Features of contraction and relaxation of smooth muscles. Smooth muscles, their structure and innervation, physiological properties, functional features
The physiological properties of smooth muscles are associated with the peculiarity of their structure, the level of metabolic processes and differ significantly from the features skeletal muscle.
Smooth muscles are found in internal organs, blood vessels and skin.
They are less excitable than striated ones. For their excitation, a stronger and longer stimulus is required. The contraction of smooth muscles is slower and longer. A characteristic feature of smooth muscles is their ability for automatic activity, which is provided by nerve elements (they are able to contract under the influence of excitation impulses born in them).
Smooth muscles, unlike striated muscles, have a high extensibility. In response to slow stretching, the muscle lengthens, but its tension does not increase. Due to this, when filling the internal organ, the pressure in its cavity does not increase. The ability to maintain the length given by stretching without changing the stress is called plastic tone. It is a physiological feature of smooth muscles.
Smooth muscles are characterized by slow movements and prolonged tonic contractions. The main irritant is the rapid and strong stretching.
Smooth muscles are innervated by sympathetic and parasympathetic nerves, which have a regulatory effect on them, and not a starting one, as on skeletal muscles, they are highly sensitive to certain biologically active substances (acetylcholine, adrenaline, norepinephrine, serotonin, etc.).
Muscle Fatigue
Physiological state a temporary decrease in performance that occurs as a result of muscle activity is called fatigue . It manifests itself in a decrease in muscle strength and endurance, an increase in the number of erroneous and unnecessary actions, a change in heart rate and respiration, an increase in blood pressure, an increase in the processing time of incoming information, and a time for visual-motor reactions. With fatigue, the processes of attention, its stability and switchability are weakened, endurance, perseverance are weakened, the possibilities of memory and thinking are reduced. The severity of changes in the state of the body depends on the depth of fatigue. Changes may be absent with slight fatigue and become extremely pronounced with deep stages of body fatigue.
Subjectively, fatigue manifests itself in the form of a feeling of fatigue, causing a desire to stop work or reduce the amount of load.
There are 3 stages of fatigue. In the first stage, labor productivity is practically not reduced, the feeling of fatigue is slightly expressed. In the second stage, labor productivity is significantly reduced, the feeling of fatigue is pronounced. In the third stage, labor productivity can be reduced to zero, and the feeling of fatigue is very pronounced, persists after rest and sometimes even before the resumption of work. This stage is sometimes characterized as the stage of chronic, pathological fatigue, or overwork.
The causes of fatigue are the accumulation of metabolic products (lactic, phosphoric acids, etc.), a decrease in the supply of oxygen and the depletion of energy resources.
Depending on the nature of work, physical and mental fatigue are distinguished, development mechanisms, which are largely similar. In both cases, the processes of fatigue first develop in the nerve centers. One of the indicators of this is a decrease in mental performance with physical fatigue, and with mental fatigue - a decrease in the efficiency of muscle activity.
The recovery period after work is called rest.. I.P. Pavlov assessed rest as a state of special activity to restore cells to their normal composition. Rest can be passive(complete motor rest) and active. Active recreation includes various forms of moderate activity, but different from that which characterized the main work. The idea of outdoor activities arose from the experiments of I.M. Sechenov, which established that better recovery The efficiency of working muscles does not occur at complete rest, but with moderate work of other muscles. I.M. Sechenov explained this by the fact that the stimulating effect of afferent impulses received during rest from other working muscles in the central nervous system contributes to a better and faster recovery of the working capacity of tired nerve centers and muscles.
Meaning of Workout
The process of systematic impact on the body of physical exercise in order to increase or maintain high level physical or mental performance and human resistance to environmental influences, adverse living conditions and changes in the internal environment is called training. The essence of the upcoming changes in the body during training is complex and versatile. It includes physiological and morphological changes. The end result of physical exercise is the development of new complex conditioned reflexes that increase the functionality of the body.
Due to trace processes in the cerebral cortex, a certain connection is created from repeated exercises - a cortical stereotype. I.P. Pavlov called the cortical stereotype, expressed in motor acts, a dynamic (mobile) stereotype. In the process of training new motor skills, muscle movements become more economical, coordinated, and motor acts are highly automated. At the same time, more correct correlations are established between the power of the work performed by the muscles and the intensity of the associated vegetative functions (circulation, respiration, excretory processes, etc.). Systematically trained muscles thicken, become denser and more resilient, and their ability to exert greater force increases.
Distinguish between general and special training. The first aims to develop the functional adaptation of the whole organism to physical activity, and the second is aimed at restoring functions impaired due to illness or injury. Special training effective only in combination with the general. Training exercise It has a multifaceted positive effect on the human body, if carried out taking into account its physiological capabilities.
Lecture 4 . Physiology of muscle tissueMuscle tissue performs the following functions:
Security motor activity Purposeful behavior is the most effective form of adaptation.
Providing special functions inherent only to a person is, first of all, a communicative function, expressed in the form of oral and written speech.
Performance respiratory function- excursion chest and diaphragms.
Participation in the processes of heat generation - thermoregulatory tone, muscle tremors.
A specific property of all muscle types is contractility - the ability to contract, that is, to shorten or develop tension. To implement this ability, the muscle uses two of its additional properties - excitability And conductivity .
Skeletal muscles are also called arbitrary , since their reduction can be controlled at will. They are completely devoid of automatism and are not able to work without control impulses from the central nervous system. Smooth muscles do not contract on their own, so they are also called involuntary .
Morphofunctional characteristics of skeletal muscle . Skeletal muscle is made up of multinucleated muscle fibers. The fiber thickness is from 10 to 100 µm. The length of the fibers varies from a few mm to several centimeters.
The number of muscle fibers becomes constant at 4-5 months of postnatal development. Subsequently, only the diameter and length of the fibers increase (for example, under the influence of training - functional hypertrophy).
The muscle fiber is covered with a sarcolemma. The muscle fiber sarcoplasm contains the following intracellular elements: nuclei, mitochondria, proteins, fat droplets, glycogen granules, phosphate-containing substances, various small molecules and electrolytes. From the surface of the sarcolemma, T-tubules extend into the fiber, which ensure its interaction with the sarcoplasmic reticulum. The sarcoplasmic reticulum is a system of interconnected cisterns and tubules extending from them in the longitudinal directionlocated between myofibrils. The extreme cisterns of the reticulum are connected with T-tubules. The tanks contain calcium ions necessary for the reduction process. Inside the muscle fiber stretches a mass of filaments - myofibrils, which are part of the mechanism of the contraction process. Each myofibril consists of protofibrils, which are parallel to each other and have a protein nature.
There are two types of intramuscular threads: thin actinic and thick myosin . actin filaments consist of two subunits - twisted fibers in the form of a spiral, each of which is formed by connected molecules of the globular actin protein. In addition to actin, thin filaments contain regulatory proteins. tropomyosin And troponin . These proteins in an unexcited muscle interfere with the binding of actin and myosin, so the muscle at rest is in a relaxed state.
Fig.1. Scheme of the spatial organization of contractile and regulatory proteins in a striated muscle.
Each myosin filament is surrounded by six actin filaments. These filaments form a kind of cylinder, inside of which the myosin filament is located. The cross bridges of the myosin filament are directed towards different sides, so they interact with all actin protofibrils. In turn, each actin filament contacts three myosin filaments.
In the body of domestic animals, smooth muscles are found in the internal organs, in the wall of blood vessels and in the skin. Smooth muscles, unlike striated muscles, do not have a pronounced transverse striation, contract relatively slowly, respond with contraction to stretching, and can be in a contracted state for a long time without fatigue. They consist of elongated spindle-shaped cells. Functionally, there are Various types smooth muscles. Some contract with a certain force in response to excitation and do not have spontaneous automatic activity (ciliary, pilomotor, ciliary; muscles of the nictitating membrane, Bladder, blood vessels); others are capable of spontaneous automatic rhythmic activity, which changes under the influence of motor nerves (muscles gastrointestinal tract, ureters and uterus).
The length of smooth muscle cells is from 30 to 500 microns, the diameter is from 2 to 10 microns. Each cell has a plasma membrane of unequal thickness in different organs, the thickness and structure of the membrane are the same as in other cells. On the surface of smooth muscle cells there are indentations into the cell in the form of small spherical pockets and lateral processes. Lateral processes provide links between smooth muscle cells. In the area of the nexus (link), the plasma membranes of neighboring cells merge with outer layers. Smooth muscle cells with the help of processes are grouped into long bundles separated by connective tissue septa. The beam diameter is about 100 µm. They branch, forming strands of transitions from one bundle to another, which is important for the activity of the muscle as a single system.
Smooth muscles are innervated by sympathetic and parasympathetic nerves. One nerve fiber can contact several cells.
The contractile apparatus of smooth muscle cells consists of protofibrils grouped into myofibrils, which are placed parallel to each other in the cell. Myofibrils contain thin filaments of protofibrils of three types: actin, myosin and intermediate. The first two types are unevenly distributed, so smooth muscle cells do not have transverse striations. Myosin filaments are short, they form dimers, from which transverse bridges with heads extend. Long actin and short myosin filaments are involved in the shortening of the smooth muscle cell during contraction. Intermediate protofibrils also take part in the contraction.
Excitability of smooth muscles. Smooth muscles are less excitable than skeletal ones: the excitability threshold is higher, and chronoxia is greater. The membrane potential of smooth muscles in various animals ranges from 40 to 70 mV. Along with Na +, K + ions, Ca ++ and Cl- ions also play an important role in creating the resting potential.
Electrical activity of many smooth muscle cells internal organs manifests itself spontaneously, i.e. cells are self-excited. Consequently, excitation is not due to the transmission of nerve impulses to the muscle, but is myogenic (as in the heart muscle) in nature. This feature is referred to as the “automaticity” of smooth muscles.
Smooth muscle contractions have significant differences compared to skeletal muscles:
1. Latent (latent) period of a single contraction smooth muscle much more than skeletal (for example, in the intestinal muscles of a rabbit, it reaches 0.25 - 1 s).
2. A single contraction of a smooth muscle is much longer than that of a skeletal one. Thus, the smooth muscles of the stomach of a frog contract for 60–80 seconds, for a rabbit, for 10–20 seconds.
3. Relaxation occurs especially slowly after contraction.
4. Due to a long single contraction, a smooth muscle can be brought into a state of long-term persistent contraction, resembling a tetanic contraction of skeletal muscles by relatively rare irritations; in this case, the interval between individual stimuli ranges from one to tens of seconds.
5. Energy expenditure during such a persistent smooth muscle contraction is very small, which distinguishes this contraction from skeletal muscle tetanus, so smooth muscles consume a relatively small amount of oxygen.
6. Slow contraction of smooth muscles is combined with great strength. For example, the muscles of the stomach of birds are capable of lifting a mass equal to 1 kg per 1 cm2 of its cross section.
7. One of the physiologically important properties of smooth muscles is the reaction to a physiologically adequate stimulus - stretching. Any stretching of smooth muscles causes them to contract. The ability of smooth muscles to respond to stretch by contraction plays an important role in the physiological function of many smooth muscle organs (eg, intestines, ureters, uterus).
Smooth muscle tone. The ability of a smooth muscle to be in tension for a long time at rest under the influence of rare impulses of irritation is called tone. Prolonged tonic contractions of smooth muscles are especially pronounced in the sphincters of hollow organs, the walls of blood vessels.
All of the above factors (tetanizing frequency of pacemaker discharges, slow filament sliding, gradual relaxation cells) contribute to long-term persistent contractions of smooth muscles without fatigue and with little energy consumption.
Plasticity and elasticity of smooth muscles. Plasticity in smooth muscles is well expressed, which is of great importance for the normal activity of smooth muscles in the walls of hollow organs: the stomach, intestines, and bladder. For example, 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. Elasticity in smooth muscles is less pronounced than in skeletal muscles, but smooth muscles are able to stretch very strongly.
Physical and physiological properties of skeletal, cardiac and smooth muscles
By morphological features There are three muscle groups:
1) striated muscles (skeletal muscles);
2) smooth muscles;
3) cardiac muscle (or myocardium).
Functions of the striated muscles:
1) motor (dynamic and static);
2) ensuring breathing;
3) mimic;
4) receptor;
5) depositor;
6) thermoregulatory.
Smooth muscle functions:
1) maintaining pressure in hollow organs;
2) regulation of pressure in blood vessels;
3) emptying of hollow organs and promotion of their contents.
Function of the heart muscle- pumping, ensuring the movement of blood through the vessels.
Physiological properties of skeletal muscles:
1) excitability (lower than in the nerve fiber, which is explained by the low value of the membrane potential);
2) low conductivity, about 10–13 m/s;
3) refractoriness (takes a longer period of time than nerve fiber);
4) lability;
5) contractility (the ability to shorten or develop tension). There are two types of reduction:
A) isotonic contraction(length changes, tone does not change);
b) isometric contraction (the tone changes without changing the length of the fiber). There are single and titanic contractions. Single contractions occur under the action of a single stimulus, and titanic contractions occur in response to a series of nerve impulses;
6) elasticity (the ability to develop stress when stretched).
Smooth muscles have the same physiological properties as skeletal muscles, but they also have their own characteristics:
1) unstable membrane potential, which maintains the muscles in a state of constant partial contraction - tone;
2) spontaneous automatic activity;
3) contraction in response to stretching;
4) plasticity (decrease in stretching with increasing stretching);
5) high sensitivity to chemicals.
Physiological feature heart muscle is her automatism. Excitation occurs periodically under the influence of processes occurring in the muscle itself. The ability to automatism have certain atypical muscle areas of the myocardium, poor in myofibrils and rich in sarcoplasm.
Structural organization of skeletal muscle. Skeletal muscle consists of many muscle fibers that have points of attachment to the bones and are parallel to each other. Each muscle fiber (myocyte) includes many subunits - myofibrils, which are built from longitudinally repeating blocks (sarcomeres). The sarcomere is the functional unit of the contractile apparatus of the skeletal muscle. Myofibrils in the muscle fiber lie in such a way that the location of the sarcomeres in them coincides. This creates a pattern of transverse striation.
motor unit. The functional unit of skeletal muscle is motor unit(DE). DE - a set of muscle fibers that are innervated by the processes of one motor neuron. Excitation and contraction of the fibers that make up one MU occur simultaneously (when the corresponding motor neuron is excited). Individual MUs can fire and contract independently of each other.
The DE includes:
1. nerve cell- mainly motor neurons, whose bodies lie in the anterior horns spinal cord;
2. motor neuron axon- myelin fibers;
3. group of muscle fibers- depending on the type of activity, the number of fibers is different. If fine work 2-4, if rough - up to several thousand.
Smooth muscles are part of the 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. The inability of tonic muscles to AP 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 varies from 20-25 ms to 1 s or more (for example, in the muscles of the 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. So 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 digestive system, the second - 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.
