Muscle: Smooth

Original Editor - Lucinda hampton

Top Contributors - Lucinda hampton  

Introduction[edit | edit source]


Smooth muscle is found throughout the body where it serves a variety of functions. eg

  • It is in the stomach and intestines where it helps with digestion and nutrient collection.
  • It is found throughout the urinary system where it functions to help rid the body of toxins and works in electrolyte balance.
  • It is found throughout arteries and veins where it plays a vital role in the regulation of blood pressure and tissue oxygenation.

Smooth muscle is an integral part of the human body; its function is required for life and is present in almost every organ system.

Smooth muscle differs from skeletal muscle in a variety of ways, perhaps the most important being its ability to be contracted and controlled involuntarily.

The nervous system can use smooth muscle to tightly regulate many of the bodies subsystems for life with no thought from the user eg A person does not need to think about their blood pressure for it to adapt to increasing oxygen demands from exercise. The nervous system instead uses hormones, neurotransmitters, and other receptors to control smooth muscle spontaneously[1].

Image 1: Digestive Tract - The smooth muscle is responsible for movement of food by peristalsis and mechanical digestion by segmentation. In some regions, the circular layer of smooth muscle enlarges to form sphincters, circular muscles that control the opening and closing of the lumen eg between the stomach and small intestine).

Structure[edit | edit source]

Smooth muscle and skeletal muscle contraction.png

Smooth Muscle cell (myocyte)

  • The shape of smooth muscle is fusiform, which is round in the center and tapering at each end (3-10 µm thick and 20-200 µm long).
  • The cytoplasm consists mainly of myofilaments.
  • The nucleus is located in the center and takes a cigar-like shape during contraction.
  • The smooth muscle cells are anchored to the surrounding connective tissue by a basal lamina[2].
Smooth muscle contraction1.png

Smooth muscle contains thick and thin filaments that do not arrange into sarcomeres, resulting in a non-striated pattern (on microscopic examination, it appears homogenous). Smooth muscle cytoplasm contains large amounts of actin and myosin. Actin and myosin act as the main proteins involved in muscle contraction.

  • Actin filaments attach to dense bodies spread throughout the cell (dense bodies can be observed under an electron microscope and appear dark).
  • Another important structure is the calcium-containing sarcoplasmic reticulum, which aids in sustaining contraction.

Smooth muscle can tense and relax but has greater elastic properties than striated muscle. This quality is important in organ systems like the urinary bladder, where the preservation of contractile tone is a necessity.[2].

Smooth muscles are often grouped into two functional categories: single-unit and multiunit.

  1. Single-unit smooth muscles have numerous gap junctions (electrical synapses) between adjacent cells that weld them together electrically; they thus behave as a single unit, much like cardiac muscle. Most smooth muscles—including those in the digestive tract and uterus—are single-unit.
  2. Cells of multiunit smooth muscles contract and relax on their own individually,  and they do not act as a single unit. Examples are vascular smooth muscles, arrector pili muscle of skin, where the contraction does not get transmitted but remains confined to one particular muscle cell only which gets the direct stimulation[3].

Function[edit | edit source]


Smooth musculature is found in (almost) all organ system such as hollow organs. It fulfills various tasks such as sealing orifices (e.g. pylorus ie the passage at the lower end of the stomach that opens into the duodenum; uterine os ie mouth of the uterus) or the transport of the chyme through wavelike contractions of the intestinal tube (peristalsis).

The basic functions of smooth muscle in the organ systems are:

  • Gastrointestinal tract - propulsion of the food bolus
  • Cardiovascular - regulation of blood flow and pressure via vascular resistance
  • Renal - regulation of urine flow
  • Genital - contractions during pregnancy, propulsion of sperm
  • Respiratory tract - regulation of bronchiole diameter
  • Integument - raises hair with erector pili muscle
  • Sensory - dilation and constriction of the pupil as well as changing lens shape[1]

Smooth muscle cells

Image: An animated image to show Peristalsis

Pathophysiology[edit | edit source]

Asthma airways.png

The pathophysiology of smooth muscle is incredibly diverse. A few examples of how different organ systems may be affected by smooth muscle pathologies are shown below.

  1. Within the cardiovascular system, smooth muscle helps to regulate blood flow by controlling the diameter of the vessel. Atherosclerosis has recently been linked to smooth muscle development. Research has even shown that continuous vascular smooth muscle activation can lead to the formation of pulmonary hypertension. Within the lungs, pathologic activation of smooth muscle can lead to the development of asthma. Asthma occurs when smooth muscle constriction leads to obstruction of the airway. Recent studies have shown that the smooth muscle layer may increase in thickness before the onset of asthma even occurs, pointing towards a potential genetic link.
  2. The gastrointestinal tract is mostly dependent on smooth muscle for motility. Any damage to the smooth muscle of the intestines may have a devastating effect on the body. The term for this loss of motility is gastroparesis. Conditions that can
    Screen Shot 2021-04-28 at 4.00.59 pm.png
    impact gastric motility include nerve dysfunction, collagen disease, muscular dystrophies, amyloidosis, thyroid disease, diabetes mellitus, neuropathy
  3. In the renal system, vascular smooth muscle is present in the kidneys, throughout the ureters, and in the bladder. At the level of the kidneys, vascular smooth muscle dysfunction is associated with chronic kidney disease and can lead to end-stage renal disease. The bladder relies almost exclusively on the unique properties of smooth muscle, damage to smooth muscle regulation in the bladder can lead to loss of tone and subsequent neurogenic bladder disease, which hugely effects a person's quality of life
  4. Multisystemic Smooth Muscle Dysfunction Syndrome is a disease in which the activity of smooth muscle throughout the body is impaired This leads to widespread problems including blood vessel abnormalities, a decreased response of the pupils to light, a weak bladder, and weakened contractions of the muscles used for the digestion of food (hypoperistalsis). A certain mutation in the ACTA2 gene has been shown to cause this condition in some individuals[4]

Summary[edit | edit source]

Smooth Muscles form the walls of the organs, glands and blood vessels within the body. It is responsible for expanding and contracting passage of blood and other fluids through the vessels and organs. Smooth muscle is an involuntary muscle.

References[edit | edit source]

  1. 1.0 1.1 Hafen BB, Burns B. Anatomy, Smooth Muscle.2018 Available: 27.4.2021)
  2. 2.0 2.1 2.2 Ken Hub Smooth Muscle Available from: (accessed 27.4.2021)
  3. TOPPR Smooth muscle units Available from: (accessed 28.4.2021)
  4. Dove med MSMD Available from: (accessed 28.4.2021)