Categories: Anatomy

A Quick Overview of Smooth Muscle

Smooth muscle

Smooth muscle is an involuntary non-striated muscle that is found in the hollow organs of the body. One of the three major types of muscles. This muscle is not under the control of the conscious mind

Characteristics of Smooth muscle

  • This is one of three types of chief muscle.
  • Involuntary type of muscle.
  • This is a non-striated muscle.
  • This muscle is divided into two smaller groups;
    • the single-unit (unitary) and
    • Multiunit smooth muscle.

Within single-unit cells, the entire bundle contracts as a syncytium.

  • The SM is a kind of muscle that is used through various systems to put on pressure to vessels and organs.
  • SM  is made up of sheets of cells of SM.
  • These cells are made up of fibers of myosin and actin which run over the cell and these are sustained by a framework of the other proteins.
  • SM cells contract under the influence of certain stimuli as ATP is unbound for use by the myosin. The amount of total ATP released hinge on the total intensity of the stimuli, permitting SM to have a classified contraction as contrasting to the “on-or-off” contraction of skeletal muscle.

Smooth muscle found in

  • Walls of blood vessels
  • Ureters
  • Walls of stomach
  • Ureters
  • Intestines
  • Iris of the eye
  • In the aorta (tunica media layer)
  • Small arteries
  • Prostate
  • Respiratory Tract
  • Gastrointestinal Tract
  • Arterioles
  • Reproductive tracts (both genders)
  • Glomeruli of the Kidneys (called mesangial cells)
  • Veins
  • Bladder
  • Erector pili of the skin
  • Uterus
  • Sphincter
  • Trachea
  • Ciliary muscle
  • Bile duct

The functions of SM rest on the parts where they are set up in the body. The SM functions are exceptionally vital to the efficacy of each organ with the most muscles are involved.

Smooth Muscle Structure

  • SM tissue, not like skeletal or cardiac tissues, does not contain distinct striations noticeable on the cells.
  • The smooth muscle cells are ordered in an altered way than other muscle cells.
  • As understood in the image underneath, the filaments of actin and myosin in the smooth muscle are organized in a fixed pattern through the cell.
  • This “staircase” organization of actin and myosin is far changed than the structure in cardiac and skeletal and muscle. The actin filaments (red lines) in the smooth muscle course from one side of the cell to the other, linking at denser bodies and on the cell membrane.
  • In cardiac and skeletal muscle, the actin filaments are attached to Z plates, which grip many actin filaments together and display up as dark bands below the microscope. In the smooth muscle, the actin and myosin fibers have settled angles to each other as they run over the cell.

Functions of Smooth Muscles

  • The SM present in uterus aids a woman to push her baby out.
  • SM in the veins and arteries are mainly responsible for the regulation of blood pressure.
  • In the human eye, the pupillary sphincter muscles are in authority for shrinking the size of the actual pupil.
  • In the bladder, the SM aids to push out the urine.
  • The SM controls airflow in the lungs.
  • SM aid sperm to transfer along the reproductive tract.
  • The SM regulates the blood flow in the arteries of the body.
  • These muscles move food from end to end the digestive tract.
  • In the human eye, the pupillary sphincter muscles are in authority for shrinking the size of the actual pupil.
  • SM  move food from end to end the digestive tract.
  • In the arteries, smooth muscle movements sustain the arteries’ diameter.
  • Skin remains on the end when piloerector muscles contract.
  • SM in the lungs aids the airways to inflate and contract when necessary.
  • The tunica dartos is an important smooth muscle that contracts or enlarges to regulate the temperature of the testicles.
  • To stop urination for the duration of ejaculation by men, the muscle constricts.
Multi-unit Smooth Muscle: Cells are less organized. Function as separate units. Fibers function independently. Iris of eye, walls of blood vessels. Stimulated by neurons, hormones. Visceral Smooth Muscle: Single-unit smooth muscle; cells respond as a unit. Sheets of spindle-shaped muscle fibers. Fibers held together by gap junctions. Exhibit rhythmicity. Conduct peristalsis. Walls of most hollow organs. More common type of smooth muscle.

Invertebrate smooth muscle

In the invertebrate SM,

  • The contraction is introduced with the binding of calcium directly to myosin and then quickly cycling cross-bridges, generating force.
  • Similar to this mechanism of vertebrate of smooth muscle, there is a low calcium and low energy utilization catch phase.
  • This sustained phase or catchphrase has been attributed to a catch protein that has similarities to myosin light-chain kinase and the elastic protein-titin called twitching.
  • Clams and other bivalve mollusks use this catch phase of smooth muscle to keep their shell closed for prolonged periods with little energy usage.


A Quick Overview Of Cardiac Muscle

Growth and rearrangement

  • The mechanism in which external factors stimulate growth and rearrangement is not yet fully understood.
  • Several growth factors and neurohumoral agents influence SM growth and differentiation.
  • The Notch receptor and cell-signaling pathway have been demonstrated to be essential to vasculogenesis and the formation of arteries and veins.
  • The proliferation is implicated in the pathogenesis of atherosclerosis and is inhibited by nitric oxide.
  • The embryological origin of SM is usually of mesodermal origin, after the creation of muscle fibers in a process known as myogenesis.
  • However, the SM within the Aorta and Pulmonary arteries (the Great Arteries of the heart) is derived from ectomesenchyme of neural crest origin, although coronary artery SM is of mesodermal origin.

Related diseases of smooth muscle

  • “Smooth muscle condition” is a condition in which the body of a developing embryo does not create enough SM for the gastrointestinal system. This condition is fatal.

Anti-smooth muscle antibodies

  • Hence,  the anti-smooth muscle antibodies (ASMA) can be a symptom of an auto-immune disorder, such as hepatitis, cirrhosis, or lupus.


  • However, the tumors of SM are most commonly benign and are then called leiomyomas. So, they can occur in any organ, but the most common forms occur in the
  • Uterus
  • small bowel

The malignant SM tumors are called leiomyosarcomas.

Leiomyosarcomas are one of the more common types of soft-tissue sarcomas.

Vascular SM tumors are very rare.

They can be malignant or benign, and morbidity can be significant with either type.

Intravascular leiomyomatosis is a benign neoplasm that extends through the veins, angioleiomyoma is a benign neoplasm of the extremities, vascular leiomyosarcomas is a malignant neoplasm that can be found in the inferior vena cava, pulmonary arteries and veins, and other peripheral vessels.

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