Smooth muscle tissue is a fundamental type of muscle found throughout the human body, performing various essential functions without conscious effort. It is distinct from skeletal muscle, which controls voluntary movements, and cardiac muscle, found only in the heart.
Defining Features
Smooth muscle cells are characterized by their spindle shape, wider in the middle and tapering at the ends. Unlike skeletal and cardiac muscle, smooth muscle lacks the striped appearance of striations when viewed under a microscope. Each smooth muscle cell typically contains a single, centrally located nucleus. This non-striated structure is due to the arrangement of its contractile proteins, actin and myosin, which are not organized into the repeating units called sarcomeres.
A key characteristic of smooth muscle is its involuntary control, meaning its actions are not subject to conscious thought. The nervous system, hormones, and local factors can all trigger smooth muscle activity. Smooth muscle cells can also maintain a contractile tone for extended periods and often contract rhythmically.
Locations in the Body
Smooth muscle tissue is present in almost every organ system within the body. It forms the walls of hollow internal organs, such as the stomach, intestines, bladder, and uterus. This tissue is also found lining the passageways of the cardiovascular system, including arteries and veins, where it regulates blood flow. Additionally, smooth muscle is located in the respiratory tract, specifically in the walls of airways like the bronchi, influencing air passage.
It is present in the urinary system, regulating urine flow. In the eyes, smooth muscle is responsible for changing the size of the pupil and the shape of the lens, affecting vision. The skin also contains smooth muscle, such as the arrector pili muscles which cause hair to stand on end.
Vital Roles
In the digestive tract, it facilitates the movement of food through a wave-like action called peristalsis. This coordinated contraction and relaxation helps transport substances through tubular structures, ensuring digestion and nutrient absorption. Smooth muscle in the walls of the stomach and intestines also aids in mechanical digestion.
In the cardiovascular system, smooth muscle regulates blood flow and pressure by controlling the diameter of blood vessels. Contraction of vascular smooth muscle narrows the vessels (vasoconstriction), increasing blood pressure, while relaxation widens them (vasodilation), decreasing pressure. This dynamic control is essential for distributing blood effectively throughout the body and maintaining stable blood pressure. Smooth muscle also manages the passage of substances in the urinary and reproductive systems, such as urine flow and uterine contractions during childbirth. It regulates the diameter of airways in the respiratory system, influencing air passage into and out of the lungs.
Contraction and Control
Smooth muscle contraction begins with an increase in calcium ion concentration within the cell. Unlike skeletal muscle, smooth muscle does not contain troponin; instead, calcium binds to a protein called calmodulin. This calcium-calmodulin complex then activates an enzyme called myosin light chain kinase (MLCK). MLCK adds a phosphate group to the myosin light chain, which enables myosin to interact with actin filaments and initiate contraction.
The actin and myosin filaments then slide past one another, generating force and causing the smooth muscle cell to shorten. This process does not involve sarcomeres, but rather relies on a network of actin filaments anchored to dense bodies within the cell. Smooth muscle contraction is primarily regulated by the autonomic nervous system, including sympathetic and parasympathetic divisions. Neurotransmitters, hormones, and local factors influence calcium levels within the muscle cells, controlling their contractile state.