Smooth muscle gets its name from the absence of striations under a microscope, unlike skeletal and cardiac muscle tissues. The cells are spindle-shaped, featuring a single, centrally located nucleus. You will find this involuntary muscle lining the walls of hollow internal organs throughout the body, including the stomach, intestines, urinary bladder, and blood vessels. Its primary job is to regulate the flow of substances by contracting and relaxing the organ walls.
Inside a smooth muscle cell, the contractile proteins actin and myosin are present, but they are not arranged in orderly sarcomeres. Instead of Z-lines, smooth muscle cells have dense bodies scattered in the cytoplasm and attached to the cell membrane. Intermediate filaments anchor these dense bodies, forming a scaffolding network. When contraction occurs, the sliding of actin over myosin pulls on these dense bodies, causing the entire cell to twist and bunch up like a rolled-up towel.
Unlike skeletal muscle, smooth muscle lacks troponin. Calcium regulation relies entirely on a protein called calmodulin.
Smooth muscle contraction is not exclusively controlled by the somatic nervous system; instead, it responds to the autonomic nervous system, hormones, and local chemical changes. Autonomic nerve fibers release neurotransmitters like acetylcholine or norepinephrine into diffuse swellings called varicosities. Furthermore, circulating hormones like oxytocin or local factors such as oxygen levels, carbon dioxide, and pH can stimulate or inhibit contraction without any nerve input at all.
Smooth muscle exhibits remarkable plasticity, meaning it can maintain tension over a wide range of resting lengths without experiencing a dramatic change in resting tone. For example, the urinary bladder can stretch significantly as it fills with urine without generating high pressure spikes. Additionally, smooth muscle consumes very little ATP during contraction and can maintain a contracted state for long periods without fatigue, a feature known as the latch state.
Disfunctions in smooth muscle activity underlie several common medical conditions. In asthma, hyper-responsiveness of airway smooth muscle leads to bronchospasm and severe narrowing of the respiratory passages. In hypertension, abnormal constriction or hypertrophy of vascular smooth muscle increases systemic resistance. Conversely, loss of smooth muscle motility in the gut can lead to conditions like gastroparesis or intestinal pseudo-obstruction.