Smooth muscle

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Smooth muscle (also known as visceral muscle due to the locations in which they are present [1]) is one of the three main types of muscle tissue that exist in the human body [2] . Smooth muscle is under involuntary control [3]  and is innervated by the autonomic nervous system [4]. It can also be stimulated without the use of nerves, this is termed Pharmomechanical Coupling. In this type of stimulation an agent (usually hormone) is used to cause contraction rather than an action potential. When certain hormones bind to Ca2+ mobilising receptors on the sarcolemma inositol 1,4,5-triphosphate (InsP3) is produced. This secondary messenger opens InsP3-gated Ca2+ channels allowing an influx of calcium ions. In turn this influx causes ryanodine receptors (RYR) to open allowing an even greater influx of calcium ions.[5]

Smooth muscle lines the walls of hollow internal organs such as the bladder and intestine [6], organs of this type are known as viscera. Smooth muscle cells compose myosin myofilaments dispersed throughout the muscle cell cytoplasm and filaments of actin held together in contracile bundles. Intermediate filaments exist between contractile bundles connecting them, and they are anchored by dense plaque-like bodies [7]. The contractile filament bundles of actin and myosin are loosely arranged in a diagonal fashion, in different directions around the perimeter of the smooth muscle cell. This arrangement of fibres causes the muscle cell to become globular upon contraction [8]. Smooth muscle cells are fusiforn in shape meaning that they are wide in the middle with tapered ends, they also have only a single nucleus. Smooth muscle cells do not contain the sarcomeres found in skeletal and cardiac muscle and therefore appear unstriated under a microscope [9]. Smooth muscle cells are unstriated because there is no regular arrangement of actin and myosin filaments.They contain only a few sarcoplasmic reticulua, instead using extracellular calcium as the source of calcium ions which initiate contraction [10].

There are two types of smooth muscle cell, multi- unit and single unit smooth muscle. Single-unit smooth muscle cells are connected by gap junctions that electrically connect cells to one another, so contract as a single unit. These can be found in the intestinal tract, the skin, and the walls of small arteries, veins and hollow organs[11]. Multi-unit cells lack gap junctions, so are not linked electrically. They must be stimulated independently, which allows fine control of contractions by selective activation of individual muscle cells. Multi-unit cells can be found in the eye [12]. Smooth muscle cell contracts in different direction because there is no regular arrangement of its contractile proteins and this is important in the movement of the intestine.

In smooth muscle contraction  Ca2+ ions enter the muscle fibre and bind to calmodulin (the secondary messenger in this process [13].  This calcium - calmodulin complex removes the caldesmon from the actin sites where myosin will attach. An enzyme called a myosin light chain kinase (MLCK) is activated. MLCK is responsible for phosphorylating myosin filaments so that it can form cross-bridges with actin filaments. Relaxation occurs when the myosin is dephosphorylated by myosin phosphatase (MP) removing it from actin [14]. This process is relatively slow (maximum contraction is often nearly a second long) and uses very little ATP. This means that smooth muscle doesn’t fatigue during sustained periods of activity [15].


  1. Rodney R., (2002) Human Physiology, 6th Edition, Pacific Grove, California; London: Brooks/Cole
  2. Barrett K. E., Barman S. M., Botiano S., Brooks H. L. (2010) Ganong’s Review of Medical Physiology, 23rd edition, New York: McGraw Hill
  3. Koeppen B. M., Stanton B. A. (2008) Berne and Levy Physiology, 6th edition, Philadelphia: Mosby Elsevier
  4. The Human Body in Health and Disease 5th edition by Thibodeau, Patton (2010), page 267
  5. 3
  6. Silverthorn D. U., Johnson B. R., Ober W. C., Garrison C. W., Silverthorn A. C. (2010) Human Physiology, 5th edition, San Francisco: Pearson
  7. . [Becker W.M, Kleinsmith L.J, Hardin J, Bertoni G.P, 2009, The World of the Cell, 7th edition, Pearson]
  8. Silverthorn.D. U (2009) Human Physiology: An Integrated Approach, 5th Edition, Cambridge, UK: Pearson
  9. Fundamentals of Anatomy and Physiology 5th edition by F H Martini (Chapter 10, Smooth muscle tissue)
  10. Tortora G. and Derrickson B., Principles of Anatomy and Physiology (13th Edition, International Student Edition), 2011, pg 356
  11. Tortora G.and Derrickson B., Principles Of Anatomy and Physiology (13th Edition, International Student Edition), 2011, pg 354
  12. Silverthorn D., Johnson B., Ober W., Garrison C., Silverthorn A. (2010) Human Physiology: An Integrated Approach, 5th edition, San Francisco: Pearson Education
  13. Walsh.MP, ( 2008): PubMed : Calmodulin and the regulation of smooth muscle contraction. Avaliable at: Accessed: 29/11/2011
  14. Bruce. A, Johnson. A., Lewis. J., Raff. M., Roberts. K., Walter. P., (2008): Molecular Biology of the Cell (5th edition) pg 1029 & 1028
  15. Biology Online(2005): Muscle Available at: Accessed: 26/11/2011
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