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Smooth Muscle: Structure & Functions

Instructor: Cheryl Rosenfeld

Cheryl has taught veterinary and medical student for over 20 years and has a DVM and PhD degree in reproductive biology.

Smooth muscle is a non-striated muscle that lines and controls many tubular organs that are under involuntary control. This lesson examines the structure and function of smooth muscle, including single-unit and multi-unit contraction. Updated: 12/27/2020

Smooth Muscle

Smooth muscle is non-striated muscle; while it contains myofilaments, such as actin and myosin, these myofilaments are not arranged uniformly like they are in skeletal and cardiac muscle. Individual smooth muscle cells are called myocytes or myofibers. Since smooth muscle is not striated, it lacks myofibrils, sarcomere to sarcomere units. Instead, actin filaments in smooth muscle are anchored to dense bodies rather than sarcomeres. Thus, the dense body is similar to the Z lines of skeletal and cardiac muscle and attaches to the sarcolemma. Dense bodies may attach to intermediate filaments, such as desmin and vimentin. As such, dense bodies may regulate cellular tension due to contraction in the cytoskeleton.

Smooth muscle forms the lamina muscularis in some tubular organs, such as the stomach or small intestine. It can also form the tunica muscularis or outer layer of muscle in tubular organs that are under involuntary control, such as the stomach and small intestine. Smooth muscle myofibers contain a single nucleus that, in a longitudinal section, appears as cigar or fusiform shaped. In cross-section, smooth muscle takes on a window-pane glass appearance. Smooth muscle cross-sectional bundles can be distinguished from skeletal muscle based on nuclear location, in the center in smooth muscle, but at the periphery in skeletal muscle. Smooth muscle cross-sectional bundles are smaller than skeletal muscle cross-sectional bundles. Smooth muscle myofibers can easily divide, and thus, possess greater regenerative capacity compared to skeletal and cardiac muscle.

This diagram shows the layers of the stomach with the tunica muscularis and lamina muscularis of the tunica mucosa comprised of smooth muscle. In between the smooth muscle layers of the tunica muscularis resides the myenteric plexus, which is part of the enteric nervous system.
layers of stomach

This diagram shows a histological section of smooth muscle in a longitudinal arrangement where the nuclei appear cigar-shaped. Actin filaments will be anchored to dense bodies within each myofiber.
histology

Single Unit vs. Multi-Unit Smooth Muscle

Most smooth muscle is single unit, meaning that the entire myofibers contract or relax together. This type of system is useful in generating the force to propel luminal contents, such as red blood cells in the case of blood vessels, or digesta in the gastrointestinal system. Some examples of where single-unit smooth muscle is found include:

  • Smooth muscle forming the tunica media layer of most blood vessels
  • Lamina muscularis and tunica muscularis layers of the stomach, small intestine, and large intestine
  • Tunica muscularis (detrusor muscle) of the urinary bladder

Multi-unit smooth muscle is where individual myocytes can contract or relax independently of neighboring cells. Such a system is useful in cases where fine motor control is needed. Examples, where multi-unit smooth muscle can be found, are:

  • Tunica media of elastic arteries, such as the aorta
  • Trachea
  • Muscles controlling the iris of the eye
  • Muscles controlling arrector pili muscle to the hair follicles

Cat owners are well aware of how multi-unit smooth muscle to the iris and hair follicles can lead to such dramatic and fine motor responses. For instance, when my cat is frightened by the dog, the pupils of his eyes dilate (due to sympathetic stimulation). At the same time, sympathetic stimulation to the arrector pili muscles causes his fur or hackles to raise up so that he appears bigger. However, once the 'threat' has passed, his pupils return to normal or constricted state (due to parasympathetic stimulation), and his fur lowers back down.

The contraction of smooth muscle fibers is caused by the sliding of myosin and actin filaments, which is regulated by ATP. Adherens junctions link myofibers together. Thus, the contraction of one myofiber spreads to surrounding fibers. To ensure uniform contraction in the case of single-unit smooth muscle, gap junctions also connect myofibers and allow for the transmission of calcium and action potential across adjacent cells. Multi-unit smooth muscle fibers have considerably fewer gap junctions, and thus, contraction of one myofiber may not radiate to surrounding cells.

Regulation of Smooth Muscle Contraction

Smooth muscle fibers are stimulated to contract or relax by:

  • Parasympathetic nervous system
  • Sympathetic nervous system
  • Gases

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