Smooth muscle is one of the three major types of vertebrate muscle tissue, the others being skeletal and cardiac muscle. It can also be found in invertebrates and is controlled by the autonomic nervous system. It is non-striated, so-called because it has no sarcomeres and therefore no striations (bands or stripes). It can be divided into two subgroups, single-unit and multi-unit smooth muscle. Within single-unit muscle, the whole bundle or sheet of smooth muscle cells contracts as a syncytium.

Smooth muscle is found in the walls of hollow organs, including the stomach, intestines, bladder and uterus. In the walls of blood vessels, and lymph vessels, (excluding blood and lymph capillaries) it is known as vascular smooth muscle. There is smooth muscle in the tracts of the respiratory, urinary, and reproductive systems. In the eyes, the ciliary muscles, iris dilator muscle, and iris sphincter muscle are types of smooth muscles. The iris dilator and sphincter muscles are contained in the iris and contract in order to dilate or constrict the pupils. The ciliary muscles change the shape of the lens to focus on objects in accommodation. In the skin, smooth muscle cells such as those of the arrector pili cause hair to stand erect in response to cold temperature and fear.

Single-unit visceral smooth muscle is myogenic; it can contract regularly without input from a motor neuron (as opposed to multiunit smooth muscle, which is neurogenic - that is, its contraction must be initiated by an autonomic nervous system neuron). A few of the cells in a given single unit may behave as pacemaker cells, generating rhythmic action potentials due to their intrinsic electrical activity. Because of its myogenic nature, single-unit smooth muscle is usually active, even when it is not receiving any neural stimulation. Multiunit smooth muscle is found in the trachea, in the iris of the eye, and lining the large elastic arteries.

However, the terms single- and multi-unit smooth muscle represent an oversimplification. This is due to the fact that smooth muscles for the most part are controlled and influenced by a combination of different neural elements. In addition, it has been observed that most of the time there will be some cell-to-cell communication and activators/inhibitors produced locally. This leads to a somewhat coordinated response even in multiunit smooth muscle.

Smooth muscle differs from skeletal muscle and cardiac muscle in terms of structure, function, regulation of contraction, and excitation-contraction coupling. However, smooth muscle tissue tends to demonstrate greater elasticity and function within a larger length-tension curve than striated muscle. This ability to stretch and still maintain contractility is important in organs like the intestines and urinary bladder. Smooth muscle in the gastrointestinal tract is activated by a composite of smooth muscle cells (SMCs), interstitial cells of Cajal (ICCs), and platelet-derived growth factor receptor alpha (PDGFRα) that are electrically coupled and work together as an SIP functional syncytium.

Microanatomy

Smooth muscle cells

thumb|350px

A smooth-muscle cell is a spindle-shaped myocyte with a wide middle and tapering ends, and a single nucleus. Like striated muscle, smooth muscle can tense and relax. In the relaxed state, each cell is 30–200 micrometers in length, some thousands of times shorter than a skeletal muscle cell.

Myosin

Myosin is primarily class II in smooth muscle.) that codes for the heavy chains myosin II, but there are splice variants of this gene that result in four distinct isoforms.

The ratio of actin to myosin is between 2:1 and 10:1

Other associated proteins

Smooth muscle does not contain the protein troponin; instead calmodulin (which takes on the regulatory role in smooth muscle), caldesmon and calponin are significant proteins expressed within smooth muscle.

  • Tropomyosin is present in smooth muscle, spanning seven actin monomers and is laid out end to end over the entire length of the thin filaments. In striated muscle, tropomyosin serves to block actin–myosin interactions until calcium is present, but in smooth muscle, its function is unknown.

The intermediate filaments are connected to other intermediate filaments via dense bodies, which eventually are attached to adherens junctions (also called focal adhesions) in the cell membrane of the smooth muscle cell, called the sarcolemma. The adherens junctions consist of large number of proteins including alpha-actinin (α-actinin), vinculin and cytoskeletal actin.

Spread of impulse

To maintain organ dimensions against force, cells are fastened to one another by adherens junctions. As a consequence, cells are mechanically coupled to one another such that contraction of one cell invokes some degree of contraction in an adjoining cell. Gap junctions couple adjacent cells chemically and electrically, facilitating the spread of chemicals (e.g., calcium) or action potentials between smooth muscle cells. Single unit smooth muscle displays numerous gap junctions and these tissues often organize into sheets or bundles which contract in bulk.

Contraction

Smooth muscle contraction is caused by the sliding of myosin and actin filaments (a sliding filament mechanism) over each other. The energy for this to happen is provided by the hydrolysis of ATP. Myosin functions as an ATPase utilizing ATP to produce a molecular conformational change of part of the myosin and produces movement. Movement of the filaments over each other happens when the globular heads protruding from myosin filaments attach and interact with actin filaments to form crossbridges. The myosin heads tilt and drag along the actin filament a small distance (10–12 nm). The heads then release the actin filament and then changes angle to relocate to another site on the actin filament a further distance (10–12 nm) away. They can then re-bind to the actin molecule and drag it along further. This process is called crossbridge cycling and is the same for all muscles (see muscle contraction). Unlike cardiac and skeletal muscle, smooth muscle does not contain the calcium-binding protein troponin. Contraction is initiated by a calcium-regulated phosphorylation of myosin, rather than a calcium-activated troponin system.

Crossbridge cycling causes contraction of myosin and actin complexes, in turn causing increased tension along the entire chains of tensile structures, ultimately resulting in contraction of the entire smooth muscle tissue.

Phasic or tonic

Smooth muscle may contract phasically with rapid contraction and relaxation, or tonically with slow and sustained contraction. The reproductive, digestive, respiratory, and urinary tracts, skin, eye, and vasculature all contain this tonic muscle type. This type of smooth muscle can maintain force for prolonged time with only little energy utilization. There are differences in the myosin heavy and light chains that also correlate with these differences in contractile patterns and kinetics of contraction between tonic and phasic smooth muscle.

Activation of myosin heads

Crossbridge cycling cannot occur until the myosin heads have been activated to allow crossbridges to form. When the light chains are phosphorylated, they become active and will allow contraction to occur. The enzyme that phosphorylates the light chains is called myosin light-chain kinase (MLCK), also called MLC<sub>20</sub> kinase.

  • Myogenic mechanism
  • List of distinct cell types in the adult human body

References

  • BBC – baby born with smooth muscle condition has 8 organs transplanted
  • Smooth muscle antibody
  • Stomach smooth muscle identified using antibody
  • "Smooth Muscle"
  • Smooth muscle histology photomicrographs
  • Where smooth muscle tissue is found in the body (medlineplus.gov)