Basic Physiological Properties
Smooth muscle 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 the nerve impulse is minimal here, as is the overall lability (the ability of the tissue to rhythmically and undistortedly respond to incoming stimuli).
A single smooth myocyte contraction develops slowly and has a long duration. Despite this, it achieves significant force. This has immense functional significance for the body: it is precisely this biomechanics that allows the walls of hollow organs (e.g., stomach, gallbladder, urinary bladder) to maintain tone for a long time without fatigue. Furthermore, the ability to generate great contractile force is absolutely necessary to ensure adequate sphincter function in the digestive and urogenital tracts.
Features of Excitation Conduction
Unlike skeletal muscle, smooth muscle does not obey the law of isolated conduction of excitation. The impulse does not remain strictly within a single muscle fiber; it is transmitted freely and relatively rapidly across the tissue scale to neighboring cells.
This transmission occurs via specialized gap junctions — nexuses. Due to this morphological structure, the entire muscular coat of a hollow organ is united into a functional syncytium. Therefore, smooth muscle obeys the 'all-or-none' law: it is excited as a whole in response to a threshold stimulus. Skeletal muscle as a whole does not obey this law because it consists of isolated motor units.
Plastic Tone and Response to Stretch
The most important unique property of smooth muscle is plastic tone, or simply plasticity. This is the ability of muscle tissue to maintain a new length imparted to it by slow stretching, without a significant change in the internal wall tension.
The significance of plasticity is enormous: it allows hollow organs to fill slowly with contents without a sharp increase in intra-organ pressure. For comparison, skeletal muscle possesses elasticity—upon any stretch, it strives to return immediately to its initial length.
The nature of the smooth muscle's response to mechanical stretch depends directly on the rate of this process:
- Slow stretch: Plasticity is prominently displayed (the organ stretches without a responsive 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 swift filling.
Automaticity and Chemical Sensitivity
Smooth muscles inherently possess automaticity—a unique ability for spontaneous excitation and contraction without any external stimuli (including neural input). Crucially, baseline tone and rhythmic contractions persist even after complete denervation, i.e., physical disruption of neural connections with the central nervous system.
Additionally, smooth myocytes exhibit heightened chemical sensitivity. While skeletal muscles are activated and contract exclusively under the influence of arriving nerve impulses, smooth muscle operates much more complexly. Its contractile activity is regulated by a combination of three factors: its own automaticity, autonomic nervous system influences, and various chemical factors (hormones, metabolites, neurotransmitters).