Structure of the Smooth Myocyte
Smooth muscle cells typically have a spindle shape, and less frequently, a stellate shape. A single rod-shaped nucleus is located in the center of the myocyte. The cytoplasm is completely devoid of sarcomeres and the transverse striations characteristic of skeletal muscle.
Externally, each myocyte is covered by a basal lamina (interrupted only at cell-to-cell contact sites) and a narrow layer of loose connective tissue known as the endomysium. Cells are rarely found in isolation; they typically aggregate into sheets or bundles.
In addition to contraction, smooth myocytes function as extracellular matrix "factories." Due to a well-developed rough endoplasmic reticulum, they synthesize collagen, elastin, and proteoglycans. Consequently, a single tissue may contain two functional cell types: predominantly contractile and predominantly synthetic (which are especially important for blood vessel walls).
Cytoskeletal and Contractile Apparatus
At rest, the cell lacks pre-formed myofibrils.
The structural framework is formed by dense bodies (analogous to the Z-lines of striated muscle), which contain $\alpha$-actinin. These include dense plaques lying directly beneath the plasmalemma (forming a ribbed network) and cytoplasmic dense bodies. Between the plaques, the membrane forms invaginations called caveolae, which are involved in ion transport.
- Thin (actin) myofilaments: Constantly present in the cytoplasm and anchored to dense bodies. Notably, they lack the regulatory proteins troponin and tropomyosin.
- Thick (myosin) myofilaments: Disassembled into individual molecules at rest, assembling into filaments only upon receiving a contraction signal.
Unique Contraction Mechanism
Smooth muscle lacks T-tubules, and its specialized sarcoplasmic reticulum is poorly developed. The contraction process differs fundamentally from skeletal muscle:
- $Ca^{2+}$ ions enter the cell primarily from the extracellular environment via caveolae or channels and bind to the protein calmodulin.
- This complex activates the enzyme myosin light-chain kinase (MLCK).
- The kinase phosphorylates myosin molecules, inducing them to assemble into thick filaments.
- Temporary myofibrils form: thick filaments interdigitate with thin filaments, and sliding occurs at the expense of ATP. Dense bodies are pulled closer together, shortening the cell.
Smooth myocytes can sustain tension for long periods without fatigue due to the "latch state" (where a fraction of myosin cross-bridges remain attached even after dephosphorylation). Relaxation occurs slowly when calcium pumps remove $Ca^{2+}$ and the enzyme myosin phosphatase breaks the bonds, disassembling the thick filaments.
Types of Innervation
The tissue is regulated by the autonomic nervous system and is thus not subject to voluntary control. Depending on the location, there are two types of impulse transmission:
- Multiunit (individual innervation): An effector nerve ending supplies each individual myocyte. This is typical for blood vessels.
- Visceral (unitary/collective innervation): Typical for hollow organs (intestine, uterus). Cells form "myocytic complexes" of 10–12 cells. The nerve contacts only one cell, and the signal is transmitted to neighboring myocytes via nexus junctions (gap junctions).
Regeneration and Growth
Under normal conditions, cells do not divide; only intracellular regeneration (subcellular renewal) occurs. However, under functional demand (such as uterine growth during pregnancy) or pathology (such as benign prostatic hyperplasia), the tissue can significantly increase in mass.
Growth is driven by two mechanisms: hypertrophy (increase in cell size) and hyperplasia (increase in cell number). The source of new myocytes remains debated: it may involve poorly differentiated stem/progenitor cells, or mature myocytes that temporarily regain the capacity for cell division under specific stimuli (phenotypic modulation).