Smooth Muscle – Anatomy and Physiology (2023)

Muscle tissue


learning objectives

At the end of this section, you will be able to:

  • Describe a dense body
  • Explain how smooth muscle works with internal organs and passages through the body
  • Explain how smooth muscle differs from skeletal and cardiac muscle
  • Explain the difference between single and multi-unit smooth muscle

Smooth muscle (so named because cells lack striations) is present in the walls of hollow organs such as the urinary bladder, uterus, stomach, intestines, and in the walls of passageways such as the arteries and veins of the circulatory system. . and the tracts of the respiratory, urinary, and reproductive systems ([link]ab). Smooth muscle is also present in the eyes, where it works by changing the size of the iris and changing the shape of the lens; and on the skin, where it causes hair to stand on end in response to cold or fear.

Smooth Muscle Tissue

Smooth muscle tissue is found around the organs of the digestive, respiratory, reproductive tracts and the iris of the eye. LM × 1600. (Micrograph provided by Regents, University of Michigan School of Medicine © 2012)

see theUniversity of Michigan WebScopeto explore the fabric swatch in more detail.

(Video) Smooth Muscle Tissue Anatomy - Mnemonic, Structure, Contraction, Single-Unit, Multi-Unit

Smooth muscle fibers are fusiform (wide in the middle and tapered at both ends, like a football) and have a single nucleus; they range from around 30 to 200eum (thousands of times shorter than skeletal muscle fibers) and produce their own connective tissue, the endomysium. Although they lack striations and sarcomeres, smooth muscle fibers have actin and myosin contractile proteins, as well as thick and thin filaments. These thin filaments are anchored by dense bodies. Adense bodyit is analogous to the Z disks of skeletal and cardiac muscle fibers and is attached to the sarcolemma. Calcium ions are provided by the SR in the fibers and by sequestration of extracellular fluid through indentations in the membrane called calveoli.

Since smooth muscle cells do not contain troponin, cross-bridge formation is not regulated by the troponin-tropomyosin complex, but by the regulatory protein.calmodulina. In a smooth muscle fiber, external Ca++ions passing through open calcium channels in the sarcolemma, and additional Ca++released from the SR, binds to calmodulin. THE CA++The -calmodulin complex then activates an enzyme called myosin (light chain) kinase, which in turn activates the myosin heads, phosphorylating them (converting ATP to ADP and P).eu, with the Peufixed on the head). The heads can then attach to the actin-binding sites and pull the thin filaments. Thin filaments are also anchored to dense bodies; the structures invested in the inner membrane of the sarcolemma (at the adherent junctions) that also have string-like intermediate filaments attached to them. As the thin filaments slide past the thick filaments, they pull on the dense bodies, structures attached to the sarcolemma, which then pull on networks of intermediate filaments throughout the sarcoplasm. This arrangement causes the entire muscle fiber to contract in such a way that the ends are pulled toward the center, causing the midsection to protrude in a corkscrewing motion ([link]).

Muscular contraction

The dense bodies and intermediate filaments are interconnected through the sarcoplasm, which causes the muscle fiber to contract.

Although smooth muscle contraction depends on the presence of Ca++ions, smooth muscle fibers have a much smaller diameter than skeletal muscle cells. T tubules are not needed to reach the interior of the cell and therefore are not needed to transmit an action potential deep into the fiber. Smooth muscle fibers have a limited RS of calcium storage, but they have calcium channels in the sarcolemma (similar to cardiac muscle fibers) that open during the action potential along the sarcolemma. The influx of extracellular Ca++ions, which diffuse into the sarcoplasm to reach calmodulin, account for most of the Ca++which triggers the contraction of a smooth muscle cell.

Muscle contraction continues until ATP-dependent calcium pumps actively transport Ca++ions back into the SR and out of the cell. However, a low concentration of calcium remains in the sarcoplasm to maintain muscle tone. This leftover calcium keeps the muscle slightly contracted, which is important in certain tracts and around blood vessels.

Since most smooth muscle must function for long periods without rest, its energy output is relatively low, but contractions can continue without the use of large amounts of energy. Some smooth muscles can also maintain contractions even when Ca++is removed and the myosin kinase is inactivated/dephosphorylated. This can happen as a subset of cross bridges between the myosin and actin heads calledlock bridges, keep the thick and thin filaments connected together for a prolonged period, and without the need for ATP. This allows maintenance of muscle “tone” in the smooth muscle that lines arterioles and other visceral organs with very little energy expenditure.

(Video) Three Types of Muscle Tissue (Skeletal, Smooth, Cardiac) Anatomy Compilation Review

Smooth muscle is not under voluntary control; therefore, it is called an involuntary muscle. Triggers for smooth muscle contraction include hormones, neural stimulation by the ANS, and local factors. In certain locations, such as the walls of visceral organs, stretching the muscle can trigger it to contract (the stress-relaxation response).

The axons of neurons in the ANS do not form the highly organized JNMs with smooth muscle, as seen between motor neurons and skeletal muscle fibers. Instead, there are a series of neurotransmitter-filled bulges called varicosities as an axon travels along smooth muscle, loosely forming motor units ([link]). Avaricositiesreleases neurotransmitters into the synaptic cleft. In addition, the visceral muscle in the walls of hollow organs (except the heart) contains pacemaker cells. Apacemaker cellcan spontaneously trigger action potentials and muscle contractions.

Motor Units

A series of axon-like swellings, called varicosities or "buds," of autonomic neurons form motor units through smooth muscle.

Smooth muscle is organized in two ways: as unitary smooth muscle, which is much more common; and as multiunit smooth muscle. The two types have different locations on the body and different characteristics. A unitary muscle has its muscle fibers held together by gap junctions, so 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 is therefore commonly calledvisceral muscle. Because muscle fibers are not constrained by the organization and elastic limits of sarcomeres, visceral smooth muscle has astress-relaxation response. This means that as the muscle of a hollow organ is stretched when it fills, the mechanical stress of stretching will trigger contraction, but this will immediately be followed by relaxation so that the organ does not empty its contents prematurely. This is important for hollow organs like the stomach or urinary bladder, which continually expand as they fill. The smooth muscle around these organs can also maintain muscle tone as the organ empties and shrinks, a feature that prevents "sagging" in the empty organ. 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 have gap junctions and therefore are not electrically coupled. As a result, the contraction does not spread from one cell to another, but is confined to the cell that was originally stimulated. Stimuli for multiunit smooth muscle come from autonomic nerves or hormones, but not from stretching. This type of tissue is found around large blood vessels, in the airways, and in the eyes.

Similar to skeletal and cardiac muscle cells, smooth muscle can undergo hypertrophy to increase in size. Unlike other muscles, smooth muscle can also divide to produce more cells, a process calledhyperplasia. This can be seen most evidently in the uterus at puberty, which responds to increased estrogen levels by producing more uterine smooth muscle fibers and greatly increasing the size of the myometrium.

(Video) Skeletal Muscle Tissue: Contraction, Sarcomere, Myofibril Anatomy Myology

Smooth muscle is found throughout the body, around various organs and tracts. Smooth muscle cells have a single nucleus and are fusiform. Smooth muscle cells can undergo hyperplasia, dividing mitotically to produce new cells. Smooth cells are not striated, but their sarcoplasm is filled with actin and myosin, along with dense bodies in the sarcolemma to anchor the thin filaments and a network of intermediate filaments involved in pulling the sarcolemma toward the middle of the fiber, shortening it at the end. process. Here++ions trigger contraction when they are released from the SR and enter through open voltage-gated calcium channels. Smooth muscle contraction is initiated when Ca++binds to intracellular calmodulin, which then activates an enzyme called myosin kinase, which phosphorylates the myosin heads so they can form cross-bridges with actin and then pull the thin filaments. Smooth muscle can be stimulated by pacemaker cells, the autonomic nervous system, hormones, spontaneously, or by stretching. Some smooth muscle fibers have locking bridges, cross bridges that circulate slowly without the need for ATP; these muscles can maintain low-level contractions for long periods. Unitary smooth muscle tissue contains gap junctions to synchronize depolarization and membrane contractions so that the muscle contracts as a single unit. The unitary smooth muscle in the walls of the viscera, called visceral muscle, has a stress-relaxation response that allows the muscle to stretch, contract, and relax as the organ expands. Multiunit smooth muscle cells do not have gap junctions, and contraction does not spread from cell to cell.

Smooth muscle differs from skeletal and cardiac muscle because ________.

  1. falta de myofibrillas
  2. are under voluntary control
  3. lack of myosin
  4. lack actin


Which of the following statements describes smooth muscle cells?

  1. They are fatigue resistant.
  2. They have a rapid onset of contractions.
  3. They cannot have tetanus.
  4. They primarily use anaerobic metabolism.
(Video) Cardiac Muscle Tissue Anatomy & Physiology Review Lecture


Why can smooth muscle contract over a wider range of resting lengths than skeletal and cardiac muscle?

Smooth muscle 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.

Describe the differences between single-unit smooth muscle and multi-unit smooth muscle.

Unitary smooth muscle is found in the walls of hollow organs; Multi-unit smooth muscle is found in the airways that go to the lungs and large arteries. Single unit smooth muscle cells contract synchronously, are coupled by gap junctions, and exhibit spontaneous action potentials. Multiunit smooth cells lack gap junctions and their contractions are not synchronous.

(Video) Structure of Smooth Muscle | Types of Smooth Muscle | Muscle Physiology | Myology


regulatory protein that facilitates smooth muscle contraction
dense body
sarcoplasmic structure that attaches to the sarcolemma and shortens the muscle as thin filaments slide past thick filaments
process in which a cell divides to produce new cells
lock bridges
subset of a cross bridge in which actin and myosin remain interconnected
pacemaker cell
cell that triggers action potentials in smooth muscle
stress-relaxation response
relaxation of smooth muscle tissue after being stretched
increase in neurons that release neurotransmitters into the synaptic clefts
visceral muscle
smooth muscle found in the walls of visceral organs


What is smooth muscle anatomy and physiology? ›

Smooth muscle consists of thick and thin filaments that are not arranged into sarcomeres giving it a non-striated pattern. On microscopic examination, it will appear homogenous. Smooth muscle cytoplasm contains a large amount of actin and myosin. Actin and myosin act as the main proteins involved in muscle contraction.

What are 3 important facts about smooth muscles? ›

Smooth muscle is present throughout the body, where it serves a variety of functions. It is in the stomach and intestines, where it helps with digestion and nutrient collection. It exists throughout the urinary system, where it functions to help rid the body of toxins and works in electrolyte balance.

What are the 3 anatomical characteristics of the smooth muscles? ›

The smooth muscle cell is 3-10 µm thick and 20-200 µm long. The cytoplasm is homogeneously eosinophilic and consists mainly of myofilaments. The nucleus is located in the center and takes a cigar-like shape during contraction.

What does smooth muscle lack that makes it easy to identify? ›

The structure of smooth muscle, as compared to other muscles, lacks sarcomeres. Lack of a tropomyosin complex. It also lacks troponin C but has calmodulin to which calcium ions bind to forming the Ca2+– calmodulin complex that activates the myosin kinase (phosphorylation enzyme).

What are the functions of smooth muscle quizlet? ›

Functional roles of smooth muscle contraction: propels contents through a hollow organ or tube. maintains pressure against the contents within a hollow organ or tube. regulates internal flow of contents by changing tube diameter (resistance).

What is smooth muscle answer? ›

Smooth muscle is an involuntary non-striated muscle, so-called because it has no sarcomeres and therefore no striations (bands or stripes). It is divided into two subgroups, single-unit and multiunit smooth muscle. Within single-unit muscle, the whole bundle or sheet of smooth muscle cells contracts as a syncytium.

What is the anatomy of smooth muscle? ›

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. 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.

Where is smooth muscle found in the body? ›

Smooth muscle fibers are located in walls of hollow visceral organs (such as the liver, pancreas, and intestines), except the heart, appear spindle-shaped, and are also under involuntary control.

How many smooth muscles are in the human body? ›

It's estimated that there are over 650 named skeletal muscles in your body. Other muscle tissue, such as smooth muscle, typically occurs on a cellular level, meaning that you can actually have billions of smooth muscle cells. The muscles of your body perform a variety of vital functions.

What are the properties of smooth muscle physiology? ›

The smooth muscle cell is a spindle-shaped fiber with a single, oval, centrally placed nucleus. The size of the fibers varies greatly according to their environment. The fibers are bound together by a dense connective tissue network. There is no true cell membrane as is found in striated muscle.

How do smooth muscles grow? ›

Vascular smooth muscle cells (VSMC) exhibit several growth responses to agonists that regulate their function including proliferation (hyperplasia with an increase in cell number), hypertrophy (an increase in cell size without change in DNA content), endoreduplication (an increase in DNA content and usually size), and ...

What are the 4 characteristics of smooth muscle tissue cells? ›

Smooth muscle cells are spindle shaped, have a single, centrally located nucleus, and lack striations. They are called involuntary muscles. Cardiac muscle has branching fibers, one nucleus per cell, striations, and intercalated disks. Its contraction is not under voluntary control.

What is the main function of the smooth muscle cell? ›

Smooth muscle cells are quite heterogeneous, depending on the organ system in which they serve their function. Their major role is to control the diameter, wall movement, and wall stiffness of hollow organs like the vascular, bronchial, gastrointestinal or urogenital system as well as the uterus.

What are the two major types of smooth muscle? ›

Describe the differences between single-unit smooth muscle and multiunit smooth muscle. 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.

What is the function of no in smooth muscle? ›

Generally, NO relaxes many types of smooth muscle by stimulating soluble guanylate cyclase to increase cGMP (Lincoln & Cornwell, 1993; Carvajal et al. 2000).

What is smooth muscle and its function? ›

The vasculature system that transports blood throughout the body is controlled by smooth muscle. Smooth muscles contract to regulate blood pressure and other cardiovascular processes. They also aid in food digestion through peristalsis, which is the rhythmic wave-like contraction of muscles around the digestive tract.

What is the smooth muscle? ›

smooth muscle, also called involuntary muscle, muscle that shows no cross stripes under microscopic magnification. It consists of narrow spindle-shaped cells with a single, centrally located nucleus. Smooth muscle tissue, unlike striated muscle, contracts slowly and automatically.

Which describes smooth muscle quizlet? ›

Smooth muscle has no T-tubules. Smooth muscle has little SR. Calcium needed for contraction is stored in the sarcoplasm. Smooth muscle is uninucleate.


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