the entire body.
Cardiac muscle cells are striated, branched, and contain many mitochondria. They are under involuntary control. Each myocyte contains a single, centrally located nucleus surrounded by a cell membrane known as the sarcolemma. The sarcolemma contains voltage-gated calcium channels, specialised ion channels that skeletal muscle does not possess. The cardiac muscle cells contain branched fibres connected via intercalated discs that contain gap junctions and desmosomes. These interconnections allow the cardiomyocytes to contract together synchronously to enable the heart to work as a pump.
Specialised modified cardiomyocytes known as pacemaker cells set the rhythm of the heart contractions. The pacemaker cells are only weakly contractile without sarcomeres and are connected to neighbouring contractile cells via gap junctions. They are distributed throughout the heart and are located in the sinoatrial node (the primary pacemaker) and the atrioventricular node (secondary pacemaker). Pacemaker cells carry the impulses that are responsible for the beating of the heart.
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Frequently asked questions
Unstriped muscles, also known as visceral, smooth, or involuntary muscles, are found in the walls of hollow organs such as the urinary bladder, uterus, lungs, intestines, arteries, veins, and the digestive, urinary, and reproductive tracts.
Unstriped muscles are small and exhibit the shape of a spindle. They are smooth and lack striations, hence the name "smooth muscles."
Unstriped muscles are uncontrolled and involuntary, meaning they contract without conscious control.
