Smooth muscles contain actin and myosin filaments that have chemical characteristics similar to the actin and myosin filaments of skeletal muscles. But in smooth muscles ah no troponin complex needed to trigger contraction skeletal muscle therefore, the contraction initiation mechanism in them is different. This mechanism is discussed in detail later in our article.
Chemical studies have shown that actin and myosin filaments extracted from smooth muscle interact with each other in much the same way as in skeletal muscle. Moreover, the contraction process is activated by calcium ions, and the energy for contraction is provided by the breakdown of ATP to ADP.
There are, however, significant differences in morphological organization of smooth and skeletal muscles, as well as in the conjugation of excitation and contraction, the mechanism of triggering the contractile process by calcium ions, the duration of contraction and the amount of energy required for contraction.
Morphological basis of smooth muscle contraction
Smooth muscles do not have such an ordered organization of actin and myosin filaments, which is found in skeletal muscles, giving them "banding". Using the technique of electron microphotography, the histological organization is revealed. One can see a large number of actin filaments attached to the so-called dense bodies. Some of these bodies are attached to the cell membrane, while others are distributed within the cell. Some of the membranous dense bodies of neighboring cells are linked together by bridges of intracellular proteins. Through these bridges, the force of contraction is mainly transmitted from one cell to another.
in muscle fiber scattered among the actin filaments are myosin filaments. Their diameter is more than 2 times the diameter of actin filaments. On electron micrographs, actin filaments are usually found 5-10 times more than myosin filaments.
The figure shows the proposed structure of a separate contractile unit inside a smooth muscle cell, where you can see a large number of actin filaments emanating from two dense bodies; the ends of these filaments overlap the myosin filament located in the middle between the dense bodies. This contractile unit is similar to the contractile unit of skeletal muscle, but without the specific regularity of its structure. In essence, smooth muscle dense bodies play the same role as Z-discs in skeletal muscle.
There is another difference. Most myosin filaments have transverse bridges with the so-called lateral polarity. The bridges are organized as follows: on one side they are hinged in one direction, and on the other - in the opposite direction. This allows myosin to pull the actin filament on one side in one direction while simultaneously propelling another actin filament on the other side in the opposite direction. This organization allows smooth muscle cells to shorten up to 80% of their length instead of the less than 30% shortening typical of skeletal muscle.
Most skeletal muscles contract and relax rapidly, but smooth muscle contractions are mostly long-lasting tonic contractions that sometimes last for hours or even days. Therefore, it can be expected that the morphological and chemical features of smooth muscles should differ from the corresponding characteristics of skeletal muscles. Some of these differences are discussed next.
Slow cyclic activity myosin cross bridges. In smooth muscle, in comparison with skeletal muscle, the rate of cyclic activity of myosin cross-bridges is much lower; the rate of their attachment to actin, detachment from actin, and reattachment for the next cycle. In fact, the cycle rate is only 1/10 to 1/300 of that in skeletal muscle. However, in smooth muscle, the relative amount of time during which the transverse bridges remain attached to the actin filaments is believed to be much greater, which is the main factor determining the force of contraction. A possible reason for the slow cycling is the much lower ATP-ase activity of the heads of the cross bridges compared to the skeletal muscle, and therefore the rate of destruction of ATP, the energy source for the movement of the heads of the cross bridges, is significantly reduced with a corresponding slowdown in the rate of their cycles.
Smooth muscles are part of internal organs. Due to the contraction, they provide the motor (motor) function of their organs (alimentary canal, genitourinary system, blood vessels, etc.). Unlike skeletal muscles, smooth muscles are involuntary.
Morpho-functional structure of smooth (not striated) muscles. The main structural unit of smooth muscles is the muscle cell, which has a spindle shape and is covered on the outside with a plasma membrane. Under an electron microscope, numerous depressions can be seen in the membrane - caveolae, which significantly increase the total surface of the muscle cell. The sarcolemma of an unfazed muscle cell includes the plasma membrane, together with the basement membrane that covers it from the outside, and adjacent collagen fibers. The main intracellular elements:
nucleus, mitochondria, lysosomes, microtubules, sarcoplasmic reticulum and contractile proteins.
Muscle cells form muscle bundles and muscle layers. The intercellular space (100 nm or more) is filled with elastic and collagen fibers, capillaries, fibroblasts, etc. In some areas, the membranes of neighboring cells lie very tightly (the gap between cells is 2-3 nm). It is assumed that these areas (nexus) serve for intercellular communication, transmission of excitation. It has been proven that some smooth muscles contain a large number of nexus (sphincter of the pupil, circular muscles of the small intestine, etc.), while others have few or none at all (vas deferens, longitudinal muscles of the intestines). There is also an intermediate, or desmotic, connection between non-smoking muscle cells (through a thickening of the membrane and with the help of cell processes). Obviously, these connections are important for the mechanical connection of cells and the transmission of mechanical force by cells.
Due to the chaotic distribution of myosin and actin protofibrils, smooth muscle cells are not striated like skeletal and cardiac cells. Unlike skeletal muscles, there is no T-system in smooth muscles, and the sarcoplasmic reticulum makes up only 2-7% of the volume of the myoplasm and has no connections with the external environment of the cell.
Physiological properties of smooth muscles. Smooth muscle cells, like striated, contract due to the sliding of actin protofibrils between myosin, however, the speed of sliding and ATP hydrolysis, and hence the rate of contraction, is 100-1000 times less than in striated muscles. Thanks to this, smooth muscles are well adapted for long-term sliding with little energy and without fatigue.
Smooth muscles, taking into account the ability to generate AP in response to threshold or supra-horn stimulation, are conditionally divided into phasic and tonic. Phasic muscles generate a full-fledged AP, tonic muscles - only local ones, although they also have a mechanism for generating full-fledged potentials. failure to tonic muscles to PD is explained by the high potassium permeability of the membrane, which prevents the development of regenerative depolarization.
The value of the membrane potential of smooth muscle cells of non-frightening muscles varies from -50 to -60 mV. As in other muscles, including nerve cells, it is formed mainly to +, Na +, Cl-. In the smooth muscle cells of the alimentary canal, uterus, and some vessels, the membrane potential is unstable, spontaneous fluctuations are observed in the form of slow depolarization waves, at the top of which AP discharges may appear. The duration of AP of smooth muscles ranges from 20-25 ms to 1 s or more (for example, in muscles Bladder), i.e. she
longer than the duration of AP of skeletal muscles. In the mechanism of AP of smooth muscles, Ca2+ plays an important role next to Na+.
Spontaneous myogenic activity. Unlike skeletal muscles, smooth muscles of the stomach, intestines, uterus, and ureters have spontaneous myogenic activity, i.e. develop spontaneous tetanohyodibne contractions. They are stored under conditions of isolation of these muscles and with pharmacological shutdown of the intrafusal nerve plexuses. So, PD occurs in the smooth muscles themselves, and is not due to the transmission of nerve impulses to the muscles.
This spontaneous activity is of myogenic origin and occurs in muscle cells that act as a pacemaker. In these cells, the local potential reaches a critical level and transforms into AP. But after the repolarization of the membrane, a new local potential spontaneously arises, which causes another AP, and so on. AP, propagating through the nexus to neighboring muscle cells at a speed of 0.05-0.1 m/s, covers the entire muscle, causing its contraction. For example, peristaltic contractions of the stomach occur with a frequency of 3 times per 1 min, segmental and pendulum movements of the colon - 20 times per 1 min in the upper sections and 5-10 per 1 min - in the lower. Thus, the smooth muscle fibers of these internal organs have automatism, which is manifested by their ability to contract rhythmically in the absence of external stimuli.
What is the reason for the appearance of potential in the cells of the smooth muscles of the pacemaker? Obviously, it occurs due to a decrease in potassium and an increase in sodium and (or) calcium permeability of the membrane. As for the regular occurrence of slow waves of depolarization, most pronounced in the muscles of the gastrointestinal tract, there is no reliable data on their ionic origin. It is possible that a decrease in the initial inactivating component of the potassium current during depolarization of muscle cells due to the inactivation of the corresponding potassium ion channels plays a certain role. Due to this, the occurrence of repeated G1D becomes possible.
Elasticity and extensibility of smooth muscles. Unlike skeletal muscles, they are smooth when stretched themselves as plastic, elastic structures. Due to plasticity, smooth muscle can be completely relaxed both in a contracted and stretched state. For example, the plasticity of the smooth muscles of the wall of the stomach or bladder, as these organs are filled, prevents an increase in intracavitary pressure. Excessive stretch often leads to stimulation of contraction, which is due to the depolarization of the pacemaker cells that occurs when the muscle is stretched, and is accompanied by an increase in the frequency of AP, and as a result, an increase in contraction. The contraction, which activates the stretching process, plays a large role in the self-regulation of the basal tone of the blood vessels.
mechanism of smooth muscle contraction. A prerequisite the occurrence of contraction of smooth muscles, as well as skeletal ones, and an increase in the concentration of Ca2 + in myoplasm (up to 10v-5 M). It is believed that the contraction process is activated mainly by extracellular Ca2 +, which enters muscle cells through voltage-dependent Ca2 + channels.
A feature of neuromuscular transmission in smooth muscles is that innervation is carried out by the autonomic nervous system and it can have both excitatory and inhibitory effects. By type, cholinergic (mediator acetylcholine) and adrenergic (mediator norepinephrine) mediators are distinguished. The former are usually found in the muscles of the digestive system, the latter in the muscles of the blood vessels.
The same mediator can be excitatory in some synapses, and inhibitory in others (depending on the properties of cytoreceptors). Adrenoreceptors are divided into a- and B-. Norepinephrine, acting on a-adrenergic receptors, constricts blood vessels and inhibits the motility of the digestive tract, and acting on B-adrenergic receptors, stimulates the activity of the heart and dilates the blood vessels of some organs, relaxes the muscles of the bronchi. Described neuromuscular. ny transfer in smooth muscles for the help and other mediators.
In response to the action of an excitatory mediator, depolarization of smooth muscle cells occurs, which manifests itself in the form of an excitatory synaptic potential (SSP). When it reaches a critical level, PD occurs. This happens when several impulses come one after another to the nerve ending. The emergence of ISGI is a consequence of an increase in the permeability of the postsynaptic membrane for Na +, Ca2 + and SI ".
The inhibitory neurotransmitter causes hyperpolarization of the postsynaptic membrane, which is manifested in the inhibitory synaptic potential (GSP). Hyperpolarization is based on an increase in membrane permeability mainly for K +. The role of an inhibitory mediator in smooth muscles excited by acetylcholine (for example, muscles of the intestine, bronchi) is played by norepinephrine, and in smooth muscles for which norepinephrine is an excitatory mediator (for example, bladder muscles) - acetylcholine.
Clinical and physiological aspect. In some diseases, when the innervation of skeletal muscles is disturbed, their passive stretching or displacement is accompanied by a reflex increase in their tone, i.e. resistance to stretching (spasticity or rigidity).
In case of circulatory disorders, as well as under the influence of certain metabolic products (lactic and phosphoric acids), toxic substances, alcohol, fatigue, decreased muscle temperature (for example, during prolonged swimming in cold water) after prolonged active contraction of the muscle, contracture may occur. The more the muscle function is disturbed, the stronger the contracture aftereffect is expressed (for example, contracture chewing muscles in pathology maxillofacial area). What is the origin of contracture? It is believed that the contracture arose due to a decrease in the concentration of ATP in the muscle, which led to the formation of a permanent connection between the transverse bridges and actin protofibrils. In this case, the muscle loses flexibility and becomes hard. The contracture subsides, the muscle relaxes when the ATP concentration reaches a normal level.
In diseases such as myotonia, muscle cell membranes are excited so easily that even slight stimulation (for example, the introduction of a needle electrode during electromyography) causes a discharge of muscle impulses. Spontaneous AP (fibrillation potentials) are also recorded at the first stage after muscle denervation (until inactivity leads to its atrophy).
Tonic contractions of some smooth muscles, especially the muscles of the vascular walls (basal or myogenic, tone) are activated mainly by extracellular Ca 2 +. Physiologically active substances and mediators can cause a decrease in smooth muscle tone by closing chemosensitive Ca2 + channels (through the activation of chemoreceptors) or hyperpolarization, which leads to the suppression of spontaneous AP and the closure of voltage-dependent Ca2 + channels.
PHYSIOLOGY OF SMOOTH MUSCLE
Smooth muscles are built from muscle fibers that have a diameter of 2 to 5 microns and a length of only 20 to 500 microns, which is much smaller than in skeletal muscles, the fibers of which are 20 times larger in diameter and thousands of times longer. They do not have transverse striations. The mechanism of contraction of smooth muscle fibers is fundamentally the same as in the lopere-swallowing. It is built on the interaction between the contractile proteins actin and myosin, although there are some differences - they are not characterized by an ordered arrangement of filaments. An analogue of Z-lines in smooth muscles is dense bodies, which are contained in the myoplasm and are connected to the cell membrane and actin filaments. The contraction of various smooth muscles lasts from 0.2 s to 30 s. Their absolute strength is 4-6 kg/cm2, in skeletal muscles - 3-17 kg/cm2.
Types of Smooth Muscles: smooth muscles are divided into visceral, or unitary, polyelemental, or multiunitary, And vascular smooth muscle, having properties of both previous types.
Visceral, or unitary muscles are contained in the walls of hollow organs - the digestive canal, uterus, ureters, gallbladder and bladder. their feature is that they transmit excitation from cell to cell with low-resistance gap junctions, which allows the muscles to respond as a functional syncytium, that is, as one cell, hence the term unitary muscles. They are spontaneously active, have pacemakers (pacemakers), which are modulated under the influence of hormones or neurotransmitters. The resting potential for these muscle fibers is not typical, since in the active state of the cell it is low, during its inhibition it is high, and at rest it is about -55 mV. They are characterized by the so-called sinusoidal slow waves of depolarization, on which peak APs are superimposed, lasting from 10 to 50 ms (Fig. 2.34).
The mechanism of AP generation in smooth muscles and their contraction is largely initiated by Ca2 ions. The contraction occurs 100–200 ms after excitation, and the maximum develops only 500 ms after the onset of the peak. Therefore, smooth muscle contraction is a slow process. However, visceral muscles have a high degree of electrical conjugation between cells, providing high coordination of their contraction.
Polyelement, or multiunitary smooth muscles are composed of individual units without connecting bridges, and the response of the whole muscle to stimulation consists of the response of individual muscle fibers. Each muscle fiber is innervated by one nerve ending, as in skeletal muscle. These include the muscles of the iris of the eye, the ciliary muscle of the eye, the piloerectoral muscles of the hair of the skin. They do not have voluntary regulation, they are reduced due to nerve impulses that are transmitted through the neuromuscular synapses of the autonomic nervous system, whose neurotransmitters can cause both excitation and inhibition.
Mechanisms of contraction and relaxation of smooth muscles
The mechanism of conjugation of excitation and contraction differs from a similar process occurring in skeletal muscles, since smooth muscles do not contain troponin.
The sequence of processes in smooth muscles that leads to contraction and relaxation has the following steps:
1. When the cell membrane is depolarized, potential deposits of calcium channels and ions open
RICE. 2.34.
Ca 2+ enter the cell with an electrochemical gradient, the concentration of Ca 2+ ions in the cell increases.
2. The entry of Ca 2+ ions through the cell membrane can cause an additional exit of Ca 2+ ions from the sarcoplasmic reticulum (SPR) through the Ca 2+ dependent gate of calcium channels. Hormones and neurotransmitters also stimulate the release of Ca 2+ ions from SBP through inositol triphosphatide (ISP) dependent calcium channel gates.
3. intracellular concentration of Ca 2+ ions increases.
4. Ca 2+ ions bind to calmodulin, a regulatory protein that has 4 Ca 2+ bindings and plays important role in the activation of enzymes. Calcium calmodulin complex activates the enzyme kinase myosin light chain, resulting in phosphorylation of myosin head molecules. Myosin hydrolyzes ATP, energy is generated, and the cycle of formation of transverse actin-myosin bridges, sliding of actin along myosin chains begins. Phosphorylated myosin bridges repeat their cycle until they are dephosphorylated. myosinphosphatase.
5. Dephosphorylation of myosin leads to relaxation of the muscle fiber, or a state of residual tension due to the formed cross bridges, until the final dissociation of the calcium-calmodulin complex occurs.
AGE CHANGES IN EXCITIVE STRUCTURES
In the process of ontogenesis, the properties of excitable structures change in connection with the development of the musculoskeletal system and its regulation.
Muscle mass increases - from 23.3% of body weight in a newborn to 44.2% at the age of 17-18 years. Muscle tissue grows due to the lengthening and thickening of muscle fibers, and not an increase in their number.
In a newborn child, the activity of sodium-potassium pumps located in myocyte membranes is still low, and therefore the concentration of K + ions in the cell is almost half that in an adult, and only at 3 months begins to increase. APs are already generated after birth, but they have a lower amplitude and a longer duration. The generation of PD of muscle fibers in newborns is not blocked by tetrodotoxin.
After birth, the length and diameter of the axial cylinders in the nerve fibers increase from 1-3 microns to 7 microns at 4 years, and their formation is completed at 5-9 years. Until the age of 9, myelination of nerve fibers ends. The rate of excitation conduction after birth does not exceed 50% of the rate in adults and increases within 5 years. The increase in conduction velocity is due to: an increase in the diameter of nerve fibers, their myelination, the formation of ion channels and an increase in the amplitude of AP. A decrease in the duration of AP and, accordingly, the phase of absolute refractoriness leads to an increase in the number of AP that a nerve fiber can generate.
The receptor apparatus of the muscles develops faster than the motor nerve endings are formed. The duration of neuromuscular transmission after birth is 4.5 ms, in an adult it is 0.5 ms. In the process of ontogenesis, the synthesis of acetylcholine, acetylcholinesterase, and the density of cholinergic receptors of the end plate increase.
In the process of aging, the duration of AP in excitable structures increases, and the number of AP generated by muscle fibers per unit time (lability) decreases. Muscle mass decreases due to a decrease in metabolic rate.
important properties of smooth muscle is its great plasticity, i.e., the ability to maintain the length given by stretching without changing the stress. The difference between skeletal muscle, which has little plasticity, and smooth muscle, with well-defined plasticity, is easily detected if they are first slowly stretched, and then the tensile load is removed. immediately shortened after the load is removed. In contrast, the smooth muscle after the removal of the load remains stretched until, under the influence of some kind of irritation, its active contraction occurs.
The property of plasticity is very great importance for the normal activity of the smooth muscles of the walls of hollow organs, such as the bladder: due to the plasticity of the smooth muscles of the walls of the bladder, the pressure inside it changes relatively little with different degrees of filling.
Excitability and arousal
Smooth muscles less excitable than skeletal ones: their thresholds of irritation are higher, and the chronaxy is longer. The action potentials of most smooth muscle fibers have a small amplitude (about 60 mV instead of 120 mV in skeletal muscle fibers) and longer duration - up to 1-3 seconds. On rice. 151 shows the action potential of a single fiber of the uterine muscle.
The refractory period lasts for the entire period of the action potential, i.e. 1-3 seconds. The rate of excitation conduction varies in different fibers from a few millimeters to several centimeters per second.
There are a large number of different types of smooth muscle in the body of animals and humans. Most of the hollow organs of the body are lined with smooth muscles that have a sensitial type of structure. The individual fibers of such muscles are very closely adjacent to each other and it seems that morphologically they form a single whole.
However, electron microscopic studies have shown that there is no membrane and protoplasmic continuity between the individual fibers of the muscular syncytium: they are separated from each other by thin (200-500 Å) slits. The concept of "syncytial structure" is currently more physiological than morphological.
syncytium- this is a functional formation that ensures that action potentials and slow waves of depolarization can freely propagate from one fiber to another. Nerve endings are located only on a small number of syncytium fibers. However, due to the unhindered spread of excitation from one fiber to another, the involvement of the entire muscle in the reaction can occur if the nerve impulse arrives at a small number of muscle fibers.
Smooth muscle contraction
With a large force of a single irritation, smooth muscle contraction may occur. The latent period of a single contraction of this muscle is much longer than that of the skeletal muscle, reaching, for example, in the intestinal muscles of a rabbit 0.25-1 second. The duration of the contraction itself is also large ( rice. 152): in the stomach of a rabbit, it reaches 5 seconds, and in the stomach of a frog - 1 minute or more. Relaxation is especially slow after contraction. The wave of contraction propagates through the smooth muscles also very slowly, it travels only about 3 cm per second. But this slowness of the contractile activity of smooth muscles is combined with their great strength. Thus, the muscles of the stomach of birds are capable of lifting 1 kg per 1 cm2 of their cross section.
Smooth muscle tone
Due to the slowness of contraction, a smooth muscle, even with rare rhythmic stimuli (for a frog's stomach, 10-12 stimuli per minute is enough), easily passes into a long-term state of persistent contraction, reminiscent of tetanus of skeletal muscles. However, the energy expenditure during such a persistent contraction of the smooth muscle is very small, which distinguishes this contraction from the tetanus of the striated muscle.
The reasons why smooth muscles contract and relax much more slowly than skeletal muscles have not yet been fully elucidated. It is known that myofibrils of smooth muscle, like those of skeletal muscle, consist of myosin and actin. However, smooth muscles do not have striation, no Z membrane, and are much richer in sarcoplasm. Apparently, these features of the structure of smooth muscle waves determine the slow pace of the contractile process. This corresponds to a relatively low level of smooth muscle metabolism.
Smooth muscle automation
A characteristic feature of smooth muscles, which distinguishes them from skeletal muscles, is the ability for spontaneous automatic activity. Spontaneous contractions can be observed in the study of the smooth muscles of the stomach, intestines, gallbladder, ureters and a number of other smooth muscle organs.
Smooth muscle automation is of myogenic origin. It is inherent in the muscle fibers themselves and is regulated by nerve elements that are located in the walls of smooth muscle organs. The myogenic nature of automaticity has been proven by experiments on strips of muscles of the intestinal wall, freed by careful dissection from the adjacent nerve plexuses. Such strips, placed in a warm Ringer-Locke solution, which is saturated with oxygen, are capable of making automatic contractions. Subsequent histological examination revealed the absence of nerve cells in these muscle strips.
In smooth muscle fibers, the following spontaneous oscillations of the membrane potential are distinguished: 1) slow waves of depolarization with a cycle duration of the order of several minutes and an amplitude of about 20 mV; 2) small rapid potential fluctuations preceding the emergence of action potentials; 3) action potentials.
Smooth muscle responds to all external influences by changing the frequency of spontaneous rhythm, which results in contraction and relaxation of the muscle. The effect of irritation of the smooth muscles of the intestine depends on the ratio between the frequency of stimulation and the natural frequency of spontaneous rhythm: with a low tone - with rare spontaneous action potentials - the applied irritation enhances the tone; with a high tone, relaxation occurs in response to irritation, since an excessive increase in impulses leads to that each next impulse falls into the refractory phase from the previous one.