Basic physiological properties
Smooth musculature differs fundamentally from striated muscle in its electrophysiological parameters. This tissue is characterized by extremely low excitability and very prolonged excitation. The conduction velocity of a nerve impulse here is minimal, as is the overall lability (the ability of the tissue to respond rhythmically and without distortion to incoming stimuli).
A single contraction of a smooth myocyte arises slowly and has a long duration. Despite this, it achieves significant force. This has colossal functional significance for the body: it is precisely this biomechanics that makes it possible to maintain the tone of hollow organ walls (e.g., stomach, gallbladder, urinary bladder) for a long time and without fatigue. In addition, the ability to generate great contraction force is absolutely necessary to ensure adequate sphincter function of various digestive and urogenital tracts.
Features of excitation propagation
Unlike skeletal muscles, smooth musculature does not obey the law of isolated conduction of excitation. The impulse does not remain strictly within a single muscle fiber. It is transmitted unimpeded and fairly quickly on the scale of the tissue to neighboring cells.
This transmission is carried out through contact via specialized gap junctions—nexuses. Due to this morphological structure, the entire muscular coat of a hollow organ is integrated into a functional syncytium. That is why smooth muscle obeys the "all-or-none" law: it is excited as a single whole in response to a threshold stimulus. Recall that skeletal muscle as a whole does not obey this law, as it consists of isolated motor units.
Plastic tone and response to stretch
The most important unique property of smooth musculature is plastic tone, or simply plasticity. This is the ability of muscle tissue to maintain a new length imparted to it as a result of slow stretching, and without significant changes in the internal tension of the wall.
The significance of plasticity is immense: it allows hollow organs to fill slowly with contents without a sharp increase in intra-organ pressure. By comparison, skeletal musculature possesses elasticity—upon any stretch, it strives to return immediately to its initial length.
The nature of a smooth muscle's response to mechanical stretch depends directly on the speed of this process:
- Slow stretch: plasticity is vividly manifested (the organ stretches without a response contraction, adapting to the volume).
- Rapid stretch: acts as an adequate stimulus. It instantly triggers a powerful contraction of the muscular coat, leading to rapid emptying of the organ in response to its rapid filling.
Automatism and chemical sensitivity
Smooth muscles are inherently characterized by automatism—a unique ability for spontaneous, self-generated excitation and contraction without the influence of any external stimuli (including neural ones). It is important to note that basal tone and rhythmic contractions persist even with complete denervation, i.e., upon physical destruction of neural connections with the central nervous system.
Furthermore, smooth myocytes are distinguished by increased chemical sensitivity. While skeletal muscles are activated and contract exclusively under the influence of incoming nerve impulses, smooth musculature operates much more complexly. Its contractile activity is regulated by a combination of three factors simultaneously: its own automatism, neural influences from the autonomic nervous system, and the effects of various chemical factors (hormones, metabolites, mediators).
Briefly: answers to topical questions
Which ion is responsible for initiating contraction in smooth myocytes?
Calcium ions ($Ca^{2+}$) are responsible for initiating contraction in smooth myocytes. Under the action of a nerve impulse, they enter from the extracellular environment (via caveolae or channels). Unlike skeletal muscles, this process proceeds more slowly.
Which contractile proteins form the myofilaments of smooth muscle tissue?
The myofilaments of smooth muscle tissue are formed by the proteins actin (thin filaments) and myosin (thick filaments). In the cytoplasm, these filaments are present in a disassembled form, and their assembly into temporary myofibrils occurs only at the moment of contraction.
What is the molecular mechanism of smooth muscle relaxation?
Smooth muscle relaxation includes the following stages:
- Calcium removal: active pumping out of $Ca^{2+}$ ions by calcium pumps.
- Dephosphorylation: myosin phosphatase (myosin light chain phosphatase) cleaves phosphate from myosin.
- Disruption of cross-bridges: gradual breaking of bonds between actin and myosin.
- Disintegration of thick filaments: breakdown into fragments and individual myosin molecules.
As a result, the cell returns to its initial state, and temporary myofibrils disappear.
To remember the difference in responses to stretch, use the rule: "Pull slowly — the wall adapts (plasticity), jerk sharply — the organ contracts (emptying)."
Class and exam questions
Why does smooth muscle obey the "all-or-none" law while skeletal muscle does not?
In smooth muscle, fibers are connected by gap junctions (nexuses), so excitation is transmitted from cell to cell, and the entire muscle reacts as a single whole. In skeletal muscle, fibers are isolated from each other.
Will stomach contractions be preserved if the nerves leading to it are completely severed?
Yes, contractions and tone will be preserved. This is due to the fact that smooth muscle tissue possesses automatism—the ability for spontaneous excitation without commands from the CNS.
What is the fundamental difference between the plasticity of smooth muscles and the elasticity of skeletal muscles?
Skeletal muscle works like a spring when stretched (elasticity), striving to return to its initial length. Smooth muscle upon slow stretching maintains the imparted length without a change in internal tension (plasticity).
What else to study on this topic
- Mechanisms of functioning of gap junctions (nexuses) during excitation transmission.
- Features of the application of the "all-or-none" law in smooth musculature.
- The nature of the high chemical sensitivity of smooth myocytes.
- Physiological significance of the sphincter function of the gastrointestinal tract.
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