Smooth muscle is a distinct type of tissue that operates constantly throughout the body without conscious direction. This involuntary nature means it is controlled automatically by the autonomic nervous system, managing many essential functions. Structurally, these cells are non-striated, lacking the striped appearance of skeletal muscle, and each spindle-shaped cell contains a single nucleus. Unlike voluntary skeletal muscle or specialized cardiac muscle, smooth muscle is responsible for slow, sustained contractions that govern the movement of substances through hollow organs.
Smooth Muscle in Visceral Organs
Smooth muscle tissue forms the muscular walls of the entire gastrointestinal (GI) tract, extending from the esophagus through the stomach and the small and large intestines. In these organs, the muscle is typically arranged in two distinct layers: an inner circular layer and an outer longitudinal layer. The coordinated, rhythmic contraction of these two muscle sheets generates peristalsis, a wave-like motion that propels food and waste through the digestive tube.
Digestive System
In the stomach, this muscular layer is robust, allowing for the churning and mixing of contents before release into the small intestine. Movement in the small intestine combines mixing contractions, called segmentation, with peristaltic propulsion to maximize nutrient absorption.
Urinary System
Smooth muscle is also fundamental to the urinary system. It forms the walls of the ureters, which contract to push urine from the kidneys to the bladder. The bladder is lined with smooth muscle that allows it to store varying volumes of fluid, while muscular sphincters control the eventual release of urine.
Controlling Circulation and Blood Flow
The cardiovascular system relies heavily on smooth muscle, which comprises the middle layer (tunica media) of nearly all blood vessels, including arteries, arterioles, and veins. Contraction (vasoconstriction) narrows the vessel diameter, while relaxation (vasodilation) increases it. This mechanism is a primary determinant of systemic blood pressure, as resistance to blood flow increases during contraction and decreases during relaxation.
By precisely controlling the caliber of blood vessels, the body regulates blood flow distribution based on immediate metabolic need. For example, during strenuous exercise, smooth muscle in arterioles supplying resting organs constricts, diverting blood to active skeletal muscles. This control maintains vascular tone, ensuring stable blood pressure and efficient delivery of oxygen and nutrients.
Regulating Air and Fluid Passageways
Smooth muscle manages the diameter of air and fluid passageways in the respiratory and reproductive systems.
Respiratory System
Within the lungs, this muscle tissue encircles the bronchi and bronchioles. Contraction and relaxation of this airway smooth muscle directly control the resistance to airflow. This is apparent in conditions like asthma, where excessive constriction can severely narrow the passages.
Reproductive System
In the female reproductive tract, a thick layer of smooth muscle, known as the myometrium, makes up the bulk of the uterine wall. This tissue undergoes the powerful, coordinated contractions required for childbirth and also contracts during menstruation. Smaller amounts of smooth muscle are present in the fallopian tubes, where gentle, wave-like contractions facilitate the movement of the egg toward the uterus.
Smooth Muscle in Sensory and Integumentary Structures
Smooth muscle is found in specialized locations where it performs precise, localized functions, particularly within sensory organs.
Ocular Structures
In the eye, the iris contains two sets of smooth muscle: the sphincter pupillae and the dilator pupillae. These opposing muscles contract and relax to control the size of the pupil, regulating the amount of light that enters the eye. The ciliary muscle, another smooth muscle structure, is responsible for visual accommodation. By changing the tension on the lens, this muscle adjusts the lens’s shape, allowing the eye to focus clearly on objects at varying distances.
Integumentary Structures
In the skin, small bundles of smooth muscle called arrector pili muscles attach to the base of hair follicles. When these muscles contract, they pull the hair upright, creating the phenomenon known as “goosebumps.”