Structure of the smooth myocyte
Smooth muscle cells are most often spindle-shaped, and less commonly stellate. A single rod-shaped nucleus is located in the center of the myocyte. The cytoplasm is completely devoid of sarcomeres and the transverse striations typical of skeletal muscles.
Externally, each myocyte is covered by a basal lamina (which is interrupted only at contact points with neighbors) and a thin layer of loose connective tissue — the endomysium. Cells are rarely found singly; they usually unite into sheets or bundles.
In addition to contraction, smooth myocytes function as extracellular matrix «factories». Due to their well-developed granular endoplasmic reticulum, they synthesize collagen, elastin, and proteoglycans. Consequently, two functional types of cells may coexist within the same tissue: predominantly contractile and predominantly synthetic (which are especially important for blood vessel walls).
Supporting and contractile apparatus
At rest, there are no preformed myofibrils in the cell.
The supporting framework is formed by dense bodies, which are the analogues of the Z-lines in striated muscles. They contain the $\alpha$-actinin protein. Dense plaques, located directly beneath the plasmalemma and forming a ribbed framework, as well as cytoplasmic dense bodies, are distinguished. In the intervals between the plaques, the membrane forms invaginations called caveolae, which participate in ion transport.
- Thin (actin) myofilaments: constantly present in the cytoplasm and attached to dense bodies. Their distinctive feature is that they lack the regulatory proteins troponin and tropomyosin.
- Thick (myosin) myofilaments: at rest, they are disassembled into individual molecules and assemble into filaments only upon receipt of a contraction signal.
Unique contraction mechanism
Smooth muscles lack T-tubules, and the specialized sarcoplasmic reticulum is poorly developed. The contraction mechanism differs fundamentally from skeletal muscle:
- $Ca^{2+}$ ions enter the cell predominantly from the extracellular environment via caveolae or channels and bind to the protein calmodulin.
- This complex activates an enzyme — myosin light-chain kinase.
- The kinase phosphorylates myosin molecules, causing them to assemble into thick filaments.
- Temporary myofibrils are formed: thick filaments interdigitate among thin ones, and sliding occurs with ATP consumption. Dense bodies move closer together, and the cell shortens.
Smooth myocytes are able to maintain tension for a very long time without fatigue due to the «latch» bridge effect (some myosin cross-bridges persist even after dephosphorylation). Relaxation occurs slowly when calcium pumps remove $Ca^{2+}$, and the enzyme myosin phosphatase breaks down the bonds, disassembling the thick filaments.
Types of innervation
The activity of the tissue is controlled by the autonomic nervous system, so contractions are not subject to voluntary control. Depending on the localization, two types of impulse transmission are distinguished:
- Vascular (individual innervation): an effector nerve ending approaches each individual myocyte. This is typical for blood vessels.
- Visceral (collective innervation): typical for hollow organs (intestine, uterus). Cells form «myocyte complexes» of 10–12 units. The nerve contacts only a single cell, and the signal is then transmitted to neighboring myocytes via nexuses (gap junctions).
Regeneration and growth
Under normal conditions, cells do not divide — only intracellular regeneration occurs (renewal at the subcellular level). However, under functional load (growth of the uterus during pregnancy) or pathology (prostate adenoma), the tissue can significantly increase in mass.
Growth is ensured by two mechanisms: hypertrophy (increase in the size of the cells themselves) and hyperplasia (increase in the number of cells). The source of new myocytes remains a matter of debate: it may be poorly differentiated cambial precursor cells, or mature myocytes that transiently acquire the ability to divide under the influence of stimuli (phenotypic modulation).
Briefly: answers to topical questions
From which embryonic sources does smooth muscle tissue develop?
Smooth muscle tissue develops from various sources:
- Mesenchyme is the source of development for most smooth muscles, including the smooth muscles of blood vessels and internal organs.
- Neural origin is characteristic of the smooth myocytes of the iris of the eye: they develop from the neural primordium of the optic cup, a derivative of the neural tube.
The origin of ciliary body myocytes remains controversial: according to some data, they are of neural origin, while according to others, they originate from mesenchyme.
Which regulatory and structural proteins are part of the thin myofilaments of smooth muscle tissue?
The thin myofilaments of smooth muscle tissue contain only actin. The regulatory proteins troponin and tropomyosin are absent in the thin myofilaments of smooth myocytes. Thin actin myofilaments are always present in the cytoplasm and attached to dense bodies.
To remember the specifics of smooth muscle activation, use the rule of two «C's»: Calcium activates Calmodulin and Kinase. In striated muscles, Troponin and T-tubules work instead.
Class and exam questions
Why is smooth muscle tissue called «smooth»?
It lacks permanent myofibrils and properly organized sarcomeres. The contractile apparatus assembles only at the moment of contraction, which is why the tissue lacks transverse striations under microscopy.
Where does the calcium for smooth myocyte contraction come from?
Unlike skeletal muscles, where calcium is stored in cisternae, in smooth tissue $Ca^{2+}$ ions come predominantly from the extracellular environment through specialized plasmalemmal channels and caveolae.
How do cells interact with each other in the intestinal wall?
They are connected via nexuses — gap junctions. This allows an instantaneous signal transmission from a single innervated cell to the entire myocyte complex, ensuring the collective work of the tissue.
What else to study on this topic
- Stages of histogenesis of smooth muscle tissue (differon)
- Molecular structure of dense bodies and their connection with the cytoskeleton
- Synthetic function of myocytes and its role in maintaining the vascular wall
- Mechanisms of prolonged tonic contraction (the «latch» effect)
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