Smooth Muscle

Smooth muscle is a specialized type of involuntary, non-striated muscle found primarily in the walls of hollow organs, blood vessels, airways, and numerous other visceral structures. Unlike skeletal and cardiac muscle, smooth muscle does not contain sarcomeres and therefore lacks the regular transverse striations seen in those tissues.

Smooth muscle is composed of elongated spindle-shaped cells, each typically containing a single centrally located nucleus. Although smooth muscle cells contain the contractile proteins actin and myosin, these proteins are arranged in an oblique network rather than in repeating sarcomeres.

Smooth muscle is adapted for slow, sustained, and often energy-efficient contraction. It performs essential functions including regulating blood vessel diameter, propelling material through the gastrointestinal tract, controlling airway caliber, emptying hollow organs, regulating pupil diameter, and producing contractions of the uterus.

Distribution of Smooth Muscle

Smooth muscle is widely distributed throughout the body and is particularly associated with tubular and hollow structures.

Location	Major Function of Smooth Muscle
Blood vessels	Regulates vessel diameter and vascular resistance
Gastrointestinal tract	Mixes and propels luminal contents
Respiratory tract	Regulates airway diameter
Urinary bladder	Produces contraction during emptying
Ureters	Propels urine toward the bladder
Uterus	Produces uterine contractions
Iris	Regulates pupil diameter
Ciliary body	Participates in accommodation
Skin	Arrector pili muscles elevate hairs

Smooth Muscle Cells

Individual smooth muscle cells, also called smooth muscle fibers, are elongated and fusiform. They are widest near the center and taper toward both ends.

Each cell usually contains a single elongated nucleus positioned centrally within the cytoplasm. Cell dimensions vary substantially according to location and physiological state.

Shape of Smooth Muscle Cells

The spindle shape allows smooth muscle cells to pack closely together within sheets and layers. Adjacent cells are often arranged so that the thicker central portion of one cell lies near the tapered ends of neighboring cells.

This arrangement contributes to the compact organization of smooth muscle within the walls of hollow organs.

Nucleus

Each smooth muscle cell normally contains one centrally positioned nucleus. In relaxed cells, the nucleus is elongated and follows the long axis of the cell.

During strong contraction, the cell shortens and thickens, and the nucleus may acquire a folded or corkscrew-like appearance in histological preparations.

Sarcolemma

The plasma membrane of a smooth muscle cell is referred to as the sarcolemma. It contains receptors, ion channels, pumps, transport proteins, and specialized membrane invaginations involved in calcium handling and cellular signaling.

Unlike skeletal and cardiac muscle cells, smooth muscle cells do not possess conventional T-tubules.

Sarcoplasm

The cytoplasm of a smooth muscle cell contains actin and myosin filaments, intermediate filaments, mitochondria, sarcoplasmic reticulum, glycogen, and other cellular organelles.

Organelles are particularly concentrated around the central nucleus, while contractile filaments occupy much of the remaining cytoplasm.

Absence of Striations

Smooth muscle appears non-striated under light microscopy because its thick and thin filaments are not organized into sarcomeres.

The absence of visible striations does not indicate absence of an organized contractile apparatus. Instead, the contractile filaments form a complex lattice anchored to structures known as dense bodies and membrane-associated dense plaques.

Contractile Filaments

Smooth muscle contains both actin-containing thin filaments and myosin-containing thick filaments. These proteins interact to generate contractile force.

The arrangement of these filaments differs substantially from the parallel sarcomeric organization of skeletal and cardiac muscle.

Thin Filaments

Thin filaments are particularly abundant in smooth muscle. They contain actin and tropomyosin together with regulatory proteins associated with the contractile apparatus.

Smooth muscle does not use the troponin-based regulatory system characteristic of skeletal and cardiac muscle.

Thick Filaments

Thick filaments are composed primarily of myosin II. Activation of myosin depends on phosphorylation of its regulatory light chains.

This regulatory mechanism allows smooth muscle contraction to be controlled through intracellular calcium and the calcium-binding protein calmodulin.

Dense Bodies

Dense bodies are cytoplasmic structures that anchor actin and intermediate filaments. Functionally, they perform some roles analogous to Z discs in striated muscle, although they are not arranged into repeating sarcomeres.

Dense bodies distribute contractile force throughout the smooth muscle cell.

Dense Plaques

Dense plaques are membrane-associated anchoring structures located along the inner surface of the sarcolemma. Actin and intermediate filaments attach to these structures.

They help transmit intracellular contractile forces to the cell membrane and through extracellular attachments to neighboring cells and connective tissue.

Intermediate Filaments

Intermediate filaments form an intracellular network connecting dense bodies and dense plaques. Depending on the smooth muscle tissue, these filaments commonly contain proteins such as desmin and vimentin.

The intermediate filament network provides structural support and distributes mechanical forces across the cell.

Contractile Network

The actin, myosin, and intermediate filament systems form an obliquely arranged contractile network. When actin and myosin interact, dense bodies are drawn toward one another and the entire cell shortens.

Because the filaments are not constrained within sarcomeres, smooth muscle can maintain useful force over a relatively broad range of cell lengths.

Caveolae

Caveolae are small flask-shaped invaginations of the smooth muscle cell membrane. They are numerous along the sarcolemma and are associated with proteins involved in signal transduction and calcium regulation.

Caveolae are sometimes considered functionally analogous in certain respects to the T-tubular system of striated muscle, although they are structurally distinct.

Sarcoplasmic Reticulum

Smooth muscle contains a relatively sparse sarcoplasmic reticulum compared with skeletal muscle. It participates in intracellular calcium storage and release.

Calcium required for contraction can originate both from intracellular stores and from extracellular fluid through plasma membrane calcium channels.

Calcium and Smooth Muscle Contraction

An increase in cytosolic calcium concentration is a major trigger for smooth muscle contraction. Calcium enters from the extracellular fluid and can also be released from the sarcoplasmic reticulum.

Unlike skeletal muscle, smooth muscle does not initiate contraction through calcium binding to troponin. Instead, calcium binds to calmodulin.

Calmodulin

Calmodulin is an intracellular calcium-binding protein that plays a central role in regulating smooth muscle contraction.

When intracellular calcium increases, calcium binds to calmodulin. The resulting calcium-calmodulin complex activates myosin light-chain kinase.

Myosin Light-Chain Kinase

Myosin light-chain kinase, commonly abbreviated MLCK, phosphorylates regulatory light chains associated with myosin.

This phosphorylation increases myosin ATPase activity and allows effective interaction between myosin and actin, initiating cross-bridge cycling.

Sequence of Smooth Muscle Contraction

1. Intracellular calcium concentration increases.
2. Calcium binds to calmodulin.
3. The calcium-calmodulin complex activates myosin light-chain kinase.
4. MLCK phosphorylates regulatory myosin light chains.
5. Activated myosin interacts with actin.
6. Cross-bridge cycling generates tension and cellular shortening.

Cross-Bridge Cycling

As in other muscle types, smooth muscle generates force through cyclic interactions between actin and myosin. ATP is required for cross-bridge cycling.

Cross-bridge cycling in smooth muscle is generally slower than in skeletal muscle, contributing to slower contraction and relaxation.

Smooth Muscle Relaxation

Relaxation occurs as intracellular calcium concentration falls and myosin regulatory light chains become dephosphorylated.

Myosin light-chain phosphatase removes phosphate groups from myosin light chains, reducing myosin activity and promoting relaxation.

Myosin Light-Chain Phosphatase

Myosin light-chain phosphatase, or MLCP, opposes the action of MLCK. The balance between MLCK and MLCP activity is an important determinant of smooth muscle contractile state.

Cellular signaling pathways can alter the activity of either enzyme and thereby modify contraction even without proportionate changes in intracellular calcium.

Calcium Sensitization

Smooth muscle force can be altered by changing the sensitivity of the contractile apparatus to calcium. Inhibition of MLCP, for example, can maintain greater myosin phosphorylation at a given calcium concentration.

This phenomenon contributes to regulation of vascular and visceral smooth muscle tone.

Latch State

Smooth muscle can maintain tension for prolonged periods while consuming relatively little ATP. This energy-efficient behavior is associated with slowly cycling or attached cross-bridges and is often described as the latch state.

This property is particularly useful in structures that must maintain sustained tone, such as blood vessels and sphincters.

Slow Contraction

Smooth muscle generally contracts and relaxes more slowly than skeletal muscle. Its slower mechanical activity reflects differences in myosin ATPase activity, calcium handling, and cross-bridge kinetics.

Slow contraction is well suited to functions such as moving intestinal contents, maintaining vascular tone, and controlling hollow-organ volume.

Length-Tension Properties

Smooth muscle can develop active force over a relatively broad range of lengths. This allows hollow organs to undergo substantial changes in volume while retaining the ability to contract effectively.

This property is especially important in organs such as the urinary bladder and uterus.

Stress-Relaxation Response

Some smooth muscle tissues can adapt to changes in length while maintaining relatively stable tension. After being stretched, tension may initially rise and then gradually decline despite persistence of the new length.

This behavior helps hollow organs accommodate changes in volume without producing excessive increases in internal pressure.

Control of Smooth Muscle

Smooth muscle is involuntary and can be regulated by several mechanisms simultaneously. Depending on the tissue, contraction may be influenced by autonomic nerves, hormones, circulating mediators, local chemical conditions, mechanical stretch, and intrinsic electrical activity.

This diverse regulation distinguishes smooth muscle from skeletal muscle, where contraction is normally initiated through somatic motor neurons at specialized neuromuscular junctions.

Autonomic Innervation

Many smooth muscles receive innervation from the autonomic nervous system. Sympathetic and parasympathetic effects vary according to the organ and receptors expressed by its smooth muscle cells.

The same autonomic neurotransmitter can therefore produce contraction in one tissue and relaxation in another.

Autonomic Varicosities

Autonomic axons supplying smooth muscle often contain multiple swellings called varicosities. These release neurotransmitter over a relatively broad region rather than forming the highly specialized motor end plates characteristic of skeletal muscle.

Neurotransmitter diffuses from the varicosities to receptors on nearby smooth muscle cells.

Neuroeffector Junction

The relationship between autonomic nerve terminals and smooth muscle is often described as a neuroeffector junction. It is generally less anatomically specialized than the skeletal neuromuscular junction.

One autonomic axon can influence numerous smooth muscle cells over a relatively broad area.

Gap Junctions

Many smooth muscle cells communicate electrically through gap junctions. These channels permit ions and small molecules to move directly between neighboring cells.

Electrical coupling through gap junctions allows excitation to spread through groups of smooth muscle cells and facilitates coordinated contraction.

Single-Unit Smooth Muscle

Single-unit smooth muscle, also called unitary or visceral smooth muscle, consists of cells that are extensively coupled through gap junctions and often contract together as a coordinated functional unit.

This type is common in the gastrointestinal tract, uterus, ureters, and many small blood vessels.

Characteristics of Single-Unit Smooth Muscle

• Numerous gap junctions between cells
• Coordinated contraction of groups of cells
• Spontaneous electrical activity may occur
• Stretch can influence contractile activity
• Common in walls of hollow visceral organs

Multiunit Smooth Muscle

Multiunit smooth muscle contains cells that function more independently and have relatively limited electrical coupling compared with single-unit smooth muscle.

Individual cells or small groups receive more direct neural regulation, permitting relatively precise control.

Locations of Multiunit Smooth Muscle

Examples of tissues with prominent multiunit characteristics include smooth muscles of the iris and ciliary body. Some larger airways and larger blood vessels also contain smooth muscle whose behavior differs from typical visceral single-unit muscle.

Single-Unit and Multiunit Comparison

Feature	Single-Unit Smooth Muscle	Multiunit Smooth Muscle
Gap junctions	Numerous	Relatively fewer
Cellular activity	Highly coordinated	More independent
Spontaneous activity	Common in some tissues	Less prominent
Neural control	Diffuse modulation	More precise
Examples	Gastrointestinal tract, uterus, ureters	Iris and ciliary muscle

Pacemaker Activity

Some visceral smooth muscle tissues display spontaneous changes in membrane potential that contribute to rhythmic activity. Specialized pacemaker mechanisms can establish patterns of electrical activity that influence nearby smooth muscle cells.

The gastrointestinal tract is an important example in which intrinsic electrical rhythms help organize muscular activity.

Slow Waves

In portions of the gastrointestinal tract, rhythmic fluctuations in membrane potential called slow waves influence the timing of smooth muscle contraction.

Slow waves are closely associated with specialized pacemaker cells called interstitial cells of Cajal rather than being generated solely by the smooth muscle cells themselves.

Response to Stretch

Stretch can stimulate contraction in some smooth muscle tissues. This property allows organs to respond automatically when their walls are distended.

Stretch-dependent responses are particularly important in hollow visceral organs and contribute to regulation of their contents.

Hormonal Regulation

Smooth muscle cells express receptors for numerous hormones and circulating mediators. Depending on the receptor type and intracellular signaling pathway, these substances can promote either contraction or relaxation.

Hormonal regulation is particularly important in vascular, gastrointestinal, reproductive, and urinary smooth muscle.

Local Regulation

Local chemical conditions can modify smooth muscle tone independently of direct neural control. Changes in oxygen, carbon dioxide, pH, metabolites, and locally released signaling molecules can alter smooth muscle activity.

This local regulation is especially important in the vascular system, where vessel diameter can be adjusted according to tissue metabolic requirements.

Vascular Smooth Muscle

Vascular smooth muscle is located primarily in the tunica media of blood vessel walls. Its contraction decreases vessel diameter, while relaxation permits dilation.

By regulating vascular caliber, smooth muscle contributes to systemic vascular resistance, blood pressure, and regional distribution of blood flow.

Vasoconstriction

Vasoconstriction occurs when vascular smooth muscle contracts and decreases the internal diameter of a vessel. This can increase vascular resistance and reduce blood flow through the affected vascular bed.

Vasodilation

Vasodilation involves relaxation of vascular smooth muscle and an increase in vessel diameter. It can reduce vascular resistance and increase blood flow.

Vascular tone reflects a dynamic balance among neural, hormonal, endothelial, metabolic, and intrinsic smooth muscle influences.

Gastrointestinal Smooth Muscle

Smooth muscle forms major components of the muscular wall of the gastrointestinal tract. Organized layers of smooth muscle generate movements that mix luminal contents and propel them through the digestive system.

Contraction is coordinated by interactions among smooth muscle cells, the enteric nervous system, autonomic inputs, gastrointestinal hormones, and interstitial cells of Cajal.

Peristalsis

Peristalsis consists of coordinated muscular contractions that propel contents along a hollow organ. It is particularly important in the esophagus, gastrointestinal tract, and ureters.

Circular and longitudinal smooth muscle layers cooperate to produce effective propulsive movements.

Urinary Tract Smooth Muscle

Smooth muscle within the ureters produces peristaltic contractions that propel urine from the kidneys toward the urinary bladder.

The muscular wall of the bladder contains the detrusor muscle, whose coordinated contraction is essential for bladder emptying.

Uterine Smooth Muscle

The muscular wall of the uterus, known as the myometrium, consists predominantly of smooth muscle. Its structure and contractile properties change substantially during pregnancy.

Uterine smooth muscle generates the powerful coordinated contractions involved in childbirth.

Respiratory Smooth Muscle

Smooth muscle is present in the walls of bronchi and bronchioles. Changes in its contractile state modify airway caliber and therefore influence resistance to airflow.

Excessive airway smooth muscle contraction contributes to airflow limitation in conditions such as asthma.

Ocular Smooth Muscle

Smooth muscle within the eye contributes to regulation of pupil diameter and lens accommodation. The sphincter pupillae and dilator pupillae alter pupil size, while the ciliary muscle participates in accommodation.

These muscles are controlled through autonomic pathways.

Arrector Pili Muscles

Arrector pili are small bundles of smooth muscle associated with hair follicles in the skin. Their contraction elevates hairs and produces localized skin dimpling commonly called goosebumps.

They receive sympathetic autonomic innervation.

Sphincters

Smooth muscle forms or contributes to several involuntary sphincters. Sustained contraction can maintain closure of an opening, while relaxation permits passage of contents.

The ability of smooth muscle to maintain prolonged tension with relatively low energy expenditure is particularly advantageous in sphincter function.

Mechanical Coupling Between Cells

Smooth muscle cells are surrounded by extracellular matrix containing collagen, elastic fibers, and other structural components. Mechanical connections between cells and the extracellular matrix distribute force through the tissue.

In sheets of visceral smooth muscle, contraction of many cells can therefore generate coordinated changes in the shape or diameter of an organ.

Basal Lamina

Individual smooth muscle cells are surrounded by a basal lamina and delicate connective tissue fibers. This extracellular support helps transmit force while allowing cells to change shape during contraction.

Regenerative Capacity

Smooth muscle generally has greater regenerative capacity than skeletal or cardiac muscle. Mature smooth muscle cells can retain the ability to undergo mitosis under appropriate conditions.

Additional smooth muscle cells may also arise from progenitor populations in certain tissues.

Smooth Muscle Hypertrophy

Smooth muscle can increase in mass through hypertrophy, an increase in individual cell size. This can occur in response to increased functional demands or hormonal stimulation.

A prominent physiological example is enlargement of uterine smooth muscle cells during pregnancy.

Smooth Muscle Hyperplasia

Unlike mature cardiac muscle and most mature skeletal muscle fibers, smooth muscle can also increase tissue mass through hyperplasia, an increase in cell number.

Both hypertrophy and hyperplasia can contribute to enlargement of smooth muscle tissues.

Histological Identification

Smooth muscle can be recognized histologically by several characteristic features:

• Spindle-shaped cells
• Single centrally positioned nuclei
• Absence of transverse striations
• Eosinophilic cytoplasm
• Closely packed cells arranged in sheets or bundles
• Variable appearance depending on the plane of section

Longitudinal Section

In longitudinal sections, smooth muscle cells appear elongated with tapered ends. Their centrally located nuclei are elongated and usually stain prominently.

Cell boundaries can be difficult to distinguish with routine light microscopy because adjacent cells are tightly packed.

Transverse Section

In transverse sections, smooth muscle cells appear as circular or polygonal profiles of different diameters because the section passes through different levels of their tapered shapes.

Only profiles passing through the central portion of a cell contain a visible nucleus, which helps explain why many cell profiles appear without nuclei in a given histological section.

Smooth Muscle Compared with Skeletal Muscle

Feature	Smooth Muscle	Skeletal Muscle
Striations	Absent	Present
Control	Involuntary	Primarily voluntary
Cell shape	Spindle-shaped	Long and cylindrical
Nuclei	One central nucleus	Multiple peripheral nuclei
Sarcomeres	Absent	Present
Troponin	Absent	Present
Primary calcium regulator	Calmodulin	Troponin C
Neural control	Autonomic modulation	Somatic motor innervation
Regenerative capacity	Relatively greater	Limited

Smooth Muscle Compared with Cardiac Muscle

Feature	Smooth Muscle	Cardiac Muscle
Striations	Absent	Present
Sarcomeres	Absent	Present
Cell shape	Spindle-shaped	Short and branching
Nucleus	Single and central	Usually single and central
Intercalated discs	Absent	Present
Control	Involuntary	Involuntary
Troponin	Absent	Present
Regenerative capacity	Relatively greater	Very limited

Comparison of the Three Muscle Types

Feature	Skeletal	Cardiac	Smooth
Striated	Yes	Yes	No
Voluntary control	Primarily yes	No	No
Sarcomeres	Yes	Yes	No
Cell branching	No	Yes	No
Nuclei	Multiple	Usually one	One
Nuclear position	Peripheral	Central	Central
Intercalated discs	No	Yes	No
Major calcium regulatory protein	Troponin C	Troponin C	Calmodulin

Clinical Importance

Abnormal smooth muscle contraction or relaxation contributes to disorders affecting many organ systems. Excessive vascular smooth muscle tone can increase vascular resistance, while inappropriate airway smooth muscle contraction can narrow the respiratory passages.

Altered gastrointestinal smooth muscle activity can disturb propulsion of luminal contents, and abnormalities of urinary tract smooth muscle can affect urine transport or bladder emptying.

Bronchoconstriction

Bronchoconstriction results from contraction of airway smooth muscle, reducing airway diameter and increasing resistance to airflow. Excessive or inappropriate bronchoconstriction is an important component of asthma and other obstructive airway conditions.

Vascular Smooth Muscle and Hypertension

Vascular smooth muscle tone is one determinant of systemic vascular resistance. Persistent changes in vascular tone and structural remodeling of vessel walls can contribute to abnormal regulation of arterial blood pressure.

Gastrointestinal Motility Disorders

Normal gastrointestinal movement depends on coordinated smooth muscle activity together with enteric neural circuits and pacemaker mechanisms. Disruption of these systems can impair propulsion or mixing of intestinal contents.

Functional Importance

Smooth muscle provides the body with a highly adaptable form of involuntary contractile tissue. Its spindle-shaped cells, non-sarcomeric arrangement of actin and myosin, dense bodies, calcium-calmodulin regulation, and energy-efficient cross-bridge activity distinguish it structurally and functionally from skeletal and cardiac muscle.

Its ability to maintain sustained tension with relatively low energy consumption is particularly important in blood vessels and sphincters. Its broad working length allows hollow organs such as the urinary bladder and uterus to undergo major changes in volume while retaining contractile function.

Smooth muscle can respond to autonomic nerves, hormones, local chemical signals, mechanical stretch, and intrinsic electrical activity. This combination of control mechanisms allows individual tissues to adapt smooth muscle behavior precisely to their physiological roles.

Understanding smooth muscle is fundamental to the study of muscle histology, visceral anatomy, vascular tone, gastrointestinal motility, airway regulation, urinary function, reproductive anatomy, autonomic physiology, calcium signaling, and the functional organization of hollow organs.