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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.
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
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 is present in the following tissues:
The walls of arteries, veins, and lymphatic ducts of the cardiovascular system ( Fig. 6.1 )
The walls of bronchi of the respiratory tract ( Fig. 6.2 )
The walls of the esophagus, stomach, small intestine, and colon of the GI tract ( Fig. 6.3 )
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 ).
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.
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 ).
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.
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).
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.
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 ).
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.
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.
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.
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.
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).
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.
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).
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.
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