Smooth, unstriated muscle forms the muscular component in the walls of hollow organs such as the gastrointestinal tract, the trachea, bronchi and bronchioles of the respiratory system, blood vessels in the cardiovascular system and the urogenital system. Smooth muscle contracts and relaxes much more slowly than skeletal muscle, but is much better adapted to sustained contractions. The load against which smooth muscle works is typically the pressure within the tubular structures that they line. In organs such as the blood vessels, they are responsible for a steady intraluminal pressure brought about by their tonic contraction. In the gastrointestinal tract, they produce a phasic contraction that propels its contents onward. They also occur in the iris, ciliary body and nictitating membrane in the eye, and are the small muscles which erect the hairs. The functions of smooth muscle in the body are thus diverse. This is reflected in their wide variations in structure and detailed physiological properties, for which this chapter only provides a brief introduction.

Structure

Smooth muscle cells (Figure 13.1) are uni-nucleate, elongated, often spindle-shaped and much smaller than the multi-nucleate skeletal muscle fibres. They are typically 3–5 μm in diameter and up to 400 µm long. Their thick myosin and thin actin filaments are arranged longitudinally in the cytoplasm, but are not aligned transversely. The cells consequently show no visible striations or sarcomeres. The actin filaments are attached in bundles at dense bodies in the cytoplasm, and to attachment plaques at the membrane.