Smooth muscle is one of the three distinct types of muscle tissue found in the human body, alongside skeletal and cardiac muscle. Its primary function involves the generation of forces that govern the movement of substances and the adjustment of tension within internal organs. This type of muscle is unique because its activity is almost entirely outside of conscious control, meaning its contractions happen automatically to manage various bodily functions. It provides the force required for internal systems to operate efficiently, maintaining a stable internal environment through autonomic processes.
Defining Characteristics and Location
Smooth muscle cells, also called myocytes, possess a distinct morphology that sets them apart from other muscle types. Each cell is spindle-shaped, featuring a wide middle section that tapers toward the ends, and contains a single, centrally located nucleus. Unlike skeletal and cardiac muscle, smooth muscle lacks the organized, repeating units called sarcomeres. This absence means it does not appear striated or striped under a microscope, which is why it is classified as “smooth.”
The contractile proteins, actin and myosin, are present but are arranged in a diagonal or crisscross pattern anchored to structures called dense bodies within the cell membrane. This non-linear arrangement allows the cell to contract in a corkscrew-like motion, leading to a large change in length. Smooth muscle tissue is widely distributed, forming the walls of almost all hollow internal organs, collectively known as viscera.
Smooth muscle lines the entire gastrointestinal, respiratory, urinary, and reproductive tracts. It is also a major component of the vascular system, forming the middle layer of arteries and veins, excluding capillaries. Specialized locations include the erector pili muscles attached to hair follicles, which cause goosebumps, and the intrinsic muscles of the eye, such as the iris and ciliary body. This widespread distribution highlights its role in regulating internal passage sizes and moving contents through tubular structures.
The Mechanism of Involuntary Contraction
The action of smooth muscle is controlled involuntarily, primarily by the Autonomic Nervous System (ANS), circulating hormones, and local chemical factors. Specialized nerve endings from the ANS release neurotransmitters that influence the muscle, either stimulating contraction or promoting relaxation. This control system ensures that internal processes, such as digestion and blood pressure regulation, are managed automatically.
Contraction is initiated by an increase in calcium ions within the muscle cell. Unlike skeletal muscle, where calcium binds to troponin, smooth muscle utilizes a different protein called calmodulin. The calcium-calmodulin complex then activates Myosin Light Chain Kinase (MLCK). MLCK phosphorylates the myosin heads, which permits the myosin to interact with actin and begin the cross-bridge cycling that generates force.
A defining characteristic is the ability to maintain a sustained contraction, known as muscle tone, with very little energy expenditure. This tone is important for organs like blood vessels, where a constant, partial contraction is necessary to maintain normal blood pressure. Smooth muscle also exhibits plasticity, or the stress-relaxation response. This allows organs such as the urinary bladder or stomach to stretch significantly as they fill without immediately triggering a forceful, expulsive contraction. The muscle gradually adjusts its tension to the new length, maintaining stable internal pressure and preventing premature emptying of the organ.
Key Functional Roles in Body Systems
The contractile activity of smooth muscle is directly responsible for regulating the internal environment and transporting substances throughout the body. In the vascular system, smooth muscle cells in the walls of arteries and arterioles control vascular resistance. By contracting (vasoconstriction) or relaxing (vasodilation), they adjust the diameter of the blood vessels, regulating systemic blood pressure and directing blood flow based on metabolic demand.
In the gastrointestinal tract, the coordinated contraction and relaxation of smooth muscle layers produce waves of movement known as peristalsis. This rhythmic action propels food and waste material from the esophagus down through the intestines and is also responsible for mixing contents within the stomach.
Smooth muscle in the respiratory system allows for the adjustment of airflow into the lungs. Contracting the muscle surrounding the bronchioles narrows the air passages (bronchoconstriction), while relaxing widens them (bronchodilation). This function is a defense mechanism against irritants and is a target for medications used in respiratory conditions like asthma.
Within the urinary and reproductive systems, smooth muscle generates powerful, expulsive forces. In the urinary tract, it facilitates urine flow and its contraction in the bladder wall initiates urination; in the uterus, it generates the intense contractions required to expel the fetus during childbirth. Specialized smooth muscles also control the iris in the eye to regulate pupil size and the arrector pili muscles in the skin to create goosebumps.