The uterus is primarily composed of smooth muscle. Uterine smooth muscle contraction provides the driving force for parturition. Smooth muscle contraction is also essential in the immediate post-partum period in securing uterine haemostasis following delivery; uterine atony is a common cause of post-partum haemorrhage.

The arteries contain layers of vascular smooth muscle within their tunica media. Contraction of vascular smooth muscle reduces the vessel radius, increasing its resistance to blood flow.

The respiratory tract, where bronchiolar smooth muscle contraction results in bronchoconstriction.

The gastrointestinal (GI) tract – coordinated contraction of longitudinal and circular smooth muscle (segmentation and peristalsis) in the intestinal wall mixes and propels the luminal contents along the gut.

Single-unit smooth muscle occurs in the viscera and the blood vessels, except the large elastic arteries, as sheets of smooth muscle cells forming syncytial units. The ANS innervates a single cell within the sheet, with action potentials rapidly propagated to neighbouring cells through gap junctions, leading to synchronous contraction.

Multi-unit smooth muscle is found in the large elastic arteries, the trachea and the iris. These smooth muscle cells are not connected by gap junctions. A single autonomic nerve branches to innervate many smooth muscle cells, in a similar way to the motor unit in skeletal muscle.

Size: skeletal muscle cells are large, cylindrical cells that span the entire length of the muscle. Smooth muscle cells are much smaller, spindle-shaped cells that are arranged in sheets, or syncytia.

Nuclei: skeletal muscle cells are multi-nucleate, whilst smooth muscle cells have only one nucleus.

Sarcomeres: like skeletal muscle, the primary function of smooth muscle is contraction. In both skeletal and smooth muscle, actin and myosin are the main contractile proteins: actin is arranged in thin filaments and myosin in thick filaments. However, in smooth muscle, the thick and thin filaments are not organised into sarcomeres – smooth muscle is therefore not striated.

Troponin complex: whilst tropomyosin is present in both smooth and skeletal muscle, troponin is absent in smooth muscle.

Transverse (T)-tubules: the tube-like invaginations of the skeletal muscle sarcolemma are absent in smooth muscle. Instead, smooth muscle has shallower, rudimentary invaginations known as caveolae, which increase the surface area-to-volume ratio of the muscle cell.

The sarcoplasmic reticulum (SR): an intracellular store of Ca²⁺. Despite the important role of the SR in skeletal muscle excitation–contraction coupling (see Chapter 54), the SR is poorly developed in smooth muscle.