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 the process by which the organism keeps the internal chemical-physical conditions constant, even when external environmental factors vary. 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 its recruitment is removed from the domain of the will, a part of our peripheral nervous system - called the autonomic or vegetative nervous system (ortho and parasympathetic) - is still able to control it in an excellent way. For the general characteristics of the autonomic nervous system, please refer to the following article.
Smooth muscle is the characteristic muscle of internal and hollow organs, such as the stomach, intestines, 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 are responsible for controlling the erection of the hair.
The adjective "smooth" derives from the "microscopic aspect of this muscle, characterized by the" absence of the transversal striations typical of the striated one, both skeletal and cardiac. The contractile filaments of the smooth fiber cells are in fact arranged in a less organized way and are not recognizable. classic sarcomeres.
The 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 fiber cells are collected in intertwined bundles, arranged in such a way that the central part of one corresponds to the terminal part of the other; their size is smaller than the voluntary counterpart.
Inside the smooth fiber cells, always in contrast with the skeletal fibers - which are multinucleated -, we recognize only one nucleus.
Furthermore, in the various tissues, 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 surrounds the lumen and a longitudinal layer that runs along its entire length.
It covers the walls of all those apparatuses devoted to the vegetative life; we find it in the wall of blood vessels (arteries, veins), in the wall of hollow organs (stomach, intestine), inside the ocular globe, in the erector muscles of the hair. Its main function is to
push materials in and out of the body.
It constitutes the skeletal muscles and the musculature of organs such as the eyeball and the tongue, thus most of the musculature.
It allows movement and maintenance of posture; it helps to determine body shapes
It is made up of smooth fibers, which under the microscope do not show the typical streaks of cardiac or skeletal muscle
The particular arrangement of the contractile proteins gives the muscle a striated appearance, characterized by streaks (alternately repeated light and dark bands); hence the term Striated Muscle.
Very slow, but prolonged and more efficient contraction (less ATP required).
They cannot remain contracted for a long time with high intensity, they are subject to fatigue
They are often intrinsic, and as such, not
they attach themselves to skeletal structures
(*) Although it is under the control of our will, in certain circumstances the skeletal muscle can be responsible for involuntary motor acts (reflexes, such as patellar or swallowing) in response to external stimuli.
Additional characteristics of smooth muscle
The propagation of the nerve impulse occurs much slower than in skeletal muscle; analogously for the speed of contraction and relaxation. The neurotransmitter, released by the autonomic neuron, depolarizes the fibrocell by simple diffusion and by the subsequent encounter with intracellular receptors ( there are no surface areas rich in receptors such as those typical of the neuromuscular plate)
Despite being slower than that of the skeletal counterpart, the contraction is more efficient and lasting (it requires less energy, therefore less ATP, to generate a certain force). Thanks also to the reduced oxygen consumption, the smooth muscle is therefore almost insensitive to fatigue and can sustain 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 one).
All these metabolic peculiarities are linked to a series of ultrastructural characteristics, such as the greater length of actomyosin myofilaments and the presence of a myosin isoform with slower ATPase activity. Furthermore, the myosin filaments are fewer than those of actin, with a ratio of 10-15: 1; their heads, moreover, are present along the entire filament and, as such, allow a sliding for greater distances than those produced by the sarcomere of the skeletal muscle.
Smooth muscle lacks troponin; in its place is calmodulin, which maintains the ability to bind calcium and trigger a cascade of events that culminate in muscle contraction. The oblique and intertwined arrangement of the contractile elements causes the cell to become rounded when it contracts.
Smooth muscle cell recruitment can be unitary or multi-unit. In the first case (eg gastrointestinal tract and blood vessels) the whole of the muscle fibers, aggregated together, contracts in their entirety, thanks to the rapid propagation of the action potential from one cell to another (gap-junction). In the multi-unit smooth muscle, on the other hand, each single fiber, quite distinct from the others, can contract autonomously, guaranteeing greater control and finesse of movement (we find it, for example, in the muscles of the iris, eyelashes and piloerectrice) .
The smooth muscle architecture is not homogeneous like the striated one, but it specializes by 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), the muscular activity. Some examples are given by histamine (responsible for the contraction of the bronchial muscles and the dyspnea typical of the asthmatic crisis ), from noradrenaline, from "oxytocin, from" angiotensin, from vasopressin, from nitric oxide, but also from the partial pressure of oxygen and carbon dioxide (which regulates the contraction of arterioles, metarterioles and precapillary sphincters by increasing or decreasing blood flow to tissues).
Smooth muscle has little chance of post-traumatic regeneration, but it can undergo significant increases in volume (hypertrophy), as happens, for example, to the uterus during pregnancy. Even the smooth muscle that lines the walls of the arteries can undergo a series of particularly deleterious structural and metabolic modifications, because they dangerously narrow the internal lumen of the vessel (atherosclerosis).
