Smooth muscle is a type of tissue found extensively throughout the human body, distinct from the skeletal muscles that control voluntary movement and the cardiac muscle of the heart. This specialized tissue forms the walls of many internal organs and structures. It operates involuntarily, performing fundamental actions essential for maintaining bodily functions and regulating numerous internal processes.
Defining Smooth Muscle
Smooth muscle distinguishes itself from other muscle types. Unlike skeletal muscle, which appears striped under a microscope, smooth muscle lacks this striated pattern, appearing homogeneous. Its cells are spindle-shaped, tapering at both ends, and each typically contains a single nucleus. This muscle type operates involuntarily.
The actions of smooth muscle are generally slow, sustained, and often rhythmic, differing from the rapid, forceful contractions of skeletal muscles. This characteristic allows smooth muscle to maintain tension for extended periods with minimal energy expenditure. It forms sheets of tissue within organs, facilitating coordinated functions such as moving substances through internal passageways.
Vital Functions Across Body Systems
Smooth muscle plays diverse and important roles across nearly every organ system. In the digestive system, it facilitates peristalsis, a wave-like contraction that propels food through the esophagus, stomach, and intestines. It also controls sphincters, ring-like muscles that regulate the passage of substances, such as the movement of digested food from the stomach to the small intestine.
Within the vascular system, smooth muscle in the walls of arteries and veins adjusts blood vessel diameter. This action, known as vasoconstriction (narrowing) and vasodilation (widening), precisely controls blood flow to different organs and helps regulate overall blood pressure. In the respiratory system, smooth muscle within the walls of bronchioles adjusts the diameter of airways, influencing airflow to and from the lungs. This regulation is important for breathing.
The urinary system relies on smooth muscle for the transport and storage of urine. Smooth muscle contractions in the ureters move urine from the kidneys to the bladder, and the bladder wall itself contains smooth muscle that contracts to expel urine during urination. In the reproductive system, smooth muscle contractions are important for processes such as uterine contractions during childbirth, which help push the baby through the birth canal. In males, smooth muscle aids in the movement of sperm through the reproductive tract.
Even the integumentary system, which includes the skin, utilizes smooth muscle. Tiny arrector pili muscles attached to hair follicles contract in response to cold or fear, causing hairs to stand erect and creating “goosebumps.” Smooth muscle is also found in the eye, where it adjusts the size of the pupil to control the amount of light entering and changes the shape of the lens to allow for focusing on objects at different distances.
The Mechanics of Contraction
Smooth muscle contraction involves a distinct mechanism compared to skeletal muscle. Unlike skeletal muscle, smooth muscle cells do not have sarcomeres, which are the highly organized contractile units that give striated muscles their appearance. Instead, the contractile proteins, actin and myosin, are arranged in a more diffuse network within the cell.
The process of smooth muscle contraction is initiated by an increase in intracellular calcium ion concentration. These calcium ions enter the cell from the outside or are released from internal stores within the cell. Once inside, calcium binds to a protein called calmodulin, forming a complex. This calcium-calmodulin complex then activates an enzyme known as myosin light chain kinase (MLCK).
MLCK phosphorylates (adds a phosphate group to) the myosin heads, which enables them to interact with the actin filaments. This interaction leads to the sliding of actin and myosin filaments past each other, generating force and causing the smooth muscle cell to shorten. The lack of highly structured sarcomeres allows smooth muscle to contract over a wider range of lengths and maintain sustained contractions with less energy, which is important for its roles in hollow organs.
How Smooth Muscle is Controlled
The regulation of smooth muscle activity is involuntary, orchestrated by a combination of neural, hormonal, and local factors. The autonomic nervous system plays a significant role, with its sympathetic and parasympathetic divisions often having opposing effects. For example, sympathetic stimulation might cause smooth muscle in blood vessels to constrict, while parasympathetic stimulation might promote contraction in the digestive tract.
Hormones circulating in the bloodstream can also exert powerful control over smooth muscle. For instance, oxytocin stimulates the smooth muscle of the uterus during labor, triggering contractions. Adrenaline, released during stress, can influence smooth muscle in various ways, such as dilating blood vessels in skeletal muscles while constricting those in the digestive system.
Local conditions within tissues can directly influence smooth muscle activity, providing fine-tuned control. Changes in oxygen levels, carbon dioxide levels, pH, or the presence of certain metabolic byproducts can cause local smooth muscle relaxation or contraction. For example, a decrease in oxygen in a tissue can lead to the relaxation of smooth muscle in local blood vessels, increasing blood flow to that area. Mechanical stretch of smooth muscle, particularly in hollow organs like the bladder, can also trigger its contraction.