Smooth muscle is one of the three primary types of muscle tissue found in the vertebrate body, alongside skeletal and cardiac muscle. This tissue is responsible for a wide array of involuntary movements essential for sustaining life. Unlike muscles that control conscious movement, smooth muscle operates without any deliberate thought or effort. Its actions are fundamental to regulating the internal environment, maintaining balance and function across multiple organ systems.
Defining Characteristics of Smooth Muscle
Smooth muscle is defined by its lack of conscious control, making it an involuntary muscle. Its function is managed by the autonomic nervous system, which regulates bodily functions such as heart rate, digestion, and respiratory rate. This allows the nervous system to precisely adjust many subsystems without requiring individual input.
The tissue is also classified as “visceral muscle” because it forms the walls of most hollow internal organs, known as the viscera. It often contracts in a coordinated fashion as a single unit.
Where Smooth Muscle Resides
Smooth muscle is distributed extensively throughout the body, primarily forming the muscular layer within the walls of hollow organs. It lines the entire gastrointestinal tract, from the esophagus to the intestines, facilitating the movement of contents. It is also a structural component of the urinary system, present in the walls of the ureters and the bladder.
In the cardiovascular system, smooth muscle forms the tunica media layer of arteries and veins. It is also found in the respiratory system, specifically in the walls of the bronchi and bronchioles, controlling airway diameter, and in the reproductive tracts (such as the uterus) and the eye (controlling pupil size and lens shape).
Unique Structure and Contraction
Smooth muscle cells, or myocytes, are distinctly different from their striated counterparts, appearing spindle-shaped with tapering ends and containing a single, centrally located nucleus. They are called “smooth” because they lack the visible striations or bands that characterize skeletal and cardiac muscle. This non-striated appearance is due to the contractile proteins, actin and myosin, not being organized into orderly repeating units called sarcomeres.
Instead of Z-discs, the thin actin filaments are anchored to structures called dense bodies, which are scattered throughout the cytoplasm and attached to the cell membrane. When contraction is initiated, calcium ions enter the cell and bind to the protein calmodulin. This complex activates an enzyme that phosphorylates the myosin heads, allowing them to interact with actin and pull on the dense bodies.
This arrangement causes the cell to shorten and bulge in a spiral fashion as the filaments slide past each other. The contraction process is slower than in striated muscle but is efficient at maintaining tension for long periods. This is achieved through the “latch state,” where the myosin heads remain attached to the actin filaments with minimal energy expenditure, allowing for sustained muscle tone.
Essential Functions in the Body
The primary role of smooth muscle is to regulate the movement of substances through internal passageways and to control the size of hollow organs. In the digestive tract, coordinated waves of contraction, known as peristalsis, propel food and waste through the intestines, which is fundamental for digestion and nutrient absorption.
Smooth muscle in blood vessel walls regulates blood flow and pressure. By contracting (vasoconstriction) or relaxing (vasodilation), these muscles adjust vessel diameter, controlling resistance and directing blood to areas with the greatest need. In the respiratory system, smooth muscle in the bronchioles controls airway size, regulating the amount of air entering and leaving the lungs.
Smooth muscle is also responsible for expulsion processes, such as the powerful contractions of the uterus during childbirth. Similarly, the smooth muscle in the bladder wall contracts to push urine out of the body.