Smooth Muscle – Anatomy and Physiology (2023)

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 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 P_eufixed 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]).

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.

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.

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.