Physical Address

304 North Cardinal St.
Dorchester Center, MA 02124

Smooth muscle


Introduction

Like skeletal muscle (see Ch. 5 ), the primary function of smooth muscle is contraction. However, although skeletal muscles often attach to bones and cross joints so that contraction produces limb movement, smooth muscle typically forms tissues within organ systems (e.g., vascular, respiratory, gastro-intestinal [GI]) to regulate the movement of liquids (blood), gases (air), and/or solids (food) within hollow tubular structures (e.g., blood vessels, bronchi, alimentary tract). Smooth muscle also has synthetic functions that are important physiologically.

System structure

Just as skeletal muscle function requires the integration of three organ systems (nervous system, skeletal muscle system, and skeletal system), smooth muscle function requires three components:

  • Neural input

  • Smooth muscle

  • Target organ systems

Smooth muscle in organs

With the possible exceptions of the urinary bladder and the uterus, there are no true smooth muscle organs. Instead, smooth muscle is a component of many organ systems.

  • Often, these organ systems are hollow, and smooth muscle is a component of the walls of the constituent organs.

  • Smooth muscle thus serves a contractile tissue that, like connective tissue, serves a roughly consistent function in many anatomic locations.

Smooth muscle tissues

Smooth muscle is present in the following tissues:

  • The walls of arteries, veins, and lymphatic ducts of the cardiovascular system ( Fig. 6.1 )

    Fig. 6.1, Smooth muscle layers in the wall of peripheral blood vessels. Longitudinal sections of an artery and an arteriole reveal the smooth muscle tissue layer beneath the endothelium. Note that smooth muscle cells are oriented circularly around the vessel lumen, so that even slight contraction will cause important changes in the radius of the lumen. The thickness of the smooth muscle layer is greater in arteries than in veins. Capillaries lack a smooth muscle layer.

  • The walls of bronchi of the respiratory tract ( Fig. 6.2 )

    Fig. 6.2, Smooth muscle in the walls of bronchi and bronchioles. Strips of smooth muscle beneath the respiratory mucosa wind obliquely down the airways of the lung.

  • The walls of the esophagus, stomach, small intestine, and colon of the GI tract ( Fig. 6.3 )

    Fig. 6.3, Smooth muscle in the walls of the gastrointestinal (GI) tract. Note that there are both circular layers, which alter the lumen radius according to Poiseuille’s law, and longitudinal layers which facilitate the movement of GI contents (peristalsis). In addition, the muscularis mucosae allows the mucosal layer of the GI tract to move. The submucosal (Meisner’s) and myenteric (Auerbach’s) plexuses are collections of autonomic neurons that modulate smooth muscle contraction.

  • The ureters and urinary bladder of the genitourinary tract

  • The myometrium of the uterus

Smooth muscle cells form sheets and layers ( Fig. 6.4 ). In longitudinal section, the individual cells appear tightly packed in a staggered array (see Fig. 6.4 ). This staggering means that in transverse section, the single plane of section “catches” different cells at different points along the longitudinal axis—some at one end, others in the middle (see Fig. 6.4 ).

Fig. 6.4, Sheets and layers of smooth muscle cells. Smooth muscle cells are packed tightly together and are staggered in a block of tissue. A longitudinal plane of section (front face of the block) displays the fusiform shape of the cells and how the thicker central part of a given smooth muscle cell is often apposed to the thinner ends of adjacent cells. A transverse or transaxial plane of section (right side face of the block) catches different cells in different places along their long axis.

Smooth muscle cells

Both the muscle cells and the nerve cells with which they are physically associated differ between the skeletal muscle system and the smooth muscle system. Their histologic differences are the basis of functional differences among these cells.

Innervation

Skeletal muscle cells and lower motor neurons make up the neuromuscular junction, or synapse, discussed in Chapter 5 . Smooth muscle cells are often associated with neurons as well, but these nerve cells are part of the autonomic nervous system (see Ch. 4 ).

  • Many smooth muscle tissues have dual innervation from both the sympathetic and parasympathetic branches of the autonomic nervous system.

  • Instead of terminal boutons, the axons of these autonomic nerves end in dilated structures called varicosities.

  • Although these varicosities are closely approximated to the cell membrane of smooth muscle cells, they do not form neuromuscular junctions ( Fig. 6.5 ).

    Fig. 6.5, Autonomic neuron endings near smooth muscle cells. Unlike the arrangement between skeletal muscle cells and lower motor neurons, there is no synapse between smooth muscle cells and autonomic neurons. Instead, multiple terminal varicosities (A and D) or single varicosities (B) of autonomic or intrinsic nerves that contain neurotransmitter laden vesicles are closely apposed to the smooth muscle cell membrane (C). In (E), the sympathetic innervation of vascular structures is shown. C, capillaries; la , large arterioles; lv, large veinules; PCV , post capillary veinules; SA , small arteries; sa , small arterioles; SV , small veins; svl , small veinules; TA , terminal arterioles.

Histology

When relaxed, a single smooth muscle cell is fusiform, or long and tapered at both ends.

  • The staggered arrangement of cells allows for close packing: the thickest middle portion of one cell typically is surrounded by the smaller ends of adjacent cells, and vice versa (see Fig. 6.4 A).

  • These cells are called “smooth” because, unlike skeletal muscle cells, they have no striations. They have a homogeneous appearance under the light microscope.

Smooth muscle organelles

When examined by electron microscopy, a number of organelles are visible in smooth muscle cells.

  • They contain a single large, centrally located nucleus.

  • For synthetic function, they often possess ribosomes, rough endoplasmic reticulum (ER), and Golgi apparati.

  • However, the organization of myofilaments is especially characteristic of smooth muscle cells.

Like skeletal muscle, smooth muscle contains myofilaments, both thick and thin.

  • Skeletal (striated) muscle is highly organized into sarcomeres, producing the striations ( Fig. 6.6 A and B).

    Fig. 6.6, Thick and thin filaments in skeletal and smooth muscle. A, In skeletal muscle, thick and thin filaments are arranged into sarcomeres. B, A transverse section of a sarcomere is characterized by an almost crystalline regularity of filaments and demonstrates the 1:2 ratio of thick to thin filaments in this type of muscle. C, Smooth muscle cells have thick and thin filaments but lack this highly organized pattern of sarcomeres. D, A transverse section of smooth muscle reveals a ratio of thick to thin filaments of approximately 1:10. Nevertheless, the contractile machinery and sliding filament model are largely the same in both types of muscle cells.

  • The absence of such striations in smooth muscle cells is a direct consequence of the different arrangement of thick and thin filaments.

    • Smooth muscle cells have no sarcomeres and no regularly ordered association or ratio between the different contractile protein polymers ( Fig. 6.6 C and D).

    • Instead of the skeletal muscle ratio of 2:1 thin to thick filaments, there is a looser ratio of approximately 10:1 thin to thick filaments.

Whereas sarcomeres in skeletal muscle are connected in series to form myofibrils parallel to the long axis of the muscle fiber, the contractile machinery of smooth muscle cells, lacking sarcomeres, is not limited to this one alignment but is often oriented obliquely.

Smooth muscle cells also have intermediate filaments that interconnect various components of the cytoskeleton ( Fig. 6.7 A).

  • Dense bodies are protein-anchoring structures to which thin filaments attach (analogous to skeletal muscle Z lines).

  • Dense bodies may be entirely within the cytoplasm, or they may be bound to the inner aspect of the cell membrane.

Fig. 6.7, Dense bodies serve as rivet-like anchoring points for thin filaments during contraction.

Skeletal and smooth muscle cells both have sarcoplasmic reticulum (SR), a collection of closed membranes that store Ca 2+ .

  • Smooth muscle cell SR is not as well developed as that of skeletal muscle.

  • As in skeletal muscle cells, the smooth muscle cell membrane is called the sarcolemma and the cytoplasm is called the sarcoplasm.

Although skeletal muscle fibers are the result of the fusion of numerous precursor cells, smooth muscle cells remain distinct. However, in different anatomic and functional contexts, these individual smooth muscle cells may be more or less interconnected.

  • Gap junctions, desmosomes, and other structures can mechanically, electrically, and chemically couple adjacent smooth muscle cells (see Fig. 6.7 A and B).

  • Although smooth muscle cells lack the T tubules of skeletal muscle, they do possess sarcolemma invaginations called caveolae, which perform a similar function ( Fig. 6.8 ).

    Fig. 6.8, Depiction of caveolae.

Smooth muscle molecules

As in skeletal muscle, thick filaments in smooth muscle are composed of myosin. However, the smooth muscle isoform is different from that of skeletal muscle. Thin filaments also contain actin and tropomyosin but lack the troponin regulatory complex seen in skeletal muscle. Extensive extracellular connective tissue networks contain elastin, collagen, and reticulin.

Smooth muscle cations

As in skeletal muscle cells, there are three cations differentially distributed between the intracellular and extracellular spaces:

  • Sodium (Na + )

  • Potassium (K + )

  • Calcium (Ca 2+ )

The electrochemical gradients driving these cations are identical between skeletal and smooth muscle cells:

  • The electrochemical gradient for Na + greatly favors its influx into a smooth muscle cell.

  • The opposite is true for K + , which exits the cell when K + - specific ion channels open.

  • The cytoplasm of a smooth muscle cell normally has low concentration of Ca 2+ relative to the extracellular space and the lumen of the SR, so that a large electrochemical gradient favors the entry of this cation into the sarcoplasm.

System function

Like skeletal muscle, the main function of smooth muscle is contraction. However, unlike skeletal muscle, smooth muscle contraction is largely under unconscious control by the autonomic nervous system. As with their structures, the contractile mechanisms of skeletal and smooth muscle are similar on many scales but differ importantly and reflect their distinct physiologic roles.

Smooth muscle cations

As in other excitable cells, such as skeletal muscle cells and neurons, the movement of cations via their electrochemical gradients can cause and result from changes in the cell membrane electrical potential. The relationship between changes in the membrane potential and contraction in smooth muscle cells, however, is not as rigid as that of excitation contraction coupling in skeletal muscle cells.

Smooth muscle membrane electrical potential

For skeletal muscle, an action potential is absolutely necessary for contraction. However, there is no consistent relationship between changes in smooth muscle membrane potential and contractility. There is great variability from one smooth muscle subtype to another.

  • Some smooth muscle cells manifest regular, intrinsic, sinusoidal alterations in membrane potential that are unaccompanied by a change in contractile state ( Fig. 6.9 A).

    Fig. 6.9, The relationship between electrical activity and contraction in smooth muscle. A, Unlike excitation-contraction coupling in skeletal muscle, spontaneous, periodic membrane depolarization may not cause contraction in some smooth muscle cells. B, In other smooth muscle tissues, a depolarization must reach or exceed a threshold value to cause contraction and rapid sequences may generate tetanus. Various stimuli may induce contraction or relaxation in smooth muscle from different anatomic locations. The critical factors determining a given smooth muscle cell’s response to an agent are the type and number of cell surface receptors and their associated signal transduction mechanisms.

  • Other smooth muscle cells are activated by an action potential. These cells often exhibit contractile summation and tetanus like skeletal muscles if action potentials “stack” in rapid succession ( Fig. 6.9 B).

As with many cell types, the smooth muscle cell’s resting membrane potential is the function of the variable permeability to and different intra- and extracellular concentrations of sodium, potassium, chloride, bicarbonate, and other ions. Ion pumps and membrane channels similar to those in skeletal muscle maintain electrochemical gradients.

Calcium

Skeletal and smooth muscle contraction and relaxation share dependence on changes in the intracellular Ca 2+ concentration. Influx and efflux of Ca 2+ into the sarcoplasm determine the sarcoplasmic Ca 2+ concentration, but the sources of Ca 2+ vary from one type of smooth muscle cell to another.

Some Ca 2+ enters from the extracellular pool by means of sarcolemmal ion channels.

  • These sarcolemmal Ca 2+ channels are often voltage-gated or ligand-gated, but others simply remain open for long periods (“leak” channels).

  • There are also mechanically-gated Ca 2+ channels that respond to stretch by allowing influx of Ca 2+ .

Other smooth muscle cells have significant Ca 2+ caches in SR.

  • When smooth muscle is stimulated, the entry of extra-cellular Ca 2+ may trigger the release of sarcoplasmic stores through calcium-gated calcium channels.

  • In other cells, stimulatory ligands may activate receptors and generate second messengers, such as inositol triphosphate (IP3) and diacylglycerol (DAG) (see appendix on signal transduction) which may liberate sarcoplasmic Ca 2+ .

  • Whatever the mechanism, stimulation increases the intra-cellular Ca 2+ concentration ( Fig. 6.10 A1-3).

    Fig. 6.10, The movement of calcium into and out of smooth muscle. A, Extracellular Ca 2+ may enter the cell through voltage-gated ( A1 ) or ligand-gated channels ( A2 ) in the sarcolemma. Leak Ca 2+ channels open independent of membrane voltage changes or the binding of ligand and spend a relatively large amount of the time open, allowing Ca 2+ to enter the cell. Ca 2+ itself, or second messengers, such as inositol triphosphate ( IP3 ), may induce the release of Ca 2+ from the sarcoplasmic reticulum. B, Specialized ion pumps in the sarcolemma remove Ca 2+ , extruding it into the extracellular space. Some of these use the energy liberated by the cleavage of adenosine triphosphate ( ATP ), while others use the energy of the steep electrochemical gradient favoring the influx of Na + . Other ATP-requiring Ca 2+ pumps sequester Ca 2+ in the sarcoplasmic reticulum. MLCK , Myosin light-chain kinase.

Smooth muscle molecules

A number of molecules are important in smooth muscle contraction and relaxation. Among them are regulatory mediators, contractile proteins, kinases, and phosphatases that modulate the contractile proteins, channels and pumps that control the movement of cations, and the small molecules that fuel contraction.

You're Reading a Preview

Become a Clinical Tree membership for Full access and enjoy Unlimited articles

Become membership

If you are a member. Log in here

Related Posts