Smooth muscle cells range from 5 to 10 µm in diameter and from 30 to 200 µm in length. Each cell is spindle-shaped and has a single, centrally located nucleus. Figure 10-22a shows typical smooth muscle fibers as seen by light microscopy, and Figure 10-22b

Smooth muscle tissue occurs within almost every organ, forming sheets, bundles, or sheaths around other tissues. Smooth muscles around blood vessels regulate blood flow through vital organs. In the digestive and urinary systems, rings of smooth muscle, called sphincters, regulate the movement of materials along internal passageways. Smooth muscles in bundles, layers, or sheets play a variety of other roles:

• Integumentary system. Smooth muscles around blood vessels regulate the flow of blood to the superficial dermis; smooth muscles of the arrector pili elevate hairs. 
• Cardiovascular system. Smooth muscles encircling blood vessels control the distribution of blood and help regulate blood pressure.
• Respiratory system. Smooth muscle contraction or relaxation alters the diameters of the respiratory passageways and changes the resistance to airflow.
• Digestive system. Extensive layers of smooth muscle in the walls of the digestive tract play an essential role in moving materials along the tract. Smooth muscle in the walls of the gallbladder contract to eject bile into the digestive tract.
• Urinary system. Smooth muscle tissue in the walls of small blood vessels alters the rate of filtration at the kidneys. Layers of smooth muscle in the walls of the ureters transport urine to the urinary bladder; the contraction of the smooth muscle in the wall of the urinary bladder forces urine out of the body.
• Reproductive system. Layers of smooth muscle help move sperm along the reproductive tract in males and cause the ejection of glandular secretions from the accessory glands into the reproductive tract. In females, layers of smooth muscle help move oocytes (and perhaps sperm) along the reproductive tract, and contraction of the smooth muscle in the walls of the uterus expels the fetus at delivery.

DIFFERENCES BETWEEN SMOOTH MUSCLE TISSUE AND OTHER MUSCLE TISSUES

Smooth muscle tissue differs from both skeletal and cardiac muscle tissues in structure and function (Table 10-4).

Structural Differences

Actin and myosin are present in all three muscle types. In skeletal and cardiac muscle cells, these proteins are organized in sarcomeres, with thin and thick filaments. The internal organization of a smooth muscle cell is very different:

• A smooth muscle fiber has no T tubules, and the sarcoplasmic reticulum forms a loose network throughout the sarcoplasm. Smooth muscle tissue has no myofibrils or sarcomeres. As a result, this tissue also has no striations and is called nonstriated muscle.
• Thick filaments are scattered throughout the sarcoplasm of a smooth muscle cell. The myosin proteins are organized differently than in skeletal or cardiac muscle cells, and smooth muscle cells have more cross-bridges per thick filament.
• The thin filaments in a smooth muscle cell are attached to dense bodies, structures distributed throughout the sarcoplasm in a network of intermediate filaments composed of the protein desmin (Figure 10-22b). Some of the dense bodies are firmly attached to the sarcolemma. The dense bodies and intermediate filaments anchor the thin filaments such that, when sliding occurs between thin and thick filaments, the cell shortens. Dense bodies are not arranged in straight lines, so when a contraction occurs, the muscle cell twists like a corkscrew.
• Adjacent smooth muscle cells are bound together at dense bodies, transmitting the contractile forces from cell to cell throughout the tissue.
• Although smooth muscle cells are surrounded by connective tissue, the collagen fibers never unite to form tendons or aponeuroses as they do in skeletal muscles.

Functional Differences

Smooth muscle tissue differs from other muscle types in terms of (1) excitation–contraction coupling, (2) length–tension relationships, (3) control of contractions, and (4) smooth muscle tone.

Excitation–Contraction Coupling The trigger for smooth muscle contraction is the appearance of free calcium ions in the cytoplasm. Most of these calcium ions enter the cell from the extracellular fluid. Once in the sarcoplasm, the calcium ions interact with calmodulin, a calcium-binding protein. Calmodulin then activates the enzyme myosin light chain kinase, which breaks down ATP and initiates the contraction. This situation is quite different from the one in skeletal and cardiac muscles, in which the trigger for contraction is the binding of calcium ions to troponin.

Length–Tension Relationships Because the thick and thin filaments are scattered and are not organized into sarcomeres, tension development and resting length in smooth muscle are not directly related. A stretched smooth muscle soon adapts to its new length and retains the ability to contract on demand. This ability to function over a wide range of lengths is called plasticity. Smooth muscle can contract over a range of lengths four times greater than that of skeletal muscle. This ability is especially important for digestive organs that undergo great changes in volume, such as the stomach. Despite the lack of sarcomere organization, smooth muscle contractions can be just as powerful as those of skeletal muscles. Like skeletal muscle fibers, smooth muscle cells often undergo sustained tetanic contractions.

Control of Contractions Many smooth muscle cells are not innervated by motor neurons, and the neurons that do innervate smooth muscles are not under voluntary control. The nature of the connection with the nervous system provides a means of categorizing smooth muscle cells as either multiunit or visceral. Multiunit smooth muscle cells are innervated in motor units comparable to those of skeletal muscles, but each smooth muscle cell may be connected to several motor neurons rather than to just one. In contrast, many visceral smooth muscle cells lack a direct contact with any motor neuron.

Multiunit smooth muscle cells resemble skeletal muscle fibers and cardiac muscle cells, in that neural activity produces an action potential that is propagated over the sarcolemma. However, the contractions of these smooth muscle cells are more leisurely than are those of skeletal or cardiac muscle cells. Multiunit smooth muscle cells are located in the iris of the eye, where they regulate the diameter of the pupil; along portions of the male reproductive tract; within the walls of large arteries; and in the arrector pili muscles of the skin. Multiunit muscle cells do not typically occur in the digestive tract.

Visceral smooth muscle cells are arranged in sheets or layers. Within each layer, adjacent muscle cells are connected by gap junctions. Because they are connected in this way, whenever one muscle cell contracts, the electrical impulse that triggered the contraction can travel to adjacent smooth muscle cells. The contraction therefore spreads in a wave that soon involves every smooth muscle cell in the layer. The initial stimulus may be the activation of a motor neuron that contacts one of the muscle cells in the region. But smooth muscle cells will also contract or relax in response to chemicals, hormones, local concentrations of oxygen or carbon dioxide, or physical factors such as extreme stretching or irritation.

Many visceral smooth muscle networks show rhythmic cycles of activity in the absence of neural stimulation. These cycles are characteristic of the smooth muscle cells in the wall of the digestive tract, where pacesetter cells undergo spontaneous depolarization and trigger the contraction of entire muscular sheets. Visceral smooth muscle cells are located in the walls of the digestive tract, the gallbladder, the urinary bladder, and many other internal organs.

Smooth Muscle Tone Both multiunit and visceral smooth muscle tissues show a normal background level of activity, or smooth muscle tone. The regulatory mechanisms detailed above stimulate contraction and increase muscle tone. Neural, hormonal, or chemical factors can also stimulate smooth muscle relaxation, producing a decrease in muscle tone. For example, smooth muscle cells at the entrances to capillaries regulate the amount of blood flow into each vessel. If the tissue becomes oxygen-starved, the smooth muscle cells relax. Blood flow increases, delivering additional oxygen. As conditions return to normal, the smooth muscle regains its normal muscle tone.

 What feature of cardiac muscle tissue allows the heart to act as a functional syncytium?

 Why are cardiac and smooth muscle contractions more affected by changes in extracellular Ca²⁺ than are skeletal muscle contractions?

 Smooth muscle can contract over a wider range of resting lengths than skeletal muscle can. Why?