Cheryl has taught veterinary and medical student for over 20 years and has a DVM and PhD degree in reproductive biology.
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.
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
- 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
Acetylcholine from the parasympathetic nervous system will induce peristalsis (rhythmic muscular contractions) in the gastrointestinal system and stimulate urination.
Norepinephrine or epinephrine from the sympathetic nervous system inhibits peristalsis and urination
In some blood vessels, gases, such as carbon dioxide or oxygen, cause vasodilation or vasoconstriction by regulating smooth muscle of the tunica media layer. For instance, carbon dioxide causes arterial dilation, whereas, oxygen causes vasoconstriction. One exception is high amounts of oxygen induce vasodilation of pulmonary blood vessels.
Smooth muscle is a non-striated muscle that regulates involuntary responses. While smooth muscle has myofibers (cells) and myofilaments, it does not contain myofibrils, which is the primary distinguishing feature from cardiac and skeletal muscle. In smooth muscle, actin filaments are anchored to dense bodies. Smooth muscle myofibers are uni-nucleated with nuclei in the center of the cells and have large regenerative capacity.
Smooth muscle is classified as single unit or multi-unit. Single unit smooth muscle is the major type and contracts or relaxes together. To allow for such uniform contraction, single-unit smooth muscle is enriched with gap junctions that allow for rapid and easy transmission of calcium ions and the action potential. Single unit smooth muscle lines the gastrointestinal system, the urinary bladder, and forms the tunica media layer of most blood vessels. In multi-unit smooth muscle, individual myofibers can contract or relax independently of their neighbors, and such myofibers have fewer gap junctions to transmit calcium and the action potential to surrounding cells. This type of smooth muscle is found in the trachea, muscles regulating the iris, and tunica media of the aorta. Contraction or relaxation of smooth muscle is regulated by acetylcholine (parasympathetic nervous system), norepinephrine/epinephrine (sympathetic nervous system), and gases (carbon dioxide and oxygen) for muscles of the tunica media layer in blood vessels.
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