Neuromuscular transmission of excitation. We have already shown above that excitation in nerve and muscle fibers is carried out using electrical impulses propagating along the surface membrane. The transfer of excitation from nerve to muscle is based on a different mechanism. It is carried out as a result of the release by nerve endings of highly active chemical compounds - mediators of the nerve impulse. At skeletal muscle synapses, such a transmitter is acetylcholine (ACh).
There are three main structural elements in the neuromuscular synapse - presynaptic membrane on the nerve postsynaptic membrane on the muscle, between them - synaptic cleft . The shape of the synapse can be varied. At rest, ACh is contained in so-called synaptic vesicles inside the end plate of the nerve fiber. The cytoplasm of the fiber with synaptic vesicles floating in it is separated from the synaptic cleft by a presynaptic membrane. When the presynaptic membrane is depolarized, its charge and permeability changes, the vesicles come close to the membrane and pour into the synaptic cleft, the width of which reaches 200-1000 angstroms. The transmitter begins to diffuse through the gap to the postsynaptic membrane.
The postsynaptic membrane is not electrogenic, but is highly sensitive to the transmitter due to the presence of so-called cholinergic receptors - biochemical groups that can selectively react with ACh. The latter reaches the postsynaptic membrane in 0.2-0.5 ms. (so-called "synaptic delay") and, interacting with cholinergic receptors, causes a change in the permeability of the membrane for Na, which leads to depolarization of the postsynaptic membrane and the generation of a depolarization wave on it, which is called excitatory postsynaptic potential, (EPSP), the value of which exceeds the Ec of neighboring, electrogenic areas of the muscle fiber membrane. As a result, an action potential (AP) arises in them, which spreads over the entire surface of the muscle fiber, then causing its contraction, initiating the so-called process. electromechanical coupling (Capling). The transmitter in the synaptic cleft and on the postsynaptic membrane works for a very short time, as it is destroyed by the enzyme cholinesterase, which prepares the synapse to receive a new portion of the transmitter. It has also been shown that part of the unreacted ACh can return to the nerve fiber.
With very frequent rhythms of stimulation, postsynaptic potentials can be summed up, since cholinesterase does not have time to completely break down the ACh released in the nerve endings. As a result of this summation, the postsynaptic membrane becomes more and more depolarized. In this case, the neighboring electrogenic areas of the muscle fiber enter a state of depression similar to that which develops during prolonged action of a direct current cathode (Verigo cathodic depression).
Functions and properties of striated muscles.
Striated muscles are an active part of the musculoskeletal system. As a result of the contractile activity of these muscles, the body moves in space, parts of the body move relative to each other, and posture is maintained. In addition, muscle work produces heat.
Each muscle fiber has the following properties: excitability , those. the ability to respond to the action of a stimulus by generating PD, conductivity - the ability to conduct excitation along the entire fiber in both directions from the point of irritation, and contractility , i.e. the ability to contract or change its tension when excited. Excitability and conductivity are functions of the surface cell membrane - the sarcolemma, and contractility - a function of myofibrils located in the sarcoplasm.
Research methods. Under natural conditions, muscle excitation and contraction are caused by nerve impulses. In order to excite a muscle in an experiment or clinical study, it is subjected to artificial irritation with an electric current. Direct irritation of the muscle itself is called direct, and irritation of the nerve is called indirect irritation. Due to the fact that the excitability of muscle tissue is less than nervous tissue, the application of electrodes directly to the muscle does not yet provide direct stimulation - the current, spreading through the muscle tissue, acts primarily on the endings of the motor nerves located in it. Pure direct irritation is obtained only with intracellular irritation or after poisoning of the nerve endings with curare. Registration of muscle contraction is carried out using mechanical devices - myographs, or special sensors. When studying muscles, electron microscopy, registration of biopotentials during intracellular abduction, and other subtle techniques are used to study the properties of muscles both experimentally and in the clinic.
Mechanisms of muscle contraction.
The structure of myofibrils and its changes during contraction. Myofibrils are the contractile apparatus of muscle fibers. In striated muscle fibers, myofibrils are divided into regularly alternating sections (discs) that have different optical properties. Some of these areas are anisotropic, i.e. are birefringent. In normal light they appear dark, but in polarized light they appear transparent in the longitudinal direction and opaque in the transverse direction. Other areas are isotropic and appear transparent in normal light. Anisotropic areas are designated by the letter A, isotropic - I. There is a light stripe in the middle of disk A N, and in the middle of disk I there is a dark stripe Z, which is a thin transverse membrane through the pores of which myofibrils pass. Due to the presence of such a supporting structure, the parallel, unambiguous discs of individual myofibrils within one fiber do not move relative to each other during contraction.
It has been established that each of the myofibrils has a diameter of about 1 μm and consists of an average of 2500 protofibrils, which are elongated polymerized molecules of myosin and actin proteins. Myosin filaments (protofibrils) are twice as thick as actin filaments. Their diameter is approximately 100 angstroms. In the resting state of the muscle fiber, the filaments are located in the myofibril in such a way that thin long actin filaments enter their ends into the spaces between thick and shorter myosin filaments. In such a section, each thick thread is surrounded by 6 thin ones. Due to this, disks I consist only of actin filaments, and disks A also consist of myosin filaments. The light stripe H represents a zone free of actin filaments during the resting period. Membrane Z, passing through the middle of disk I, holds actin filaments together.
An important component of the ultramicroscopic structure of myofibrils are also numerous cross-bridges on myosin. In turn, actin filaments have so-called active centers, which at rest are covered, like a cover, by special proteins - troponin and tropomyosin. Contraction is based on the process of sliding of actin filaments relative to myosin filaments. This sliding is caused by the work of the so-called. "chemical gear", i.e. periodically occurring cycles of changes in the state of cross bridges and their interaction with active centers on actin. ATP and Ca+ ions play an important role in these processes.
When a muscle fiber contracts, the actin and myosin filaments do not shorten, but begin to slide over each other: the actin filaments move between the myosin filaments, as a result of which the length of the I disks is shortened, and the A disks maintain their size, moving closer to each other. The H strip almost disappears because the ends of actin touch and even overlap each other.
The role of PD in the occurrence of muscle contraction (the process of electromechanical coupling). In skeletal muscle under natural conditions, the initiator of muscle contraction is the action potential, which, when excited, propagates along the surface membrane of the muscle fiber.
If the tip of the microelectrode is applied to the surface of the muscle fiber in the region of membrane Z, then when a very weak electrical stimulus is applied that causes depolarization, disks I on both sides of the site of stimulation will begin to shorten. in this case, the excitation spreads deep into the fiber, along the membrane Z. Irritation of other parts of the membrane does not cause such an effect. It follows from this that depolarization of the surface membrane in the area of disk I during AP propagation is the triggering mechanism for the contractile process.
Further studies showed that an important intermediate link between membrane depolarization and the onset of muscle contraction is the penetration of free CA++ ions into the interfibrillar space. At rest, the majority of Ca++ in muscle fiber is stored in the sarcoplasmic reticulum.
In the mechanism of muscle contraction, a special role is played by that part of the reticulum that is localized in the region of membrane Z. Electron microscopy reveals the so-called reticulum. triad (T-system), each of which consists of a thin transverse tube centrally located in the region of the membrane Z, running across the fiber, and two lateral cisterns of the sarcoplasmic reticulum, which contain bound Ca++. The PD, spreading along the surface membrane, is carried deep into the fiber along the transverse tubes of the triads. Then the excitation is transmitted to the cisterns, depolarizes their membrane and it becomes permeable to CA++.
It has been experimentally established that there is a certain critical concentration of free Ca++ ions at which contraction of myofibrils begins. It is equal to 0.2-1.5 * 10 6 ions per fiber. An increase in Ca++ concentration to 5*10 6 already causes a maximum contraction.
The onset of muscle contraction is confined to the first third of the ascending limb of the AP, when its value reaches approximately 50 mV. It is believed that it is at this magnitude of depolarization that the Ca++ concentration becomes the threshold for the onset of interaction between actin and myosin.
The process of Ca++ release stops after the end of the AP peak. Nevertheless, the contraction continues to increase until the mechanism that ensures the return of Ca++ to the reticulum cisterns comes into play. This mechanism is called the “calcium pump”. To carry out its work, the energy obtained from the breakdown of ATP is used.
In the interfibrillar space, Ca++ interacts with proteins that cover the active centers of actin filaments - troponin and tropomyosin, providing the opportunity for the reaction of myosin cross bridges and actin filaments.
Thus, the sequence of events leading to contraction and then relaxation of the muscle fiber is currently depicted as follows:
Irritation - the occurrence of AP - its conduction along the cell membrane and deep into the fiber along the tubules of T-systems - depolarization of the membrane sarcoplasmic reticulum - release of Ca++ from triads and its diffusion to myofibrils - interaction of Ca++ with troponin and release of ATP energy - interaction (sliding) of actin and myosin filaments - muscle contraction - decrease in the concentration of Ca++ in interfibrillar space due to the work of the Ca pump - muscle relaxation .
The role of ATP in the mechanism of muscle contraction. In the process of interaction between actin and myosin filaments in the presence of Ca++ ions, an important role is played by an energy-rich compound - ATP. Myosin has the properties of the ATPase enzyme. When ATP is broken down, about 10,000 calories are released. per 1 mol. Under the influence of ATP, the mechanical properties of myosin filaments also change - their extensibility sharply increases. It is believed that the breakdown of ATP is the source of energy necessary for the sliding of the filaments. Ca++ ions increase the ATPase activity of myosin. In addition, ATP energy is used to operate the calcium pump in the reticulum. Accordingly, enzymes that break down ATP are localized in these membranes, and not just in myosin.
Resynthesis of ATP, which is continuously broken down during muscle work, is carried out in two main ways. The first consists of the enzymatic transfer of a phosphate group from creatine phosphate (CP) to ADP. CP is contained in the muscle in much larger quantities than ATP, and ensures its resynthesis within thousandths of a second. However, with prolonged muscle work, CP reserves are depleted, so the second pathway is important - slow resynthesis of ATP associated with glycolysis and oxidative processes. The oxidation of lactic and pyruvic acids formed in the muscle during its contraction is accompanied by phosphorylation of ADP and creatine, i.e. resynthesis of CP and ATP.
Disruption of ATP resynthesis by poisons that suppress glycolysis and oxidative processes leads to the complete disappearance of ATP and CP, as a result of which the calcium pump stops working. The concentration of Ca++ in the area of myofibrils increases greatly and the muscle enters a state of prolonged irreversible shortening - the so-called. contractures.
Heat generation during the contractile process. According to its origin and development time, heat generation is divided into two phases. The first is many times shorter than the second and is called initial heat generation. It begins from the moment the muscle is excited and continues throughout the entire contraction, including the relaxation phase. The second phase of heat generation occurs within a few minutes after relaxation, and is called delayed or recovery heat generation. In turn, the initial heat generation can be divided into several parts - activation heat, shortening heat, relaxation heat. The heat generated in the muscles maintains the temperature of the tissues at a level that ensures the active occurrence of physical and chemical processes in the body.
Types of abbreviations. Depending on the conditions under which the reduction occurs
tion, there are two types of it - isotonic and isometric . Isotonic contraction is a muscle contraction in which its fibers are shortened, but the tension remains the same. An example is shortening without load. An isometric contraction is a contraction in which the muscle cannot shorten (when its ends are motionless). In this case, the length of the muscle fibers remains unchanged, but their tension increases (lifting an unbearable load).
Natural muscle contractions in the body are never purely isotonic or isometric.
Single cut. Stimulation of a muscle or the motor nerve innervating it with a single stimulus causes a single contraction of the muscle. It distinguishes two main phases: the contraction phase and the relaxation phase. Contraction of the muscle fiber begins already during the ascending branch of the action potential. The duration of contraction at each point of the muscle fiber is tens of times longer than the duration of the AP. Therefore, there comes a moment when the AP has passed along the entire fiber and has ended, but the wave of contraction has engulfed the entire fiber and it continues to be shortened. This corresponds to the moment of maximum shortening or tension of the muscle fiber.
The contraction of each individual muscle fiber during single contractions obeys the law " all or nothing". This means that the contraction that occurs during both threshold and superthreshold stimulation has a maximum amplitude. The magnitude of a single contraction of the entire muscle depends on the strength of the stimulation. With threshold stimulation, its contraction is barely noticeable, but with increasing strength of stimulation it increases, until it reaches a certain height, after which it remains unchanged (maximum contraction). This is explained by the fact that the excitability of individual muscle fibers is not the same, and therefore only part of them is excited with weak stimulation. With maximum contraction, they are all excited. The speed of the muscle contraction wave is the same with the speed of propagation of the action force. In the biceps brachii muscle it is 3.5-5.0 m/sec.
Summation of contractions and tetanus. If in an experiment two strong single stimulations act on a single muscle fiber or the entire muscle in rapid succession, the resulting contraction will have a greater amplitude than the maximum single contraction. The contractile effects caused by the first and second stimulation seem to add up. This phenomenon is called summation of contractions. For summation to occur, it is necessary that the interval between irritations have a certain duration - it must be longer than the refractory period, but shorter than the entire duration of a single contraction, so that the second irritation affects the muscle before it has time to relax. In this case, two cases are possible. If the second stimulus arrives when the muscle has already begun to relax, on the myographic curve the apex of the second contraction will be separated from the first by a retraction. If the second stimulation acts when the first contraction has not yet reached its peak, then the second contraction seems to merge with the first, forming together with it a single summed peak. For both full and incomplete summation, PDs are not summed up. This summed contraction in response to rhythmic stimulation is called tetanus. Depending on the frequency of irritation, it can be jagged or smooth.
The reason for the summation of contractions during tetanus lies in the accumulation of Ca++ ions in the interfibrillar space to a concentration of 5*10 6 mmol/l. After reaching this value, further accumulation of Ca++ does not lead to an increase in the amplitude of tetanus.
After the cessation of tetanic stimulation, the fibers do not relax completely at first, and their original length is restored only after some time. This phenomenon is called post-tetanic, or residual contracture. It's related to that. that it takes more time to remove from the interfibrillar space all the Ca++ that got there during rhythmic stimuli and did not have time to be completely removed into the cisterns of the sarcoplasmic reticulum by the work of Ca pumps.
If, after achieving smooth tetanus, the frequency of stimulation is increased even more, then the muscle at a certain frequency suddenly begins to relax. This phenomenon is called pessimum. It occurs when each subsequent impulse falls into refractoriness from the previous one.
Motor units. We have examined the general scheme of the phenomena underlying tetanic contraction. In order to become more familiar with how this process occurs under the conditions of natural activity of the body, it is necessary to dwell on some features of the innervation of the skeletal muscle by the motor nerve.
Each motor nerve fiber, which is a process of the motor cell of the anterior horns of the spinal cord (alpha motor neuron), branches in the muscle and innervates a whole group of muscle fibers. Such a group is called a motor unit of a muscle. The number of muscle fibers that make up a motor unit varies widely, but their properties are the same (excitability, conductivity, etc.). Due to the fact that the speed of propagation of excitation in the nerve fibers innervating skeletal muscles is very high, the muscle fibers that make up the motor unit enter a state of excitation almost simultaneously. The electrical activity of a motor unit has the appearance of a palisade, in which each peak corresponds to the total action potential of many simultaneously excited muscle fibers.
It should be said that the excitability of various skeletal muscle fibers and the motor units consisting of them varies significantly. She is more in the so-called. fast and less in slow fibers. Moreover, the excitability of both is lower than the excitability of the nerve fibers that innervate them. This depends on the fact that in the muscles the E0-E difference is greater, and, therefore, the rheobase is higher. PD reaches 110-130 mV, its duration is 3-6 ms. The maximum frequency of fast fibers is about 500 per second, most skeletal ones - 200-250 per second. The duration of AP in slow fibers is approximately 2 times longer, the duration of the contraction wave is 5 times longer, and its speed is 2 times slower. In addition, fast fibers are divided depending on the speed of contraction and lability into phasic and tonic.
Skeletal muscles in most cases are mixed: they consist of both fast and slow fibers. But within one motor unit, all fibers are always the same. Therefore, motor units are divided into fast and slow, phasic and tonic. The mixed type of muscle allows the nerve centers to use the same muscle both to carry out fast, phasic movements and to maintain tonic tension.
There are, however, muscles consisting predominantly of fast or slow motor units. Such muscles are often also called fast (white) and slow (red). The duration of the contraction wave of the fastest muscle - the internal rectus muscle of the eye - is only 7.5 ms, for the slow soleus - 75 ms. The functional significance of these differences becomes obvious when considering their responses to rhythmic stimuli. To obtain smooth tetanus of the slow muscle, it is enough to irritate it with a frequency of 13 stimuli per second. in fast muscles, smooth tetanus occurs at a frequency of 50 stimuli per second. In tonic motor units, the duration of contraction per single stimulus can reach 1 second.
Summation of motor unit contractions in a whole muscle. Unlike muscle fibers in a motor unit, which fire synchronously and simultaneously in response to an incoming impulse, muscle fibers of different motor units in a whole muscle fire asynchronously. This is explained by the fact that different motor units are innervated by different motor neurons, which send impulses at different frequencies and at different times. Despite this, the total contraction of the muscle as a whole has a continuous character under normal activity conditions. This happens because the neighboring motor unit (or units) always manage to contract before those that are already excited have time to relax. The strength of muscle contraction depends on the number of motor units involved simultaneously in the reaction and on the frequency of excitation of each of them.
Skeletal muscle tone. At rest, outside of work, the muscles in the body are not
completely relaxed, but retain some tension, called tone. The external expression of tone is a certain elasticity of the muscles.
Electrophysiological studies show that tone is associated with the arrival of rare nerve impulses to the muscle, which alternately excite different muscle fibers. These impulses arise in the motor neurons of the spinal cord, the activity of which, in turn, is supported by impulses emanating from both higher centers and from proprioceptors (muscle spindles, etc.) located in the muscles themselves. The reflex nature of skeletal muscle tone is evidenced by the fact that cutting the dorsal roots, through which sensitive impulses from the muscle spindles enter the spinal cord, leads to complete relaxation of the muscle.
Muscle work and strength. The magnitude of contraction (degree of shortening) of a muscle at a given strength of stimulation depends both on its morphological properties and on the physiological state. Long muscles contract by a greater amount than short ones. Moderate stretching of the muscle increases its contractile effect; with strong stretching, the contracted muscles relax. If, as a result of prolonged work, muscle fatigue develops, then the magnitude of its contraction decreases.
To measure muscle strength, one determines either the maximum load that it is able to lift or the maximum tension that it can develop under conditions of isometric contraction. This force can be very great. Thus, it has been established that a dog with its jaw muscles can lift a load that exceeds its body weight by 8.3 times.
A single muscle fiber can develop tension reaching 100-200 mg. Considering that the total number of muscle fibers in the human body is approximately 15-30 million, they could develop a tension of 20-30 tons if they were all pulling in the same direction at the same time.
The strength of the muscle, other things being equal, depends on its cross section. The greater the sum of the cross sections of all its fibers, the greater the load that it is able to lift. This means the so-called physiological cross-section, when the section line runs perpendicular to the muscle fibers, and not to the muscle as a whole. The strength of muscles with oblique fibers is greater than with straight ones, since its physiological cross-section is greater with the same geometric cross-section. To compare the strength of different muscles, the maximum load (absolute muscle strength) that the muscle is able to lift is divided by the physiological cross-sectional area (kg/cm2). This way the specific absolute strength of the muscle is calculated. For the human gastrocnemius muscle it is equal to 5.9 kg/cm2, the shoulder flexor muscle - 8.1 kg/cm2, the triceps brachii muscle - 16.8 kg/cm2.
Muscle work is measured by the product of the lifted load times the amount of muscle shortening. There is the following pattern between the load that a muscle lifts and the work it performs. The external work of a muscle is zero if the muscle contracts without load. As the load increases, the work first increases and then gradually decreases. The muscle does the most work at some average loads. Therefore, the dependence of work and power on the load is called rules (law) medium loads .
The work of muscles, during which the load moves and the bones move in the joints, is called dynamic. Muscle work in which the muscle fibers develop tension but almost do not shorten is static. An example is hanging on a pole. Static work is more tiring than dynamic work.
Muscle fatigue. Fatigue is a temporary decrease in performance
damage to a cell, organ or whole organism that occurs as a result of work and disappears after rest.
If you irritate an isolated muscle for a long time with rhythmic electrical stimuli, to which a small load is suspended, then the amplitude of its contractions gradually decreases until it drops to zero. A fatigue curve is recorded. Along with a change in the amplitude of contractions during fatigue, the latent period of contraction increases, the period of muscle relaxation lengthens and the threshold of irritation increases, i.e. excitability decreases. All these changes do not occur immediately after the start of work; there is a certain period during which an increase in the amplitude of contractions and a slight increase in muscle excitability are observed. At the same time, it becomes easily stretchable. In such cases, they say that the muscle is “worked in,” i.e. adapts to work in a given rhythm and strength of irritation. After a period of workability, a period of stable performance begins. With further prolonged irritation, muscle fiber fatigue occurs.
The decrease in the performance of a muscle isolated from the body during prolonged irritation is due to two main reasons. The first of them is that during contractions, metabolic products accumulate in the muscle (phosphoric acid, Ca++ binding, lactic acid, etc.), which have a depressing effect on muscle performance. Some of these products, as well as Ca ions, diffuse from the fibers out into the pericellular space and have a suppressive effect on the ability of the excitable membrane to generate AP. So, if an isolated muscle placed in a small volume of Ringer’s fluid is brought to the point of complete fatigue, then it is enough just to change the solution washing it to restore muscle contractions.
Another reason for the development of fatigue in an isolated muscle is the gradual depletion of its energy reserves. With prolonged work, the glycogen content in the muscle sharply decreases, as a result of which the processes of resynthesis of ATP and CP necessary for contraction are disrupted.
It should be noted that under the natural conditions of the organism’s existence, fatigue of the motor system during prolonged work develops completely differently than in an experiment with an isolated muscle. This is due not only to the fact that in the body the muscle is continuously supplied with blood, and, therefore, receives the necessary nutrients with it and is freed from metabolic products. The main difference is that in the body, exciting impulses come to the muscle from the nerve. The neuromuscular synapse gets tired much earlier than the muscle fiber, due to the rapid depletion of the accumulated neurotransmitter reserves. This causes a blockade of the transmission of excitations from the nerve to the muscle, which protects the muscle from exhaustion caused by prolonged work. In the whole organism, the nerve centers (nervous-nervous contacts) become tired even earlier during work.
The role of the nervous system in the fatigue of the entire organism is proven by studies of fatigue in hypnosis (weight-basket), establishing the influence of “active rest” on fatigue, the role of the sympathetic nervous system (Orbeli-Ginetzinsky phenomenon), etc.
Ergography is used to study muscle fatigue in humans. The shape of the fatigue curve and the amount of work performed varies extremely among different individuals and even among the same subject under different conditions.
Working muscle hypertrophy and atrophy from inactivity. Systematic intensive work of the muscle leads to an increase in the mass of muscle tissue. This phenomenon is called working muscle hypertrophy. It is based on an increase in the mass of protoplasm of muscle fibers and the number of myofibrils contained in them, which leads to an increase in the diameter of each fiber. At the same time, the synthesis of nucleic acids and proteins is activated in the muscle and the content of ATP and KPA, as well as glycogen, increases. As a result, the strength and speed of contraction of the hypertrophied muscle increases.
An increase in the number of myofibrils during hypertrophy is facilitated mainly by static work, which requires high tension (strength load). Even short-term exercises performed daily under isometric conditions are sufficient to increase the number of myofibrils. Dynamic muscular work, performed without much effort, does not lead to muscle hypertrophy, but can have an impact on the entire body as a whole, increasing its resistance to adverse factors.
The opposite phenomenon to working hypertrophy is muscle atrophy from inactivity. It develops in all cases when muscles for some reason lose the ability to perform their normal work. This happens, for example, when a limb is immobilized for a long time in a plaster cast, the patient stays in bed for a long time, a tendon is cut, etc. With muscle atrophy, the diameter of muscle fibers and the content of contractile proteins, glycogen, ATP and other substances important for contractile activity in them decrease sharply. When normal muscle function is resumed, atrophy gradually disappears. A special type of muscle atrophy is observed when the muscle is denervated, i.e. after cutting her motor nerve.
Smooth muscles Functions of smooth muscles in different organs.
Smooth muscles in the body are found in internal organs, blood vessels, and skin. Smooth muscles are capable of relatively slow movements and prolonged tonic contractions.
Relatively slow, often rhythmic contractions of the smooth muscles of the walls of hollow organs (stomach, intestines, ducts of the digestive glands, ureters, bladder, gall bladder, etc.) ensure the movement of contents. Prolonged tonic contractions of smooth muscles are especially pronounced in the sphincters of hollow organs; their reduction prevents the content from escaping.
The smooth muscles of the walls of blood vessels, especially arteries and arterioles, are also in a state of constant tonic contraction. The tone of the muscular layer of the artery walls regulates the size of their lumen and thereby the level of blood pressure and blood supply to organs. The tone and motor function of smooth muscles is regulated by impulses arriving through the autonomic nerves and humoral influences.
Physiological characteristics of smooth muscles. An important property of smooth muscle is its large plastic , those. the ability to maintain the length given by stretching without changing the tension. Skeletal muscle, on the contrary, immediately shortens after the load is removed. The smooth muscle remains stretched until, under the influence of some irritation, its active contraction occurs. The property of plasticity is of great importance for the normal functioning of hollow organs - thanks to it, the pressure inside a hollow organ changes relatively little with different degrees of its filling.
There are different types of smooth muscles. In the walls of most hollow organs there are muscle fibers with a length of 50-200 microns and a diameter of 4-8 microns, which are very closely adjacent to each other, and therefore, when examined under a microscope, it seems that they morphologically form one whole. Electron microscopic examination shows, however, that they are separated from each other by intercellular gaps, the width of which can be 600-1500 angstroms. Despite this, smooth muscle functions as one unit. This is expressed in the fact that AP and slow waves of depolarization propagate unhindered from one fiber to another.
In some smooth muscles, for example, in the ciliary muscle of the eye, or the muscles of the iris, the fibers are located separately, and each has its own innervation. In most smooth muscles, motor nerve fibers are located on only a small number of fibers.
The resting potential of smooth muscle fibers, which have automaticity, exhibits constant small fluctuations. Its value during intracellular abduction is 30-70 mV. The resting potential of smooth muscle fibers that do not have automaticity is stable and equal to 60-70 mV. In both cases, its value is less than the resting potential of skeletal muscle. This is due to the fact that the membrane of smooth muscle fibers at rest is characterized by a relatively high permeability to Na ions. Action potentials in smooth muscles are also slightly lower than in skeletal muscles. The excess over the resting potential is no more than 10-20 mV.
The ionic mechanism of AP occurrence in smooth muscles is somewhat different from that in skeletal muscles. It has been established that regenerative membrane depolarization, which underlies the action potential in a number of smooth muscles, is associated with an increase in membrane permeability for Ca++ ions, rather than Na+.
Many smooth muscles exhibit spontaneous, automatic activity. It is characterized by a slow decrease in the resting membrane potential, which, when a certain level is reached, is accompanied by the occurrence of AP.
Conduction of excitation through smooth muscle. In nerve and skeletal muscle fibers, excitation propagates through local electrical currents arising between the depolarized and adjacent resting areas of the cell membrane. The same mechanism is also characteristic of smooth muscles. However, unlike what occurs in skeletal muscles, in smooth muscles an action potential arising in one fiber can spread to neighboring fibers. This is due to the fact that in the membrane of smooth muscle cells in the area of contacts with neighboring ones there are areas of relatively low resistance, through which current loops that arise in one fiber easily pass to neighboring ones, causing depolarization of their membranes. In this respect, smooth muscle is similar to cardiac muscle. The only difference is that in the heart, the entire muscle is excited from one cell, and in smooth muscles, the PD that arises in one area spreads from it only to a certain distance, which depends on the strength of the applied stimulus.
Another significant feature of smooth muscles is that a spreading action potential occurs downward only if the applied stimulus simultaneously excites a certain minimum number of muscle cells. This "critical zone" has a diameter of about 100 microns, which corresponds to 20-30 parallel cells. The speed of excitation in various smooth muscles ranges from 2 to 15 cm/sec. those. significantly less than in skeletal muscle.
Just as in skeletal muscles, in smooth muscles action potentials have a trigger value for the onset of the contractile process. The connection between excitation and contraction here is also carried out with the help of Ca++. However, in smooth muscle fibers the sarcoplasmic reticulum is poorly expressed, so the leading role in the mechanism of contraction is assigned to those Ca++ ions that penetrate into the muscle fiber during the generation of action potential.
With a large force of single irritation, contraction of the smooth muscle may occur. The latent period of its contraction is much longer than the skeletal one, reaching 0.25-1 sec. The duration of the contraction itself is also long - up to 1 minute. Relaxation occurs especially slowly after contraction. The contraction wave propagates through the smooth muscles at the same speed as the excitation wave (2-15 cm/sec). But this slowness of contractile activity is combined with great force of smooth muscle contraction. Thus, the muscles of the stomach of birds are capable of lifting 2 kg per 1 sq. mm. its cross section.
Due to the slowness of contraction, smooth muscle, even with rare rhythmic stimulation (10-12 per minute), easily goes into a long-term state of persistent contraction, reminiscent of skeletal muscle tetanus. However, the energy costs for such a reduction are very low.
The ability to automate smooth muscles is inherent in their muscle fibers 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 intestinal wall muscles freed from nervous elements. Smooth muscle reacts to all external influences by changing the frequency of spontaneous rhythms, resulting in contractions or relaxations of the muscle. The effect of irritation of intestinal smooth muscles depends on the relationship between the frequency of stimulation and the natural frequency of spontaneous rhythms: with low tone - rare spontaneous PD - the applied irritation increases the tone; with high tone, relaxation occurs in response to irritation, since excessive acceleration of impulses leads to each subsequent impulse falls into a refractory phase from the previous one.
Smooth muscle irritants. One of the important physiologically adequate stimuli of smooth muscles is their rapid and strong stretching. It causes depolarization of the muscle fiber membrane and the occurrence of a spreading action potential. As a result, the muscle contracts. A characteristic feature of smooth muscles is their high sensitivity to certain chemical stimuli, in particular to acetylcholine, norepinephrine, adrenaline, histamine, serotonin, and prostaglandins. The effects caused by the same chemical agent may be different in different muscles and under different conditions. Thus, ACh excites the smooth muscles of most organs, but inhibits the vascular muscles. Adrenaline relaxes the non-pregnant uterus, but contracts the pregnant one. These differences are due to the fact that these agents react on the membrane with different chemical receptors (cholinergic receptors, alpha and beta adrenergic receptors), and ultimately change the ionic permeability and membrane potential of smooth muscle cells differently. In cases where an irritating agent causes depolarization of the membrane, excitation occurs, and, conversely, hyperpolarization of the membrane under the influence of a chemical agent leads to inhibition of activity and relaxation of the smooth muscle.
Introduction
The basis of all types of muscle contraction is the interaction of actin and myosin. In skeletal muscle, myofibrils (about two-thirds of the dry weight of muscle) are responsible for contraction. Myofibrils are structures 1 - 2 µm thick, consisting of sarcomeres - structures about 2.5 µm long, consisting of actin and myosin (thin and thick) filaments and Z-disks connected to actin filaments. Contraction occurs with an increase in the concentration of Ca 2+ ions in the cytoplasm as a result of the sliding of myosin filaments relative to actin filaments. The source of contraction energy is ATP. The efficiency of a muscle cell is about 50%.
Myosin sliding relative to actin
Myosin heads break down ATP and, due to the released energy, change conformation, sliding along actin filaments. The cycle can be divided into 4 stages:
- The free myosin head binds to ATP and hydrolyzes it to ADP and phosphate and remains associated with them. (A reversible process - the energy released as a result of hydrolysis is stored in the changed conformation of myosin).
- The heads bind weakly to the next actin subunit, the phosphate is released, and this leads to strong binding of the myosin head to the actin filament. This reaction is already irreversible.
- The head undergoes a conformational change that pulls the thick filament toward the Z-disc (or, equivalently, the free ends of the thin filaments toward each other).
- ADP is released, due to this the head is separated from the actin filament. A new ATP molecule attaches.
The cycle is then repeated until the concentration of Ca 2+ ions decreases or the ATP supply is exhausted (as a result of cell death). The speed of myosin sliding along actin is ≈15 μm/sec. There are many (about 500) myosin molecules in the myosin filament and, therefore, during contraction, the cycle is repeated by hundreds of heads at once, which leads to fast and strong contraction. It should be noted that myosin behaves like an enzyme - an actin-dependent ATPase. Since each repetition of the cycle is associated with ATP hydrolysis, and therefore with a positive change in free energy, the process is unidirectional. Myosin moves along actin only towards the plus end.
Successive stages
Source of energy for reduction
To contract a muscle, the energy of ATP hydrolysis is used, but the muscle cell has an extremely efficient system for regenerating the ATP supply, so that in a relaxed and working muscle the ATP content is approximately equal. The enzyme phosphocreatine kinase catalyzes the reaction between ADP and creatine phosphate, the products of which are ATP and creatine. Creatine phosphate contains more stored energy than ATP. Thanks to this mechanism, during a burst of activity in the muscle cell, the content of creatine phosphate drops, but the amount of the universal energy source - ATP - does not change. Mechanisms for regenerating ATP reserves may vary depending on the partial pressure of oxygen in surrounding tissues (see Anaerobic Organisms).
Regulatory mechanism
Mostly neurons are involved in the regulation of muscle activity, but there are cases where hormones (for example, adrenaline and oxytocin) also control smooth muscle contraction. The contraction signal can be divided into several stages:
From cell membrane to sarcoplasmic reticulum
The effect of a transmitter released from a motor neuron causes an action potential on the cell membrane of the muscle cell, which is transmitted further using special membrane invaginations called T-tubules, which extend from the membrane into the cell. From the T-tubules, the signal is transmitted to the sarcoplasmic reticulum - a special compartment of flattened membrane vesicles (endoplasmic reticulum of the muscle cell) surrounding each myofibril. This signal causes the opening of Ca 2+ channels in the reticulum membrane. Back Ca 2+ ions enter the reticulum with the help of membrane calcium pumps - Ca 2+ -ATPase.
From the release of Ca 2+ ions to the contraction of myofibrils
The mechanism of muscle contraction taking into account troponin and tropomyosin
In order to control contraction, the protein tropomyosin and a complex of three proteins - troponin (the subunits of this complex are called troponins T, I and C) are attached to the actin filament. Troponin C is a close homologue of another protein, calmodulin. There is only one troponin complex located every seven actin subunits. The binding of actin to troponin I moves tropomyosin to a position that interferes with the binding of myosin to actin. Troponin C binds to four Ca 2+ ions and weakens the effect of troponin I on actin, and tropomyosin occupies a position that does not interfere with the connection of actin with myosin.
Major proteins of myofibrils
| Protein | Protein % | His pier. mass, kDa | Its function |
|---|---|---|---|
| Myosin | 44 | 510 | Main component of thick filaments. Forms bonds with actin. Moves along actin due to ATP hydrolysis. |
| Actin | 22 | 42 | Main component of thin filaments. During muscle contraction, myosin moves along it. |
| Titin | 9 | 2500 | A large flexible protein that forms a chain to bind myosin to the Z-disc. |
| Troponin | 5 | 78 | A complex of three proteins that regulates contraction when bound to Ca 2+ ions. |
| Tropomyosin | 5 | 64 | A rod-shaped protein associated with actin filaments that blocks myosin movement. |
| Nebulin | 3 | 600 | A long, inextensible protein associated with the Z-disk and running parallel to actin filaments. |
Literature
- B. Alberts, D. Bray, J. Lewis, M. Reff, K. Roberts, J. Watson, Molecular biology of the cell - In 3 volumes - Trans. from English - T.2. - M.: Mir, 1994. - 540 p.
- M. B. Berkinblit, S. M. Glagolev, V. A. Furalev, General biology - In 2 parts - Part 1. - M.: MIROS, 1999. - 224 p.: ill.
see also
| Muscular system | |
|---|---|
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See what “Muscle contraction” is in other dictionaries:
MUSCLE CONTRACTION- shortening or tension of muscles in response to irritation caused by motor discharge. neurons. The M. s model has been adopted, according to which, when the surface of the muscle fiber membrane is excited, the action potential first spreads through the system... ... Biological encyclopedic dictionary
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muscle contraction- see Abbreviation... Large medical dictionary
Muscle contraction- shortening of a muscle, as a result of which it produces mechanical work. M. s. provides the ability of animals and humans to voluntary movements. The most important component of muscle tissue (See Muscle tissue) proteins (16.5... ...
MUSCLE CONTRACTION- the main function of muscle tissue is the shortening or tension of muscles in response to irritation caused by the discharge of motor neurons. M. s. underlies all movements of the human body. There are M. s. isometric, when the muscle develops force... ... Psychomotorics: dictionary-reference book
Isometric muscle contraction- contraction of a muscle, expressed in an increase in its tension with a constant length (for example, contraction of a muscle of a limb, both ends of which are fixed motionless). In the body to I. m.s. the tension developed by the muscle when attempting... is approaching. Great Soviet Encyclopedia
Muscle contraction is a complex mechano-chemical process during which the chemical energy of the hydrolytic breakdown of ATP is converted into mechanical work performed by the muscle.
At present, this mechanism has not yet been fully disclosed. But the following is certain:
1. The source of energy necessary for muscle work is ATP;
2. ATP hydrolysis, accompanied by the release of energy, is catalyzed by myosin, which, as already noted, has enzymatic activity;
3. The trigger mechanism for muscle contraction is an increase in the concentration of Ca 2+ ions in the sarcoplasm of myocytes, caused by a motor nerve impulse;
4. During muscle contraction, cross bridges or adhesions arise between thick and thin filaments of myofibrils;
5. During muscle contraction, thin filaments slide along thick filaments, which leads to shortening of myofibrils and the entire muscle fiber as a whole.
There are many hypotheses trying to explain the molecular mechanism of muscle contraction. The most justified at present is hypothesis « rowing boat» or « rowing hypothesis» H. Huxley. In a simplified form, its essence is as follows.
In a muscle at rest, thick and thin filaments of myofibrils are not connected to each other, since the binding sites on actin molecules are covered by tropomyosin molecules.
Muscle contraction occurs under the influence of a motor nerve impulse, which is a wave of increased membrane permeability propagating along the nerve fiber. This wave of increased permeability is transmitted through the neuromuscular junction to the T-system of the sarcoplasmic reticulum and ultimately reaches cisterns containing high concentrations of calcium ions. As a result of a significant increase in the permeability of the tank walls ( this is also a membrane!) Calcium ions leave the tanks and their concentration in the sarcoplasm in a very short time ( about 3 ms) increases approximately 1000 times. Calcium ions, being in high concentration, attach to the protein of thin filaments - troponin and change its spatial shape ( conformation). A change in the conformation of troponin, in turn, leads to the fact that tropomyosin molecules are displaced along the groove of fibrillar actin, which forms the basis of thin filaments, and release that portion of actin molecules that is intended for binding to myosin heads. As a result, between myosin and actin ( those. between thick and thin threads) a transverse bridge appears, located at an angle of 90 º . Since thick and thin filaments contain a large number of myosin and actin molecules (about 300 each), then a fairly large number of transverse bridges or adhesions are formed between the muscle threads. In the electron micrograph ( rice. 15) it is clearly visible that between the thick and thin threads there are a large number of transversely located bridges.
Rice. 15. Electron micrograph of longitudinal cut
myofibril area(magnification 300,000 times)(L. Streiner, 1985)
The formation of a bond between actin and myosin is accompanied by an increase in the ATPase activity of the latter ( those. actin acts as an allosteric enzyme activator), resulting in ATP hydrolysis:
ATP + H 2 O ¾® ADP + H 3 PO 4 + energy
Muscle contraction is based on the movement of actin filaments relative to myosin filaments. Actin filaments move, as if through a tunnel, between myosin fibrils, due to the formation of bonds with myosin. As a result of this, the sarcomere is shortened (A. Huxley’s “sliding threads” hypothesis) (Fig. 7.29). At the same time, the length of the 1-discs decreases, while the A-discs retain their size.
Sliding of actin and myosin filaments relative to each other is possible only in the presence of Ca 2+ ions and ATP, which is formed during the breakdown of glycogen, glucose and fatty acids. Muscles are characterized by active metabolism. They are connected by a large number of blood and lymphatic vessels, as well as nerves. The latter form synaptic contacts with muscle fibers.
The entire chain of events during muscle contraction can be represented as follows: at the neuromuscular synapse, under the influence of impulses coming from the central nervous system, a mediator is released to the nerve fiber acetylcholine, depolarizing the muscle fiber membrane. The resulting impulse spreads along the fiber membrane and T-tubules and is transmitted to the membrane of the sarcoplasmic reticulum, from which calcium is released into the sarcoplasm. Calcium ions promote the formation of the actomyosin complex and the breakdown of ATP; The energy released during this process ensures the sliding of thin actin filaments along the myosin filaments.
Rice. 7.29.
Changes in the relative position of myofibrils during relaxation (b) and contraction (c) of muscle fiber
Muscle relaxation is associated with the return of Ca 2+ into the sarcoplasmic reticulum, which occurs with the participation of active mechanisms associated with the operation of ion pumps. If the concentration of calcium ions in the sarcoplasm decreases and they are pumped into the endoplasmic reticulum, then the contraction of the muscle fiber stops.
Human skeletal muscle consists of several types of muscle fibers with different structural and functional characteristics. There are four main types of muscle fibers: slow phasic oxidative fibers, fast phasic oxidative fibers, fast phasic oxidative fibers with glycolytic oxidation type, and tonic fibers.
Slow phasic muscle fibers of the oxidative type contain a large amount of myoglobin protein, which binds 0 2 . This protein is similar to red blood cell hemoglobin and gives muscle fibers a dark red color. Muscles consisting predominantly of these fibers are involved in maintaining human posture. Fatigue develops very slowly in them, and functions are restored very quickly.
Muscles consisting primarily of fast phasic fibers of the oxidative type, perform quick contractions without noticeable fatigue. This is due to the presence of a large number of mitochondria in the fibers and a good ability to synthesize ATP. The main purpose of such fibers is to perform fast, energetic movements.
Tonic fibers contract and relax slowly, since ATP activity in them is low. Such fibers are part of some eye muscles.
Most human skeletal muscles consist of muscle fibers of various types with a predominance of one of them, depending on the functions that a particular muscle performs.
The main physiological property of muscles is contractility - manifests itself in the ability of a muscle to shorten or develop tension. There are two types of muscle contractions - isotonic and isometric. At isotonic contraction, the muscle fibers are shortened, but the tension remains constant. At isometric - the muscle cannot shorten, the length of the muscle fibers remains unchanged, since both ends are fixed, but the tension increases as they contract.
In relation to the whole organism, a different classification of contraction types is used: isometric is a contraction in which the length of the muscle does not change, concentric is a contraction in which the muscle is shortened, eccentric is a contraction that lengthens (for example, when a load is slowly lowered). Natural movements usually involve all three types of muscle contraction.
The functional unit of skeletal muscle is not considered to be an individual muscle fiber, but neuromotor, or motor unit, which includes several muscle fibers innervated by the spinal cord motor neuron (Fig. 7.30, 7.31). In response to impulses coming from the motor neuron, all muscle fibers included in the neuromotor unit contract.
The number of muscle fibers that make up a motor unit, the speed of their contraction, and their resistance to fatigue are not the same. Depending on their properties, motor units are divided into fast ( phase) ) slow ( tonyFig. 7.30.Motor units
Czech) And transitional. The motor units of each muscle are not the same. Muscles that provide precise and rapid movements (for example, the muscles of the fingers) consist mainly of several hundred or thousands of fast motor units. In most mixed-type muscles, slow motor units are the first to be activated, developing
Rice. 7.31
a,6 - neuromuscular junction; V - electronic scanning
microscopy shows a small contraction force, and with increasing excitation, muscle fibers are involved in contraction, developing greater force. Activation of fast neural units ensures precise motor responses.
Under natural conditions, the muscle receives from the central nervous system not single impulses, but a series of impulses, to which it reacts not with a single, but with a long duration ( tetanic) abbreviation. It is due to the fact that each subsequent impulse comes at a moment when the previous wave of contraction has not yet ended. The latter, summed up with the previous one, prolongs muscle contraction. If each new wave of contraction occurs at a moment when the muscle has already begun to relax under the influence of previous irritation, serrated tetanus. With a shorter interval between stimulation, when each new wave of contraction occurs before the onset of muscle relaxation, a continuous or smooth, tetanus. Individual muscle fibers, when naturally stimulated by a nerve, respond to each impulse with a single contraction. Fusion tetanus is obtained due to the summation of contractions of individual muscle fibers. Typically, the muscle fibers of one muscle do not contract simultaneously, since impulses from different motor neurons of the central nervous system also do not arrive at the same time. This contributes to the formation and maintenance of a continuous tetanic contraction of the muscle.
By contracting, a muscle does work. Job muscles depends on the strength of their contraction, and the strength of contraction of the same muscle depends on the number of neuromotor units participating in it. The more there are, the more intense the contraction. The strength of contraction also depends on the frequency of stimulation. Up to a certain limit, an increase in the frequency of stimulation is accompanied by an increase in the force of muscle contraction. This is due to the fact that with increasing frequency of stimulation, an increasing number of muscle fibers are included in the reaction. The maximum tension that a muscle can develop is determined by the number of fibers that form it: the greater it is, the greater the muscle strength. In this regard, the pennate muscles, consisting of many fibers, are more powerful.
The manifestation of muscle strength also depends on the characteristics of its attachment to the bones. Muscles with a larger area of attachment or support have a greater ability to exert force. The place where muscle force is applied is also important. Bones, together with the muscles attached to them, are levers, so the closer to the point of application of gravity or the further from the fulcrum of the lever and closer to the point of application of gravity a muscle is attached, the greater the force it can develop (Fig. 7.32).
The dependence of muscle strength on such factors is clearly manifested in the activity of the muscles of the upper and lower extremities. The upper limb is designed to perform a variety of precise and quick movements. The function of the lower extremities requires great muscle strength. These functional tasks also correspond to the nature of the attachment of the corresponding muscles. Thus, the deltoid muscle, located in the area of the shoulder joint, has a small support surface and is attached to the humerus near the place where the lever rests. The muscles of the lower extremities have a large support area and the point of application of force is far from the fulcrum. The gluteal muscle has a support area 23 times larger than the deltoid muscle, and the attachment area is 4.5 times larger.
There is no directly proportional relationship between muscle strength and the amount of its shortening. The maximum shortening of the muscle, and therefore the intensity of the contraction caused by this shortening of the movement, in a particular joint depends on the length of the muscle fibers. It is greatest in muscles with parallel fibers, while pennate muscles have greater strength. When an initially stretched muscle contracts, it shortens by a greater amount.
The work of a muscle during contraction is equal to the product of the mass of the load and the height raised. It follows that the maximum work performed during a single contraction of a muscle depends on its strength (the greater the force, the greater the load that can be lifted) and the degree of shortening of the muscle. In pro-
Rice. 732.
A - balance lever; b - speed lever. The triangle is the fulcrum; dark arrows show the direction of muscle traction forces; dotted arrows - direction of gravity; dotted arrow - movement during natural human activity; the amount of work performed by a particular muscle largely depends on its ability to remain in a contracted state for a long time (endurance muscles). There is a distinction between endurance to static and dynamic forces. Static force endurance is determined by the time during which a given force is maintained. It is different for different muscles. The least endurance is characterized by the triceps brachii muscle (1 min - with an effort equal to 50% of the maximum), the greatest - by the calf muscle (7 min).
Endurance for long-term work depends not only on the size of the load being lifted, but also on the pace of work. The work is greatest at some average load and frequency of movements. For each type of muscle activity, you can select a certain average (optimal) rhythm and load value, at which the work will become maximum, and fatigue will develop gradually.
Muscle work is a necessary condition for their contraction. Prolonged inactivity leads to muscle atrophy and loss of performance. Moderate systematic work of muscles helps to increase their volume, increase strength and performance, which is important for the physical development of the whole organism.
With prolonged dynamic or static work, muscle fatigue occurs. Fatigue is a temporary decrease in the performance of a cell, organ or entire organism that occurs as a result of work and disappears after rest. Under natural conditions, fatigue is associated primarily with changes occurring in the nervous system, in particular with disruption of the conduction of excitation in interneuronal and neuromuscular synaptic contacts. The rate of onset of fatigue depends on the state of the nervous system, the rhythm in which the work is performed, and the magnitude of the load. After rest, performance is restored. I.M. Sechenov was the first to show (in 1903) that the restoration of the performance of tired muscles of a person’s arm after prolonged work on lifting a load occurs faster if, during the rest period, work is done with the other arm or leg. This kind of rest was called active.
Alternating mental and physical labor, dynamic pauses before and during classes help improve the performance of children and adults. The smaller the child, the faster he develops fatigue. In infancy, fatigue occurs after 1.5-2 hours of normal wakefulness. Children also become tired when they are immobilized or have their movements limited for a long time.
Human muscles, even at rest, are in a somewhat contracted state. Long-term holding voltage is called muscle tone. During sleep or under anesthesia, muscle tone decreases and as a result the body relaxes. Tonic muscle contractions do not lead to the development of fatigue. Complete disappearance of muscle tone is observed only after death. The preservation of tone is due to the constant supply to the muscle of nerve impulses following each other at large intervals from motor neurons C11C. The activity of these neurons is supported by impulses coming from the overlying parts of the central nervous system and from muscle receptors - muscle spindles.
Muscle tone plays an important role in the coordination of movements. In newborns and infants, the tone of the flexor muscles predominates, caused by increased excitability of the red nucleus of the midbrain. As the pyramidal system of the brain and neurons of the cerebral cortex functionally mature, muscle tone in children decreases. This clearly manifests itself in the second half of a child’s life and is a necessary condition for the development of walking. By three to five years of age, a balance in the tone of the antagonist muscles is established.
As soon as nerve impulses stop entering the muscle fiber, Ca2 ions, under the action of the so-called calcium pump, due to the energy of ATP, go into the cisterns of the sarcoplasmic reticulum and their concentration in the sarcoplasm decreases to the initial level. This causes changes in the conformation of troponin, which, by fixing tropomyosin in a certain area of actin filaments, makes it impossible for the formation of cross bridges between thick and thin filaments. Due to the elastic forces that arise during muscle contraction in the collagen threads surrounding the muscle fiber, it returns to its original state upon relaxation. Thus, the process of muscle relaxation, or relaxation, as well as the process of muscle contraction, is carried out using the energy of ATP hydrolysis.
During muscle activity, the processes of contraction and relaxation alternately occur in the muscles and, therefore, the speed-strength qualities of the muscles equally depend on the speed of muscle contraction and on the ability of the muscles to relax.
CONCLUSION
Having considered the concepts of “muscle” and “muscle contraction”, a number of conclusions can be drawn.
Muscle fiber is a multinuclear structure surrounded by a membrane and containing a specialized contractile apparatus - myofibrils.
During the process of muscle contraction, potential chemical energy is converted into potential mechanical energy of tension and kinetic energy of movement. The basis of all types of muscle contraction is the interaction of primeactin and myosin.
In skeletal muscles, two-thirds of the dry weight of the muscle is responsible for contraction. Contraction occurs with an increase in the concentration of Ca 2+ ions in the cytoplasm as a result of the sliding of myosin filaments relative to actin filaments.
The immediate source of energy for muscle contraction is the breakdown of the high-energy substance ATP. An intermediate reaction also occurs in the muscle, involving a second high-energy substance - creatine phosphate (CP). It cannot act as a direct source of energy because its breakdown does not affect the contractile proteins of the muscle. CP provides energy for ATP resynthesis. In turn, the energy for the resynthesis of CP is provided by oxidation.
Muscle fiber contraction involves shortening the myofibrils within each sarcomere. Thick (myosin) and thin (actin) filaments, in a relaxed state, connected only by the terminal sections, at the moment of contraction carry out sliding movements towards each other. The release of energy necessary for contraction occurs as a result of the conversion of ATP into ADP under the influence of myosin. The enzymatic activity of myosin manifests itself under conditions of optimal Ca2+ content, which accumulate in the sarcoplasmic reticulum.
The entire process from the appearance of a muscle action potential to the contraction of a muscle fiber is called electromechanical coupling (or electromechanical coupling).
The efficiency of a muscle cell is about 50%, the muscle as a whole is no more than 20%. Maximum muscle strength is not achieved under real-life conditions; Not all muscle cells are used at the same time and contract with maximum force, otherwise, when many skeletal muscles contract, tendons or bones will be damaged (which is sometimes observed with severe cramps). Muscle efficiency also depends on external conditions; for example, in the cold it decreases significantly, since it is more important for the body to maintain body temperature.
In dynamic mode, muscle performance is determined by the rate of breakdown and resynthesis of ATP. In this case, the rate of ATP breakdown can increase 100 times or more. ATP resynthesis can be achieved through the oxidative breakdown of glucose. Indeed, under moderate loads, ATP resynthesis is ensured by increased muscle consumption of glucose and oxygen. This is accompanied by an increase in blood flow through the muscles by approximately 20 times, an increase in cardiac output and respiration by 2-3 times. In trained individuals (for example, an athlete), an increase in the activity of mitochondrial enzymes plays a major role in ensuring the body’s increased need for energy.
At maximum physical activity, additional breakdown of glucose occurs through anaerobic glycolysis. During these processes, ATP resynthesis occurs several times faster and the mechanical work performed by the muscles is also greater than during aerobic oxidation. The maximum time for this type of work is about 30 seconds, after which an accumulation of lactic acid occurs, i.e., metabolic acidosis, and fatigue develops.
Anaerobic glycolysis also occurs at the beginning of long-term physical work, until the rate of oxidative phosphorylation increases so that ATP resynthesis again equals its breakdown. After metabolic restructuring, the athlete gains a kind of second wind. Detailed diagrams of metabolic processes are given in biochemistry manuals.
The basis of muscle contraction is biochemical processes that occur in 2 phases: the first, anaerobic (oxygen-free), and the second, aerobic (oxygen). In each of these phases, substances are broken down with the release of energy and their restoration (resynthesis). Therefore, a muscle deprived of oxygen can work for a long time, provided that residual metabolic products are removed.
During muscle activity, the processes of contraction and relaxation alternately occur in the muscles and, therefore, the speed-strength qualities of the muscles equally depend on the speed of muscle contraction and on the ability of the muscles to relax.
GLOSSARY
Actin - a muscle fiber protein involved in contractile processes in the cell. Contained mainly in muscle tissue cells.
ATP – adenylpyrophosphoric acid, a nucleotide containing adenine, ribose and three phosphoric acid residues, a universal transporter and
the main accumulator of chemical energy in living cells, released during the transfer of electrons in the respiratory chain.
Afferent fiber - centripetal nerve fiber (nerve cell processes) through which excitation is transmitted from tissues to the central nervous system.
Smooth muscle- contractile tissue, consisting of cells and without transverse striations.
Dephosphorylation - cleavage of a phosphoric acid residue from a molecule of a phosphorus-containing compound.
Kinesthesia – sensation of the position and movement of individual parts of the body, resistance and heaviness of external objects.
Myosin - muscle fiber protein; forms the main contractile element of muscles, actomyosin, with actin.
Myofibril - organelles of striated muscle cells that ensure their contraction and serve for contraction of muscle fibers.
Muscle contraction - the reaction of muscle cells to the influence of a neurotransmitter, less often a hormone, manifested in a decrease in cell length.
Muscle tissue - tissues that are different in structure and origin, but similar in their ability to undergo pronounced contractions, consisting of elongated cells that receive irritation from the nervous system and respond to it with contraction.
Muscles - organs of the body of animals and humans, consisting of elastic, elastic muscle tissue, capable of contracting under the influence of nerve impulses, designed to perform various actions: body movement, contraction of the vocal cords, breathing.
Persynaptic membrane - a section of the surface membrane of a nerve fiber through which the transmitter is released into the synaptic cleft; structural element of a synapse.
Postsynaptic membrane - a thick cell surface membrane in the synapse area that is sensitive to transmitters.
Relaxation - a state of rest, relaxation that occurs in the subject as a consequence of the release of tension after strong experiences or physical effort.
Resynthesis - the process of reverse restoration of the original complex chemical compound from “fragments” formed during its breakdown or metabolism.
Synapse - the site of contact between two neurons or between a neuron and a signal-receiving effector cell.
Phosphorylation - the process of transferring a phosphoric acid residue from a phosphorylating donor agent.
CNS – central nervous system
Efferent fiber – centrifugal nerve fibers along which excitation is transmitted from the central nervous system (from the cell) to the tissues.
BIBLIOGRAPHY
1. Physical culture of a student: Textbook / Ed. IN AND. Ilyinich. M.: Gardariki, 2000. - 448 p.
2. Physical culture. Series "Textbooks, teaching aids". Rostov-n/D: Phoenix, 2003. - 384 p.
3. www.wikipedia.ru
