Smooth muscle is one of the three types of muscle tissue found in the human body. Its action is essential to the control of homeostasis, that is to say of that process with which the organism maintains constant internal chemical and physical conditions, even when external environmental factors change. Smooth muscle is in fact synonymous with involuntary muscle, that is, a tissue capable of contracting and relaxing without the intentional participation of brain activity. Even if his recruitment is subtracted from the will of the will, a part of our peripheral nervous system - called the autonomic or vegetative nervous system (vegetable garden and parasympathetic) - is anyway able to control it in an excellent way. For the general characteristics of the autonomic nervous system we refer to the reading of the following article.
Smooth muscle is the characteristic muscle of internal and hollow organs, such as the stomach, intestine, bladder, bronchioles, uterus and blood and lymphatic vessels; we also find it in the internal muscles of the eye - which regulate the diameter of the pupil - and in the dermal ones, which control the piliferous erection.
The adjective "smooth" derives from the microscopic aspect of this muscle, characterized by the absence of the transverse striations typical of the striated one, both skeletal and cardiac. The contractile filaments of smooth fibrocellules are in fact arranged in a less organized way and the classic sarcomeres are not recognizable.
Smooth muscle cells, called fibrocellules, have a spindle shape (with a slightly dilated central area and thin and pointed ends); unlike the striated ones, which are organized in parallel bundles, the smooth fibrocellules collect in twisted bundles, arranged in such a way that at the central part of the one corresponds the terminal one of the other; their size is lower than the voluntary counterpart.
Within smooth fibrocellulas, always in contrast with skeletal fibers - which are multinucleated - we recognize only one nucleus.
In the various tissues, moreover, the smooth myofibrillar bundles can be arranged in several layers and orient themselves in different directions. In the intestine, for example, there is a circular layer that wraps around the lumen and a longitudinal layer that covers its entire length.
| SMOOTH MUSCLE | STRIPED SKELETAL MUSCLE | ||
| Involuntary | Volunteer* | ||
It covers the walls of all those apparatuses devoted to vegetative life; we find it in the wall of blood vessels (arteries, veins), in the wall of hollow organs (stomach, intestine), inside the eyeball, in the erector muscles of the hairs. Its main function is to push materials in and out of the body. | It consists of the skeletal muscles and the musculature of organs such as the bulboocular and the tongue, therefore most of the musculature. Allows movement and maintenance of posture, helps to determine body shapes | ||
It consists of smooth fibers, which do not have the typical streaks of the cardiac or skeletal muscle under the microscope | The particular disposition of the contractile proteins gives the muscle a striated aspect, characterized by streaks (alternately repeated light and dark bands); hence the term striped muscle. | ||
Contraction very slow, but prolonged and more efficient (less ATP required). | Responds with exceptional speed to nerve impulses, contracting rapidly and intensely. | ||
| They are not involved in the onset of muscle fatigue. | They cannot remain contracted for a long time with high intensity, they are subject to fatigue | ||
They are often intrinsic, and as such, not stick to skeletal structures | As a rule, they connect to the skeleton by means of tendons | ||
(*) Although it is under the control of our will, in certain circumstances the skeletal muscle may be responsible for involuntary motor acts (reflexes, such as patellar or swallowing) in response to external stimuli.
Further characteristics of smooth muscle
The propagation of the nerve impulse occurs much more slowly than in skeletal muscle; analogous speech for the speed of contraction and relaxation. The neurotransmitter, released by the autonomous neuron, depolarizes the fibrocellula by simple diffusion and for the subsequent meeting with intracellular receptors (there are no superficial areas rich in receptors such as those typical of the neuromuscular plaque)
Despite being slower than that of the skeletal counterpart, the contraction is more efficient and lasting (it requires less energy, and therefore less ATP, to generate a certain force). Thanks also to the reduced consumption of oxygen, the smooth muscle is therefore almost insensitive to fatigue and can sustain the contraction for long periods. Particular smooth muscles, the sphincters, can even remain contracted for the vast majority of the day (think for example of the two esophageal sphincters or the internal anal sphincter).
All these metabolic peculiarities are linked to a series of ultrastructural features, such as the greater length of actomyosinic myofilaments and the presence of a myosin isoform with slower ATPase activity. Furthermore, myosin filaments are fewer in number than those in actin, with a ratio of 10-15: 1; their heads, moreover, are present along the entire filament and, as such, allow a flow for greater distances than those produced by the sarcomere of skeletal muscle.
The troponin is missing in the smooth muscle; in its place is calmodulin, which maintains the ability to bind calcium and initiate a cascade of events that culminate in muscle contraction. The oblique and intertwined arrangement of the contractile elements causes the cell to become round when it contracts.
Recruitment of smooth muscle cells can be unitary or multi-unitary. In the first case (eg gastrointestinal tract and blood vessels) the set of muscle fibers, aggregated together, contracts in its entirety, thanks to the rapid propagation of the action potential from one cell to another (gap-junction). In the multi-unitary smooth muscle, on the other hand, every single fiber, quite distinct from the others, can contract independently, guaranteeing greater control and finesse of movement (we find it, for example, in the muscles of the iris, eyelashes and in that pilo-radiator) .
The architecture of the smooth muscle is not homogeneous like the striated one, but it specializes in acquiring specific functional characteristics in relation to the controlled organ or tissue.
The regulation of smooth muscle contractility is modulated by various mechanisms, not only electrical but also chemical; these impulses - of various kinds - can integrate with each other and modulate, sometimes in the opposite direction (excitatory / inhibitory), muscle activity. Some examples are histamine (responsible for the contraction of bronchial musculature and dyspnea typical of the asthmatic crisis), noradrenaline, oxytocin, angiotensin, vasopressin, nitric oxide, but also partial oxygen pressure and carbon dioxide (which regulates the contraction of arterioles, metarterioles and precapillary sphincters by increasing or decreasing blood flow to the tissues).
The smooth muscle has little possibility of post-traumatic regeneration, but it may undergo significant increases in volume (hypertrophy), as happens, for example, to the uterus during pregnancy. Even the smooth muscles that line the walls of the arteries can undergo a series of particularly deleterious structural and metabolic modifications, because they dangerously restrict the inner lumen of the vessel (atherosclerosis).
