SKELETAL MUSCLES

There are three types of muscle tissue in the human body: skeletal (striated), smooth and cardiac muscle. Here we will look at the skeletal muscles that form the muscles of the musculoskeletal system, make up the walls of our body and some internal organs(esophagus, pharynx, larynx). If all muscle tissue is taken as 100%, then skeletal muscle accounts for more than half (52%), smooth muscle tissue accounts for 40%, and cardiac muscle accounts for 8%. Skeletal muscle mass increases with age (up to mature age), and in older people, muscles atrophy, since there is a functional dependence of muscle mass on their function. In an adult, skeletal muscles make up 40-45% of the total body weight, in a newborn - 20-24%, in the elderly - 20-30%, and in athletes (especially representatives of speed-strength sports) - 50% or more. The degree of muscle development depends on the characteristics of the constitution, gender, profession and other factors. In athletes, the degree of muscle development is determined by the nature of motor activity. Systematic physical activity leads to structural restructuring of muscles, increasing their mass and volume. This process of muscle restructuring under the influence of physical activity is called functional (working) hypertrophy. Physical exercise associated with various sports cause working hypertrophy of those muscles that are most loaded. Properly dosed physical exercise causes proportional development of the muscles of the whole body. The active activity of the muscular system affects not only the muscles, it also leads to the restructuring of bone tissue and bone joints, affecting external forms human body and its internal structure.

Together with bones, muscles make up the musculoskeletal system. If bones are its passive part, then muscles are the active part of the movement apparatus.

Functions and properties of skeletal muscles . Thanks to muscles, all the variety of movements between the parts of the skeleton (torso, head, limbs), movement of the human body in space (walking, running, jumping, rotation, etc.), fixation of body parts in certain positions, in particular maintaining a vertical position of the body, is possible .

With the help of muscles, the mechanisms of breathing, chewing, swallowing, speech are carried out; muscles influence the position and function of internal organs, promote the flow of blood and lymph, and participate in metabolism, in particular heat exchange. In addition, muscles are one of the most important analyzers that perceive the position of the human body in space and the relative position of its parts.

Skeletal muscle has the following properties:

1) excitability- ability to respond to a stimulus:

2) contractility- the ability to shorten or develop tension when excited;

3) elasticity- the ability to develop tension when stretching;

4) tone- under natural conditions, skeletal muscles are constantly in a state of some contraction, called muscle tone, which is of reflex origin.

The role of the nervous system in regulating muscle activity . The main property of muscle tissue is contractility. The contraction and relaxation of skeletal muscles is subject to human will. Muscle contraction is caused by an impulse coming from the central nervous system, to which each muscle is connected by nerves containing sensory and motor neurons. Sensitive neurons, which are conductors of “muscular feeling,” transmit impulses from receptors in the skin, muscles, tendons, and joints to the central nervous system. Motor neurons carry impulses from the spinal cord to the muscle, causing the muscle to contract, i.e. Muscle contractions in the body occur reflexively. At the same time, the motor neurons of the spinal cord are influenced by impulses from the brain, in particular from the cerebral cortex. This makes the movements voluntary. By contracting, muscles move parts of the body, cause the body to move or maintain a certain posture. Sympathetic nerves also approach the muscles, thanks to which the muscle in a living organism is always in a state of some contraction, called tone. When performing sports movements, the cerebral cortex receives a stream of impulses about the place and degree of tension of certain muscle groups. The resulting sensation of parts of your body, the so-called “muscular-joint feeling,” is very important for athletes.

The muscles of the body should be considered from the point of view of their function, as well as the topography of the groups into which they are folded.

Muscle as an organ. Structure of skeletal muscle . Each muscle is a separate organ, i.e. a holistic formation that has its own specific form, structure, function, development and position in the body. The composition of a muscle as an organ includes striated muscle tissue, which forms its basis, loose and dense connective tissue, blood vessels, and nerves. However, the predominant muscle tissue in it is the main property of which is contractility.

Rice. 69. Muscle structure:

1- muscular abdomen; 2,3 - tendon ends;

4-striated muscle fiber.

Each muscle has a middle part that can contract and is called belly, And tendon ends(tendons), which do not have contractility and serve to attach muscles (Fig. 69).

Belly of muscle(Fig. 69 - 71) contains bundles of muscle fibers of varying thickness. Muscle fiber(Fig. 70, 71) is a layer of cytoplasm containing nuclei and covered with a membrane.

Rice. 70. The structure of muscle fiber.

Along with the usual components of the cell, the cytoplasm of muscle fibers contains myoglobin, which determines the color of muscles (white or red) and organelles special meaning - myofibrils(Fig. 70), making up the contractile apparatus of muscle fibers. Myofibrils consist of two types of proteins - actin and myosin. In response to a nerve signal, actin and myosin molecules react, causing contraction of the myofibrils, and, consequently, the muscle. Individual sections of myofibrils refract light differently: some of them in two directions - dark disks, others in only one direction - light disks. This alternation of dark and light areas in the muscle fiber determines the transverse striation, which is where the muscle gets its name - striated. Depending on the predominance of fibers with high or low myoglobin content (red muscle pigment) in the muscle, red and white muscles are distinguished (respectively). White muscles have high contractile speed and the ability to develop great force. Red fibers contract slowly and have good endurance.

Rice. 71. Structure of skeletal muscle.

Each muscle fiber is enveloped in a connective tissue sheath - endomysium containing blood vessels and nerves. Groups of muscle fibers, uniting with each other, form muscle bundles, surrounded by a thicker connective tissue membrane called perimysium. Outside, the belly of the muscle is covered with an even more dense and durable cover, which is called fascia, formed by dense connective tissue and having a rather complex structure (Fig. 71). Fascia divided into superficial and deep. Superficial fascia lie directly under the subcutaneous fat layer, forming a kind of case for it. Deep (proper) fascia cover individual muscles or groups of muscles, and also form sheaths for blood vessels and nerves. Due to the presence of connective tissue layers between the bundles of muscle fibers, the muscle can contract not only as a whole, but also as a separate part.

All connective tissue formations of the muscle pass from the muscle belly to the tendon ends (Fig. 69, 71), which consist of dense fibrous connective tissue.

Tendons in the human body are formed under the influence

the magnitude of muscle force and the direction of its action. The greater this force, the more the tendon grows. Thus, each muscle has a characteristic tendon (both in size and shape).

Tendons are very different in color from muscles. The muscles are red-brown in color, and the tendons are white and shiny. The shape of muscle tendons is very diverse, but long narrow or flat wide tendons are more common (Fig. 71, 72, 80). Flat, wide tendons are called aponeuroses(abdominal muscles, etc.), they are mainly found in the muscles involved in the formation of the walls of the abdominal cavity. The tendons are very strong and strong. For example, the calcaneal tendon can withstand a load of about 400 kg, and the quadriceps tendon can withstand a load of 600 kg.

The tendons of the muscle are fixed or attached. In most cases, they are attached to bone parts of the skeleton, movable in relation to each other, sometimes to the fascia (forearm, lower leg), to the skin (in the face) or to organs (muscles of the eyeball). One end of the tendon is the beginning of the muscle and is called head, the other is the place of attachment and is called tail. The origin of the muscle is usually taken to be its proximal end (proximal support), located closer to the midline of the body or to the torso, and the place of attachment is the distal part (distal support), located further from these formations. The origin of the muscle is considered a stationary (fixed) point, and the insertion of the muscle is considered a moving point. This refers to the most commonly observed movements, in which the distal parts of the body, located further from the body, are more mobile than the proximal ones, located closer to it. But there are movements in which the distal links of the body are fixed (for example, when performing movements on sports equipment), in this case the proximal links approach the distal ones. Therefore, the muscle can perform work either with proximal or distal support.

Muscles, being an active organ, are characterized

intensive metabolism, well supplied with blood vessels that deliver oxygen, nutrients, hormones and carry away muscle metabolic products and carbon dioxide. Blood enters each muscle through arteries, flows through numerous capillaries in the organ, and flows out of the muscle through veins and lymphatic vessels. The blood flow through the muscle is continuous. However, the amount of blood and the number of capillaries passing it through depend on the nature and intensity of muscle work. In a state of relative rest, approximately 1/3 of the capillaries function.

Muscle classification . The classification of muscles is based on the functional principle, since the size, shape, direction of muscle fibers, and the position of the muscle depend on the function it performs and the work performed (Table 4).

Table 4

Muscle classification

1. Depending on the location of the muscles, they are divided into corresponding topographic groups: muscles of the head, neck, back, chest, abdomen, muscles of the upper and lower extremities.

2. By shape the muscles are very diverse: long, short and wide, flat and fusiform, rhomboid, square, etc. These differences are associated with the functional significance of the muscles (Fig. 72).

IN long muscles the longitudinal dimension prevails over the transverse one. They have a small area of ​​attachment to the bones, are located mainly on the limbs and provide a significant amplitude of their movements (Fig. 72a).

Figure 72. Shape of skeletal muscles:

a-fusiform, b-biceps, c-digastric, d-ribbonoid, d-bipinnate, e-unipennate: 1-muscle belly, 2-tendon, 3-intermediate tendon, 4-tendon bridges.

U short muscles longitudinal dimension is only slightly larger

transverse They occur in those areas of the body where the range of motion is small (for example, between individual vertebrae, between the occipital bone, the atlas and the axial vertebra).

Latissimus muscles are located mainly in the body area

sha and limb girdles. These muscles have bundles of muscle fibers running in different directions and contract both as a whole and in their individual parts; they have a significant area of ​​attachment to the bones. Unlike other muscles, they have not only a motor function, but also a supporting and protective function. Thus, the abdominal muscles, in addition to participating in the movements of the body, the act of breathing, and when straining, strengthen the abdominal wall, helping to retain the internal organs. There are muscles that have an individual shape, trapezius, quadratus lumborum, pyramidal.

Most muscles have one belly and two tendons (head and tail, Fig. 72a). Some long muscles have not one, but two, three or four bellies and a corresponding number of tendons starting or ending at

various bones. In some cases, such muscles begin with proximal tendons (heads) from different bone points, and then merge into one abdomen, which is attached by one distal tendon - the tail (Fig. 72b). For example, biceps and triceps brachii, quadriceps femoris, calf muscles. In other cases, the muscles begin with one proximal tendon, and the belly ends with several distal tendons attached to different bones (flexors and extensors of the fingers and toes). There are muscles where the abdomen is divided by one intermediate tendon (digastric muscle of the neck, Fig. 72c) or several tendon bridges (rectus abdominis muscle, Fig. 72d).

3. The direction of their fibers is essential for muscle function. By grain direction Functionally determined, muscles with straight, oblique, transverse and circular fibers are distinguished. IN rectus muscles muscle fibers are located parallel to the length of the muscle (Fig. 65 a, b, c, d). These muscles are usually long and do not have much strength.

Muscles with oblique fibers can be attached to the tendon on one side ( unipinnate, rice. 65 e) or on both sides ( bipinnate, rice. 65 d). When contracted, these muscles can develop significant force.

Muscles having circular fibers, are located around the openings and, when contracting, narrow them (for example, the orbicularis oculi muscle, the orbicularis oris muscle). These muscles are called compressors or sphincters(Fig. 83). Sometimes muscles have a fan-shaped course of fibers. Most often these are broad muscles, located in the area of ​​\u200b\u200bthe spherical joints and provide a variety of movements (Fig. 87).

4. By position In the human body, muscles are divided into superficial And deep, external And internal, medial And lateral.

5. In relation to the joints, through which (one, two or several) muscles are thrown, one-, two- and multi-joint muscles are distinguished. Single-joint muscles are fixed to neighboring bones of the skeleton and pass through one joint, and multi-joint muscles pass through two or more joints, producing movements in them. Multi-joint muscles, being longer ones, are located more superficially than single-joint muscles. Throwing over a joint, the muscles have a certain relationship to the axes of its movement.

6. By function performed muscles are divided into flexors and extensors, abductors and adductors, supinators and pronators, elevators and depressors, mastication, etc.

Patterns of muscle position and function . Muscles are thrown over a joint; they have a certain relationship to the axis of a given joint, which determines the function of the muscle. Usually the muscle overlaps one or the other axis at a right angle. If the muscle lies in front of the joint, then it causes flexion, behind - extension, medially - adduction, lateral - abduction. If a muscle lies around the vertical axis of rotation of a joint, then it causes inward or outward rotation. Therefore, knowing how many and what movements are possible in a given joint, you can always predict what muscles are located by function and where they are located.

Muscles have a vigorous metabolism, which increases even more with increasing muscle work. At the same time, blood flow through the vessels increases to the muscle. Increased muscle function causes improved nutrition and increased muscle mass (working hypertrophy). At the same time, the absolute mass and size of the muscle increases due to the increase in muscle fibers. Physical exercises associated with various types of work and sports cause working hypertrophy of those muscles that are most loaded. Often, by the figure of an athlete, you can tell what kind of sport he is involved in - swimming, athletics or weightlifting. Occupational and sports hygiene requires universal gymnastics, which promotes the harmonious development of the human body. Proper physical exercise causes proportional development of the muscles of the whole body. Since increased muscle work affects the metabolism of the whole body, then Physical Culture is one of the powerful factors of beneficial influence on it.

Accessory muscle apparatus . Muscles, contracting, perform their function with the participation and with the help of a number of anatomical formations, which should be considered as auxiliary. The auxiliary apparatus of skeletal muscles includes tendons, fascia, intermuscular septa, synovial bursae and sheaths, muscle blocks, and sesamoid bones.

Fascia cover both individual muscles and muscle groups. There are superficial (subcutaneous) and deep fascia. Superficial fascia lie under the skin, surrounding all the muscles of the area. Deep fascia cover a group of synergistic muscles (i.e., performing a homogeneous function) or each individual muscle (own fascia). Processes extend deep from the fascia - intermuscular septa. They separate muscle groups from each other and attach to bones. Tendon retinaculum located in the area of ​​some joints of the limbs. They are ribbon-shaped thickenings of the fascia and are located transversely over the muscle tendons like belts, fixing them to the bones.

Synovial bursae- thin-walled connective tissue sacs filled with fluid similar to synovium and located under the muscles, between muscles and tendons or bone. They reduce friction.

Synovial vaginas develop in those places where the tendons are adjacent to the bone (i.e., in the osteofibrous canals). These are closed formations, in the form of a coupling or cylinder, covering the tendon. Each synovial vagina consists of two layers. One leaf, the inner one, covers the tendon, and the second, the outer one, lines the wall of the fibrous canal. Between the sheets there is a small gap filled with synovial fluid, which facilitates the sliding of the tendon.

Sesamoid bones located in the thickness of the tendons, closer to the place of their attachment. They change the angle of approach of the muscle to the bone and increase the leverage of the muscle. The largest sesamoid bone is the patella.

The auxiliary apparatus of the muscles forms an additional support for them - a soft skeleton, determines the direction of muscle traction, promotes their isolated contraction, prevents them from moving during contraction, increases muscle strength and promotes blood circulation and lymphatic drainage.

Performing numerous functions, muscles work in concert, forming functional working groups. Muscles are included in functional groups according to the direction of movement in a joint, according to the direction of movement of a body part, according to changes in the volume of the cavity and according to changes in the size of the hole.

When moving the limbs and their links, functional groups of muscles are distinguished - flexor, extension, abductor and adductor, pronating and supinating.

When moving the body, functional muscle groups are distinguished - flexors and extensions (tilting forward and backward), tilting to the right or left, turning to the right or left. In relation to the movement of individual parts of the body, functional groups of muscles are distinguished, raising and lowering, moving forward and backward; by changing the size of the hole - narrowing and expanding it.

In the process of evolution, functional muscle groups

developed in pairs: the flexor group was formed together with the extensor group, the pronating group - together with the supinating group, etc. This is clearly demonstrated by examples of the development of joints: each axis of rotation in the joint, expressing its shape, has its own functional pair of muscles. Such pairs usually consist of muscle groups that are opposite in function. Thus, uniaxial joints have one pair of muscles, biaxial joints have two pairs, and triaxial joints have three pairs or, respectively, two, four, six functional muscle groups.

Synergism and antagonism in muscle action . Muscles included in a functional group are characterized by the fact that they exhibit the same motor function. In particular, all of them either attract bones - they shorten, or release them - they lengthen, or they exhibit relative stability of tension, size and shape. Muscles that act together in one functional group are called synergists. Synergy manifests itself not only during movements, but also when fixing parts of the body.

Muscles of functional muscle groups that are opposite in action are called antagonists. So, flexor muscles will be antagonists of extensor muscles, pronators will be antagonists of supinators, etc. However, there is no true antagonism between them. It appears only in relation to a certain movement or a certain axis of rotation.

It should be noted that during movements in which one

muscle, there can be no synergy. At the same time, antagonism always takes place, and only the coordinated work of synergist and antagonist muscles ensures smooth movements and prevents injuries. So, for example, with each flexion, not only the flexor acts, but also the extensor, which gradually gives way to the flexor and keeps it from excessive contraction. Therefore, antagonism ensures smoothness and proportionality of movements. Every movement, therefore, is the result of the action of antagonists.

Motor function of muscles . Since each muscle is fixed primarily to the bones, its external motor function is expressed in the fact that it either attracts bones, holds them, or releases them.

A muscle attracts bones, when it actively contracts, its abdomen shortens, the attachment points come closer, the distance between the bones and the angle at the joint decrease in the direction of the muscle pull.

Bone retention occurs with relatively constant muscle tension and an almost imperceptible change in its length.

If the movement is carried out under the effective action of external forces, for example gravity, then the muscle lengthens to a certain limit and releases the bones; they move away from each other, and their movement occurs in the opposite direction compared to that which took place when the bones were attracted.

To understand the function of a skeletal muscle, it is necessary to know which bones the muscle is connected to, which joints it passes through, which axes of rotation it crosses, on which side the axis of rotation crosses, and at what support the muscle acts.

Muscle tone. In the body, every skeletal muscle is always

is in a state of certain tension, readiness for action. The minimum involuntary reflex muscle tension is called muscle tone. Physical exercise increases muscle tone and influences the specific background from which the action of the skeletal muscle begins. Children have less muscle tone than adults, women have less muscle tone than men, and those who do not engage in sports have less muscle tone than athletes.

For the functional characteristics of muscles, such indicators as their anatomical and physiological diameter are used. Anatomical diameter- cross-sectional area perpendicular to the length of the muscle and passing through the abdomen in its widest part. This indicator characterizes the size of the muscle, its thickness (in fact, it determines the volume of the muscle). Physiological diameter represents the total cross-sectional area of ​​all muscle fibers that make up the muscle. And since the strength of a contracting muscle depends on the size of the cross-section of the muscle fibers, the physiological cross-section of the muscle characterizes its strength. In fusiform and ribbon-shaped muscles with parallel fibers, the anatomical and physiological diameters coincide. It’s different for the feathery muscles. Of two equal muscles that have the same anatomical diameter, the pennate muscle will have a larger physiological diameter than the fusiform muscle. In this regard, the pennate muscle has greater strength, but the range of contraction of its short muscle fibers will be less than that of the fusiform muscle. Therefore, pennate muscles are present where significant force of muscle contractions is required with a relatively small range of movements (muscles of the foot, lower leg, some muscles of the forearm). Fusiform, ribbon-shaped muscles, built from long muscle fibers, shorten by a large amount when contracted. At the same time, they develop less force than the pennate muscles, which have the same anatomical diameter.

Types of muscle work . The human body and its parts

contractions of the corresponding muscles change their position, begin to move, overcome the resistance of gravity or, conversely, yield to this force. In other cases, when muscles contract, the body is held in a certain position without performing a movement. Based on this, a distinction is made between overcoming, yielding and holding muscle work. Overcoming work is performed when the force of muscle contraction changes the position of a body part, limb or its link with or without a load, overcoming the force of resistance. For example, the biceps brachii muscle, when flexing the forearm, performs overcoming work; the deltoid muscle (mainly its middle bundles), when abducting the arm, also performs overcoming work.

Inferior is called work in which a muscle, remaining tense, gradually relaxes, yielding to the force of gravity of a part (limb) of the body and the load it holds. For example, when adducting the abducted arm, the deltoid muscle performs yielding work, it gradually relaxes and the arm lowers.

holding called work in which the force of gravity

is balanced by muscle tension and the body or load is held in a certain position without moving in space. For example, when holding an arm in an abducted position, the deltoid muscle performs holding work.

Overcoming and yielding work, when the force of muscle contractions is determined by the movement of the body or its parts in space, can be considered as dynamic work. Holding work, in which no movement of the whole body or part of the body occurs, is static. Using one type of work or another, you can significantly diversify your training and make it more effective.

Skeletal (somatic) muscles are represented by a large number (more than 200) muscles. Each muscle has a supporting part - the connective tissue stroma and a working part - the muscle parenchyma. The more static load a muscle performs, the more developed its stroma is.

On the outside, the muscle is covered with a connective tissue sheath, which is called the external perimysium - perimysium. It has different thicknesses on different muscles. Connective tissue septa extend inward from the external perimysium - the internal perimysium, surrounding muscle bundles of various sizes. The greater the static function of a muscle, the more powerful the connective tissue partitions are located in it, the more of them there are. On the internal partitions in the muscles, muscle fibers can be attached, vessels and nerves pass through. Between the muscle fibers there are very delicate and thin connective tissue layers called endomysium.

In this stroma of the muscle, represented by the external and internal perimysium and endomysium, muscle tissue (muscle fibers forming muscle bundles) is naturally packed, forming a muscle belly of various shapes and sizes. The muscle stroma at the ends of the muscle belly forms continuous tendons, the shape of which depends on the shape of the muscles. If the tendon is cord-shaped, it is simply called a tendon - tendo. If the tendon is flat, coming from a flat muscular belly, then it is called an aponeurosis.

The tendon is also distinguished between outer and inner sheaths (mesotendineum). The tendons are very dense, compact, form strong cords that have high tensile strength. Collagen fibers and bundles in them are located strictly longitudinally, due to which the tendons become a less fatigued part of the muscle. The tendons are attached to the bones, penetrating into the thickness of the bone tissue in the form of Sharpey's fibers (the connection with the bone is so strong that the tendon is more likely to rupture than it is torn off from the bone). Tendons can move to the surface of the muscle and cover them at a greater or lesser distance, forming a shiny sheath called the tendon mirror.

In certain areas, the muscle includes vessels that supply it with blood and nerves that innervate it. The place where they enter is called the organ gate. Inside the muscle, vessels and nerves branch along the internal perimysium and reach its working units - muscle fibers, on which the vessels form networks of capillaries, and the nerves branch into:

1) sensory fibers - come from the sensitive nerve endings of the proprioceptors, located in all parts of the muscles and tendons, and carry out an impulse sent through the spinal ganglion cell to the brain;

2) motor nerve fibers that carry impulses from the brain: a) to muscle fibers, ending on each muscle fiber with a special motor plaque, b) to muscle vessels - sympathetic fibers, carrying impulses from the brain through the sympathetic ganglion cell to the smooth muscles of the blood vessels, c) trophic fibers ending on the connective tissue base of the muscle.

Since the working unit of muscles is the muscle fiber, it is their number that determines the strength of the muscle; The strength of the muscle depends not on the length of the muscle fibers, but on the number of them in the muscle. The more muscle fibers there are in a muscle, the stronger it is. The length of muscle fibers usually does not exceed 12-15 cm, the lifting force of the muscle is on average 8-10 kg per 1 cm 2 of physiological diameter. When contracting, the muscle shortens by half its length. To count the number of muscle fibers, a cut is made perpendicular to their longitudinal axis; the resulting area of ​​transversely cut fibers is the physiological diameter. The area of ​​the cut of the entire muscle perpendicular to its longitudinal axis is called the anatomical diameter. In the same muscle there can be one anatomical and several physiological diameters, formed if the muscle fibers in the muscle are short and have different directions. Since muscle strength depends on the number of muscle fibers in them, it is expressed by the ratio of the anatomical diameter to the physiological one. There is only one anatomical diameter in the muscle belly, but physiological ones can have different numbers (1:2, 1:3,..., 1:10, etc.). A large number of physiological diameters indicates muscle strength.

Muscles are light and dark. Their color depends on their function, structure and blood supply. Dark muscles are rich in myoglobin (myohematin) and sarcoplasm, they are more resilient. Light muscles are poorer in these elements; they are stronger, but less resilient. In different animals, at different ages, and even in different parts of the body, the color of the muscles can be different: they are the darkest in horses, much lighter in pigs; young animals are lighter than adults; darker on the limbs than on the body; wild animals are darker than domestic ones; In chickens the pectoral muscles are white, in wild birds they are dark.

Muscles form the active part of the musculoskeletal system. They are attached to the bones of the skeleton, act on bone levers, and set them in motion. Therefore they are also called skeletal muscles.

Skeletal muscles constructed from striated muscle tissue. They perform the following functions: 1) maintain the position of the body and its parts in space; 2) provide movement of the body (running, walking and other types of movements);

3) move body parts relative to each other; 4) carry out breathing and swallowing movements; 5) participate in the articulation of speech and the formation of facial expressions; 6) generate heat; 7) convert chemical energy into mechanical energy.

There are about 600 muscles in the human body. The total mass of skeletal muscles in newborn children averages 22% of body weight; at 17–18 years old it reaches 35–40%. In older and older people relative mass skeletal muscles decreases to 25 - 30%. In trained athletes, muscles can account for up to 50% of the total body weight.

The main functional properties of muscles: 1) excitability - the ability to quickly respond to a stimulus with excitation, as a result of which the muscle is able to contract; 2) conductivity - the ability to conduct excitation from nerve endings to the contractile structures of muscle fibers;

3) contractility - the ability to contract, shorten or change tension.

Excitation and contraction of muscles occur under the influence of nerve impulses coming along the nerves from the central nervous system, from the brain and spinal cord. In order for a muscle to be excited and respond with contraction, the strength of the nerve impulse must be sufficient. The force of stimulation that can cause muscle contraction is called threshold irritation.

The wave of excitation that arises in the muscle quickly spreads throughout the muscle, as a result the muscle contracts and acts on the bone levers, causing them to move.

In the muscle there are abdomen, consisting of striated muscle tissue, and tendon ends (tendons), formed by dense fibrous connective tissue. With the help of tendons, muscles are attached to the bones of the skeleton (Fig. 28).

Rice. 28. Scheme of origin and attachment of muscles:

1 – muscle, 2 – tendon, 3 – bone

However, some muscles can also attach to other organs (skin, eyeball).

The end of the muscle located closer to the median plane of the body. usually called the beginning of the muscle the other end, spaced from the median plane, is called muscle attachment. The origin of the muscle usually remains stationary as the length of the muscle changes. This place on the bone is called a fixed point. The attachment point of the muscle located on the bone that is set in motion is called the moving point.

The main working tissue of skeletal muscle is striated muscle tissue. Its main structural and functional element is the complex muscle fiber. Muscle fibers - these are multinucleate formations. One fiber can have more than 100 rice cores. 29). The length of the muscle fibers reaches several centimeters.

On the outside, the muscle fiber is undermined by the sheath - sarcolemma. In the cytoplasm of the muscle fiber - sarcoplasm, along with cellular organelles of a general nature, there are also specialized organelles - myofibrils. These are the main structures of muscle fiber, consisting of the contractile proteins actin and myosin. Each myofibril consists of contractile sections - sarcomeres. At the boundaries of sarcomeres, protein molecules are located across the muscle fiber. These areas attached to the sarcolemma are called telophragm. In the middle of the sarcomeres are mesophragm, also representing a transverse protein network. Actin filaments are attached to the telophragm, and myosin filaments are attached to the mesophragm.

Due to the different structure of protein molecules and the refraction of light rays, light and dark areas are visible in the sarcomeres and at their boundaries in the muscle fibers, creating the impression of striations.

Muscle contraction is based on the sliding of actin and myosin filaments relative to each other. Actin filaments, moving towards each other when excited, reduce the length of sarcomeres.

Muscle contractility manifests itself either in its shortening, or in tension, at which the length of the muscle fibers does not change. In the body, muscle contraction occurs under the influence of nerve impulses that the muscle receives from the central nervous system along the nerves that connect to it.

Motor nerve fibers, approaching muscle fibers, form endings on them - motor plates. Nerve impulses arriving at the area of ​​neuromuscular endings stimulate the release of a biologically active substance - acetylcholine, which causes an action potential. The action potential spreads across the muscle fiber membrane, the membranes of the sarcoplasmic reticulum, causing the release of calcium ions into the sarcoplasm, the formation of actomiazin, and the breakdown of ATP molecules. The energy released in this process is used to slide protein filaments and contract the muscle.

Receptors in skeletal muscles are represented by neuromuscular spindles. Each neuromuscular spindle is surrounded by a connective tissue capsule and contains specialized muscle fibers on which sensory nerve endings - receptors - are located. They sense muscle stretches and transmit nerve impulses to the central nervous system.

Each muscle consists of a large number of muscle fibers interconnected by thin layers of loose fibrous connective tissue in bundles. Groups of bundles are covered with a thicker and denser connective tissue membrane and form a muscle. The connective tissue fibers surrounding the muscle fibers and their bundles, extending beyond the muscle, form the tendon. The tendons of different muscles are not the same. In muscles located on the limbs, the tendons are usually narrow and long. The tendons of the muscles involved in the formation of the walls of the cavities are wide, they are called aponeuroses.

Muscles are rich in blood vessels, through which the blood brings nutrients and oxygen to them, and carries out metabolic products. The source of energy for muscle contraction is glycogen. In the process of its breakdown, adenosine triphosphate acid (ATP) is produced, which is the source of energy for muscle contraction.

1. What percentage of the total body weight is muscle in a newborn child, in adolescence, in old people?

2. What functions do skeletal muscles perform?

Related information.

Skeletal muscles - the active part of the musculoskeletal system, which also includes bones, ligaments, tendons and their joints. From a functional point of view, motor neurons that cause excitation of muscle fibers can also be classified as the motor system. The axon of a motor neuron branches at the entrance to the skeletal muscle, and each branch participates in the formation of the neuromuscular synapse on a separate muscle fiber.

A motor neuron, together with the muscle fibers it innervates, is called a neuromotor (or motor) unit (MU). In the eye muscles, one motor unit contains 13-20 muscle fibers, in the trunk muscles - from 1 tons of fibers, in the soleus muscle - 1500-2500 fibers. Muscle fibers of one motor unit have the same morphofunctional properties.

Functions of skeletal muscles are: 1) movement of the body in space; 2) movement of body parts relative to each other, including the implementation of respiratory movements that provide ventilation of the lungs; 3) maintaining body position and posture. In addition, striated muscles are important in the production of heat, which maintains temperature homeostasis, and in the storage of certain nutrients.

Physiological properties of skeletal muscles highlight:

1)excitability. Due to the high polarization of the membranes of striated muscle fibers (90 mV), their excitability is lower than that of nerve fibers. Their action potential amplitude (130 mV) is greater than that of other excitable cells. This makes it quite easy to record the bioelectrical activity of skeletal muscles in practice. The duration of the action potential is 3-5 ms. This determines the short period of absolute refractoriness of muscle fibers;

conductivity. The speed of excitation along the muscle fiber membrane is 3-5 m/s;

contractility. Represents the specific property of muscle fibers to change their length and tension with the development of excitation.

Skeletal muscles also have elasticity and viscosity.

Modes and types of muscle contractions. Isotonic regime - the muscle shortens in the absence of an increase in its tension. Such a contraction is possible only for an isolated (removed from the body) muscle.

Isometric mode - muscle tension increases, but the length practically does not decrease. This reduction is observed when trying to lift an overwhelming load.

Auxotonic mode the muscle shortens and its tension increases. This reduction is most often observed during human labor activity. Instead of the term "auxotonic mode" the name is often used concentric mode.

There are two types of muscle contractions: single and tetanic.

Single muscle contraction manifests itself as a result of the development of a single wave of excitation in muscle fibers. This can be achieved by applying a very short (about 1 ms) stimulus to the muscle. The development of a single muscle contraction is divided into a latent period, a shortening phase and a relaxation phase. Muscle contraction begins to appear 10 ms from the beginning of the stimulus. This time interval is called the latent period (Fig. 5.1). This will be followed by the development of shortening (duration about 50 ms) and relaxation (50-60 ms). It is believed that an average of 0.1 s is spent on the entire cycle of a single muscle contraction. But it should be borne in mind that the duration of a single contraction in different muscles can vary greatly. It also depends on the functional state of the muscle. The rate of contraction and especially relaxation slows down as muscle fatigue develops. Fast muscles that have a short period of single contraction include the muscles of the tongue and the muscles that close the eyelid.

Rice. 5.1. Temporal relationships between different manifestations of skeletal muscle fiber excitation: a - ratio of action potential, release of Ca 2+ into the sarcoplasm and contraction: / - latent period; 2 - shortening; 3 - relaxation; b - ratio of action potential, contraction and level of excitability

Under the influence of a single stimulus, an action potential first arises and only then does a period of shortening begin to develop. It continues after the end of repolarization. The restoration of the original polarization of the sarcolemma also indicates the restoration of excitability. Consequently, against the background of developing contraction in muscle fibers, new waves of excitation can be caused, the contractile effect of which will be cumulative.

Tetanic contraction or tetanus called a muscle contraction that appears as a result of the occurrence of numerous waves of excitation in motor units, the contractile effect of which is summarized in amplitude and time.

There are serrated and smooth tetanus. To obtain dentate tetanus, it is necessary to stimulate the muscle with such a frequency that each subsequent impact is applied after the shortening phase, but before the end of relaxation. Smooth tetanus occurs with more frequent stimulation, when subsequent impacts are applied during the development of muscle shortening. For example, if the shortening phase of a muscle is 50 ms, and the relaxation phase is 60 ms, then to obtain serrated tetanus it is necessary to irritate this muscle with a frequency of 9-19 Hz, to obtain smooth tetanus - with a frequency of at least 20 Hz.

Despite

Pessimum

200 Hz

50 Hz

Rice. 5.2. Dependence of the contraction amplitude on the frequency of stimulation (the strength and duration of the stimuli are unchanged)

For demonstration various types Tetanus usually involves recording contractions of the isolated frog gastrocnemius muscle on a kymograph. An example of such a kymogram is shown in Fig. 5.2. The amplitude of a single contraction is minimal, increases with serrated tetanus and becomes maximum with smooth tetanus. One of the reasons for this increase in amplitude is that when frequent waves of excitation occur, Ca 2+ accumulates in the sarcoplasm of muscle fibers, stimulating the interaction of contractile proteins.

With a gradual increase in the frequency of stimulation, the strength and amplitude of muscle contraction increases only to a certain limit - optimal response. The frequency of stimulation that causes the greatest muscle response is called optimal. A further increase in the frequency of stimulation is accompanied by a decrease in the amplitude and force of contraction. This phenomenon is called pessimism of the response, and irritation frequencies exceeding the optimal value are pessimal. The phenomena of optimum and pessimum were discovered by N.E. Vvedensky.

When assessing the functional activity of muscles, they talk about their tone and phasic contractions. Muscle tone called a state of prolonged continuous tension. In this case, visible shortening of the muscle may be absent due to the fact that excitation does not occur in all, but only in some motor units of the muscle and they are not excited synchronously. Phasic muscle contraction called short-term shortening of the muscle, followed by its relaxation.

Structurally-functional characteristics of muscle fiber. The structural and functional unit of skeletal muscle is the muscle fiber, which is an elongated (0.5-40 cm long) multinucleated cell. The thickness of muscle fibers is 10-100 microns. Their diameter can increase with intense training loads, but the number of muscle fibers can increase only until 3-4 months of age.

The muscle fiber membrane is called sarcolemma, cytoplasm - sarcoplasm. The sarcoplasm contains nuclei, numerous organelles, the sarcoplasmic reticulum, which includes longitudinal tubules and their thickenings - cisterns that contain Ca 2+ reserves. The cisterns are adjacent to transverse tubules that penetrate the fiber in the transverse direction (Fig. 5.3).

In the sarcoplasm, about 2000 myofibrils (about 1 µm thick) run along the muscle fiber, which include filaments formed by the interweaving of contractile protein molecules: actin and myosin. Actin molecules form thin filaments (myofilaments) that lie parallel to each other and penetrate a kind of membrane called the Z-line or stripe. Z-lines are located perpendicular to the long axis of the myofibril and divide the myofibril into sections 2-3 µm long. These areas are called sarcomeres.

Sarcolemma Cistern

Transverse tube

Sarcomere

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The sarcomere is shortened

The sarcomere is relaxed

Rice. 5.3. The structure of the muscle fiber sarcomere: Z-lines - limit the sarcomere,/! - anisotropic (dark) disk, / - isotropic (light) disk, H - zone (less dark)

The sarcomere is the contractile unit of the myofibril. In the center of the sarcomere, thick filaments formed by myosin molecules lie in a strictly ordered manner one above the other, and thin filaments of actin are similarly located at the edges of the sarcomere. The ends of the actin filaments extend between the ends of the myosin filaments.

The central part of the sarcomere (width 1.6 µm), in which the myosin filaments lie, appears dark under a microscope. This dark area can be traced across the entire muscle fiber, since the sarcomeres of neighboring myofibrils are located strictly symmetrically above each other. The dark areas of sarcomeres are called A-disks from the word “anisotropic.” These areas are birefringent in polarized light. The areas at the edges of the A-disc, where the actin and myosin filaments overlap, appear darker than in the center, where only the myosin filaments are located. This central area is called the H strip.

The areas of the myofibril in which only actin filaments are located do not exhibit birefringence; they are isotropic. Hence their name - I-discs. In the center of the I-disc there is a narrow dark line formed by the Z-membrane. This membrane keeps the actin filaments of two neighboring sarcomeres in an ordered state.

In addition to actin molecules, the actin filament also includes the proteins tropomyosin and troponin, which influence the interaction of actin and myosin filaments. The myosin molecule has sections called the head, neck and tail. Each such molecule has one tail and two heads with necks. Each head has a chemical center that can bind ATP and a site that allows it to bind to the actin filament.

During the formation of the myosin filament, myosin molecules are intertwined with their long tails, located in the center of this filament, and the heads are located closer to its ends (Fig. 5.4). The neck and head form a protrusion protruding from the myosin filaments. These projections are called cross bridges. They are mobile, and thanks to such bridges, myosin filaments can establish connections with actin filaments.

When ATP attaches to the head of the myosin molecule, the bridge is briefly positioned at an obtuse angle relative to the tail. At the next moment, partial cleavage of ATP occurs and, due to this, the head rises and moves to an energized position in which it can bind to the actin filament.

Actin molecules form a double helix Trolonin

ATF Communications Center

Myosin molecule at high magnification

Section of a thick filament (the heads of myosin molecules are visible)

Actin filament

Head

+Ca 2+

ADF-F

Rice. 5.4. The structure of actin and myosin filaments, the movement of myosin heads during muscle contraction and relaxation. Explanation in the text: 1-4 - stages of the cycle

The mechanism of muscle fiber contraction. Excitation of skeletal muscle fibers under physiological conditions is caused only by impulses coming from motor neurons. The nerve impulse activates the neuromuscular synapse, causes the occurrence of PC.P, and the end plate potential ensures the generation of an action potential at the sarcolemma.

The action potential propagates both along the surface membrane of the muscle fiber and deeper along the transverse tubules. In this case, the cisterns of the sarcoplasmic reticulum are depolarized and Ca 2+ channels open. Since in the sarcoplasm the concentration of Ca 2+ is 1(G 7 -1(G b M, and in the tanks it is approximately 10,000 times greater), then when the Ca 2+ channels open, calcium along the concentration gradient leaves the tanks into the sarcoplasm and diffuses to myofilaments and triggers processes that ensure contraction. Thus, the release of Ca 2+ ions

into the sarcoplasm is a factor that couples electrical skies and mechanical phenomena in muscle fiber. Ca 2+ ions bind to troponin and this, with the participation of tropomyo- zina, leads to the opening (unblocking) of actino sites howl filaments that can bind to myosin. After this, the energized myosin heads form bridges with actin, and the final breakdown of ATP previously captured and held by the myosin heads occurs. The energy obtained from the breakdown of ATP is used to rotate the myosin heads towards the center of the sarcomere. With this rotation, the myosin heads pull the actin filaments along with them, moving them between the myosin filaments. In one stroke, the head can advance the actin filament by -1% of the sarcomere length. For maximum contraction, repeated rowing movements of the heads are required. This occurs when there is a sufficient concentration of ATP and Sa 2+ in the sarcoplasm. For the myosin head to move again, a new ATP molecule must be attached to it. The addition of ATP causes a break in the connection between the myosin head and actin, and it momentarily takes its original position, from which it can move on to interact with a new section of the actin filament and make a new rowing movement.

This theory of the mechanism of muscle contraction was called theory of "sliding threads"

To relax the muscle fiber, it is necessary that the concentration of Ca 2+ ions in the sarcoplasm becomes less than 10 -7 M/l. This occurs due to the functioning of the calcium pump, which drives Ca 2+ from the sarcoplasm into the reticulum. In addition, for muscle relaxation, the bridges between the myosin heads and actin must be broken. This rupture occurs when ATP molecules are present in the sarcoplasm and bind to myosin heads. After the heads detach, elastic forces stretch the sarcomere and move the actin filaments to their original position. Elastic forces are formed due to: 1) elastic traction of spiral-shaped cellular proteins included in the structure of the sarcomere; 2) elastic properties of the membranes of the sarcoplasmic reticulum and sarcolemma; 3) elasticity of connective tissue of muscles, tendons and the effects of gravity.

Muscle strength. The strength of a muscle is determined by the maximum value of the load that it can lift, or by the maximum force (tension) that it can develop under conditions of isometric contraction.

A single muscle fiber is capable of developing a tension of 100-200 mg. There are approximately 15-30 million fibers in the body. If they acted in parallel in the same direction and at the same time, they could create a voltage of 20-30 tons.

Muscle strength depends on a number of morphofunctional, physiological and physical factors.

Muscle strength increases with increasing geometric and physiological cross-sectional area. To determine the physiological cross-section of a muscle, find the sum of the cross-sections of all muscle fibers along a line drawn perpendicular to the course of each muscle fiber.

In a muscle with parallel fibers (sartorius), the geometric and physiological cross sections are equal. In muscles with oblique fibers (intercostal) the physiological cross-section is larger than the geometric one and this helps to increase muscle strength. The physiological cross-section and strength of muscles with a pennate arrangement (most muscles of the body) of muscle fibers increases even more.

To be able to compare the strength of muscle fibers in muscles with different histological structures, the concept of absolute muscle strength was introduced.

Absolute muscle strength- the maximum force developed by the muscle, calculated per 1 cm 2 of physiological cross-section. Absolute strength of biceps - 11.9 kg/cm2, triceps brachii - 16.8 kg/cm2, gastrocnemius 5.9 kg/cm2, smooth muscle - 1 kg/cm2

The strength of a muscle depends on the percentage of different types of motor units that make up that muscle. The ratio of different types of motor units in the same muscle varies among people.

The following types of motor units are distinguished: a) slow, non-fatiguing (have a red color) - they have low strength, but can be in a state of tonic contraction for a long time without signs of fatigue; b) fast, easily fatigued (white in color) - their fibers have a great contraction force; c) fast, resistant to fatigue - have a relatively large force of contraction and fatigue develops slowly in them.

In different people, the ratio of the number of slow and fast motor units in the same muscle is determined genetically and can vary significantly. Thus, in the human quadriceps muscle, the relative content of copper fibers can vary from 40 to 98%. The greater the percentage of slow fibers in a person’s muscles, the more they are adapted to long-term, but low-power work. People with a high content of fast strong motor units are able to develop great strength, but are prone to fatigue quickly. However, we must keep in mind that fatigue depends on many other factors.

The strength of a muscle increases with moderate stretching. This is due to the fact that with moderate stretching of the sarcomere (up to 2.2 μm), the number of bridges that can form between actin and myosin increases. When a muscle is stretched, elastic traction also develops in it, aimed at shortening. This thrust is added to the force developed by the movement of the myosin heads.

Muscle strength is regulated by the nervous system by changing the frequency of impulses sent to the muscle, synchronizing the excitation of a large number of motor units, and selecting the types of motor units. The strength of contractions increases: a) with an increase in the number of excited motor units involved in the response; b) with an increase in the frequency of excitation waves in each of the activated fibers; c) when synchronizing excitation waves in muscle fibers; d) upon activation of strong (white) motor units.

First (if it is necessary to develop a small effort), slow, non-fatiguing motor units are activated, then fast, resistant to fatigue. And if it is necessary to develop a force of more than 20-25% of the maximum, then fast, easily fatigued motor units are involved in the contraction.

At a voltage of up to 75% of the maximum possible, almost all motor units are activated and a further increase in strength occurs due to an increase in the frequency of impulses arriving at the muscle fibers.

With weak contractions, the frequency of impulses in the axons of motor neurons is 5-10 impulses/s, and with a strong contraction force it can reach up to 50 impulses/s.

IN childhood the increase in strength occurs mainly due to an increase in the thickness of muscle fibers, and this is associated with an increase in the number of myofibrils. The increase in the number of fibers is insignificant.

When training adult muscles, an increase in their strength is associated with an increase in the number of myofibrils, while an increase in endurance is due to an increase in the number of mitochondria and the intensity of ATP synthesis due to aerobic processes.

There is a relationship between force and speed of shortening. The greater the length of a muscle, the higher the speed of muscle contraction (due to the summation of the contractile effects of sarcomeres) and depends on the load on the muscle. As the load increases, the contraction speed decreases. A heavy load can only be lifted by moving slowly. The maximum contraction speed achieved during human muscle contraction is about 8 m/s.

The force of muscle contraction decreases as fatigue develops.

Fatigue and its physiological basis.Fatigue called a temporary decrease in performance, caused by previous work and disappearing after a period of rest.

Fatigue is manifested by a decrease in muscle strength, speed and accuracy of movements, changes in the performance of the cardiorespiratory system and autonomic regulation, and a deterioration in the functions of the central nervous system. The latter is evidenced by a decrease in the speed of simple mental reactions, weakening of attention, memory, deterioration of thinking indicators, and an increase in the number of erroneous actions.

Subjectively, fatigue can be manifested by a feeling of tiredness, muscle pain, palpitations, symptoms of shortness of breath, a desire to reduce the load or stop working. Symptoms of fatigue may vary depending on the type of work, the intensity of the work, and the degree of fatigue. If fatigue is caused by mental work, then, as a rule, symptoms of decreased functionality of mental activity are more pronounced. With very heavy muscular work, symptoms of disorders at the level of the neuromuscular system may come to the fore.

Fatigue, which develops under conditions of normal work activity, both during muscular and mental work, has largely similar development mechanisms. In both cases, the processes of fatigue develop first in the nervous centers One indicator of this is a decrease in intelligence national performance during physical fatigue, and when mental fatigue- decrease in efficiency we cervical activities.

Rest called a state of rest or performing a new activity, in which fatigue is eliminated and performance is restored. THEM. Sechenov showed that restoration of performance occurs faster if, when resting after fatigue of one muscle group (for example, the left arm), work is performed by another muscle group ( right hand). He called this phenomenon "active recreation"

Recovery are processes that ensure the elimination of shortages of energy and plastic substances, the reproduction of structures spent or damaged during work, the elimination of excess metabolites and deviations of homeostasis indicators from the optimal level.

The length of the period required to restore the body depends on the intensity and duration of the work. The greater the intensity of work, the shorter the period of rest required.

Various indicators of physiological and biochemical processes are restored after different times from the end of physical activity. One important test of recovery rate is to determine the time it takes for your heart rate to return to resting levels. The recovery time for heart rate after a moderate exercise test in a healthy person should not exceed 5 minutes.

With very intense physical activity, the phenomena of fatigue develop not only in the central nervous system, but also in the neuromuscular synapses, as well as in the muscles. In the system of the neuromuscular preparation, the nerve fibers have the least fatigue, the neuromuscular synapse has the greatest fatigue, and the muscle occupies an intermediate position. Nerve fibers can conduct high frequency action potentials for hours without signs of fatigue. With frequent activation of the synapse, the efficiency of excitation transmission first decreases, and then a blockade of its conduction occurs. This occurs due to a decrease in the supply of transmitter and ATP in the presynaptic terminal and a decrease in the sensitivity of the postsynaptic membrane to acetylcholine.

A number of theories have been proposed for the mechanism of development of fatigue in a very intensely working muscle: a) the theory of “exhaustion” - the consumption of ATP reserves and the sources of its formation (creatine phosphate, glycogen, fatty acids), b) the theory of “suffocation” - the lack of oxygen delivery comes first into the fibers of the working muscle; c) the “clogging” theory, which explains fatigue by the accumulation of lactic acid and toxic metabolic products in the muscle. It is currently believed that all these phenomena occur during very intense muscle work.

It has been established that maximum physical work before the development of fatigue is performed at an average level of difficulty and pace of work (the rule of average loads). In the prevention of fatigue, the following are also important: the correct ratio of periods of work and rest, alternation of mental and physical work, taking into account circadian, annual and individual biological rhythms.

Muscle power is equal to the product of muscle force and the rate of shortening. Maximum power develops at an average speed of muscle shortening. For the arm muscle, maximum power (200 W) is achieved at a contraction speed of 2.5 m/s.

5.2. Smooth muscle

Physiological properties and characteristics of smooth muscles.

Smooth muscles are integral part some internal organs and participate in ensuring the functions performed by these organs. In particular, they regulate the patency of the bronchi for air, blood flow in various organs and tissues, the movement of fluids and chyme (in the stomach, intestines, ureters, urinary and gall bladders), expel the fetus from the uterus, dilate or constrict the pupils (by contracting the radial or circular muscles of the iris), change the position of hair and skin relief. Smooth muscle cells are spindle-shaped, 50-400 µm long, 2-10 µm thick.

Smooth muscles, like skeletal muscles, have excitability, conductivity and contractility. Unlike skeletal muscles, which have elasticity, smooth muscles are plastic (capable of long time maintain the length given to them by stretching without increasing tension). This property is important for performing the function of depositing food in the stomach or liquids in the gall and bladder.

Peculiarities excitability smooth muscle fibers are to a certain extent associated with their low transmembrane potential (E 0 = 30-70 mV). Many of these fibers are automatic. The duration of their action potential can reach tens of milliseconds. This happens because the action potential in these fibers develops mainly due to the entry of calcium into the sarcoplasm from the intercellular fluid through the so-called slow Ca 2+ channels.

Speed carrying out the initiation in smooth muscle cells small - 2-10 cm/s. Unlike skeletal muscles, excitation in smooth muscle can be transmitted from one fiber to another nearby. This transmission occurs due to the presence of nexuses between smooth muscle fibers, which have low resistance to electric current and ensure the exchange between cells of Ca 2+ and other molecules. As a result, smooth muscle has the properties of functional syncytium.

Contractility smooth muscle fibers are distinguished by a long latent period (0.25-1.00 s) and a long duration (up to 1 min) of a single contraction. Smooth muscles have a low contractile force, but are able to remain in tonic contraction for a long time without developing fatigue. This is due to the fact that smooth muscle spends 100-500 times less energy to maintain tetanic contraction than skeletal muscle. Therefore, the ATP reserves consumed by smooth muscle have time to be restored even during contraction, and the smooth muscles of some body structures are in a state of tonic contraction throughout their lives.

Conditions for smooth muscle contraction. The most important feature of smooth muscle fibers is that they are excited under the influence of numerous stimuli. Normal skeletal muscle contraction is initiated only by a nerve impulse arriving at the neuromuscular junction. Contraction of smooth muscle can be caused by both nerve impulses and biologically active substances (hormones, many neurotransmitters, prostaglandins, some metabolites), as well as the influence of physical factors, such as stretching. In addition, excitation of smooth muscle can occur spontaneously - due to automation.

The very high reactivity of smooth muscles and their ability to respond with contraction to the action of various factors create significant difficulties for correcting disturbances in the tone of these muscles in medical practice. This can be seen in the examples of the treatment of bronchial asthma, arterial hypertension, spastic colitis and other diseases that require correction of the contractile activity of smooth muscles.

The molecular mechanism of smooth muscle contraction also has a number of differences from the mechanism of skeletal muscle contraction. The filaments of actin and myosin in smooth muscle fibers are located less orderly than in skeletal fibers, and therefore smooth muscle does not have cross-striations. Smooth muscle actin filaments do not contain the protein troponin, and the molecular centers of actin are always open to interact with myosin heads. For this interaction to occur, ATP molecules must be broken down and phosphate transferred to the myosin heads. Then the myosin molecules are woven into filaments and bind with their heads to the myosin. This is followed by the rotation of the myosin heads, during which the actin filaments are pulled between the myosin filaments and contraction occurs.

Phosphorylation of myosin heads is carried out using the enzyme myosin light chain kinase, and dephosphorylation is carried out by myosin light chain phosphatase. If myosin phosphatase activity predominates over kinase activity, the myosin heads are dephosphorylated, the myosin-actin bond is broken, and the muscle relaxes.

Therefore, for smooth muscle contraction to occur, an increase in the activity of myosin light chain kinase is necessary. Its activity is regulated by the level of Ca 2+ in the sarcoplasm. When the smooth muscle fiber is excited, the calcium content in its sarcoplasm increases. This increase is due to the intake of Ca^+ from two sources: 1) intercellular space; 2) sarcoplasmic reticulum (Fig. 5.5). Next, Ca 2+ ions form a complex with the protein calmodulin, which converts myosin kinase into an active state.

The sequence of processes leading to the development of smooth muscle contraction: Ca 2 entry into the sarcoplasm - acti

calmodulin activation (by formation of the 4Ca 2+ - calmodulin complex) - activation of myosin light chain kinase - phosphorylation of myosin heads - binding of myosin heads to actin and rotation of the heads, in which actin filaments are pulled between myosin filaments.

Conditions necessary for smooth muscle relaxation: 1) decrease (to 10 M/l or less) Ca 2+ content in the sarcoplasm; 2) disintegration of the 4Ca 2+ -calmodulin complex, leading to a decrease in the activity of myosin light chain kinase - dephosphorylation of myosin heads, leading to the rupture of bonds between actin and myosin filaments. After this, elastic forces cause a relatively slow restoration of the original length of the smooth muscle fiber and its relaxation.

Test questions and assignments

Cell membrane

Rice. 5.5. Scheme of the pathways of Ca 2+ entry into the sarcoplasm of smooth muscle-

of the cell and its removal from the plasma: a - mechanisms that ensure the entry of Ca 2+ into the sarcoplasm and the initiation of contraction (Ca 2+ comes from the extracellular environment and the sarcoplasmic reticulum); b - ways to remove Ca 2+ from the sarcoplasm and ensure relaxation

The influence of norepinephrine through α-adrenergic receptors

Ligand-dependent Ca 2+ channel

Leakage channels

Potential dependent Ca 2+ channel

Smooth muscle cell

a-adreno! receptorfNorepinephrineG

Name the types of human muscles. What are the functions of skeletal muscles?

Describe the physiological properties of skeletal muscles.

What is the relationship between action potential, contraction and excitability of a muscle fiber?

What modes and types of muscle contractions exist?

Give the structural and functional characteristics of muscle fiber.

What are motor units? List their types and features.

What is the mechanism of muscle fiber contraction and relaxation?

What is muscle strength and what factors influence it?

What is the relationship between the force of contraction, its speed and work?

Define fatigue and recovery. What are their physiological basis?

What are the physiological properties and characteristics of smooth muscles?

List the conditions for contraction and relaxation of smooth muscle.

The first includes all human skeletal muscles, which provide the ability to perform voluntary movements, the muscles of the tongue, the upper third of the esophagus and some others, the heart muscle (myocardium), which has its own characteristics (protein composition, nature of contraction, etc.). Smooth muscles include the muscular layers of internal organs and the walls of human blood vessels, which provide the ability to perform a number of important physiological functions.

The structural elements of all types of muscles are muscle fibers. Striated muscle fibers in skeletal muscles form bundles connected to each other by layers of connective tissue. At their ends, muscle fibers are intertwined with tendon fibers, through which muscle traction is transmitted to the bones of the skeleton. Striated muscle fibers are giant multinucleated cells, the diameter of which varies from 10 to 100 microns, and the length often corresponds to the length of the muscles, reaching, for example, 12 cm in some human muscles. The fiber is covered with an elastic membrane - the sarcolemma and consists of sarcoplasm, the structural elements of which are organelles such as mitochondria, ribosomes, tubes and vesicles of the sarcoplasmic reticulum and the so-called T-systems, various inclusions, etc. In the sarcoplasm, usually in the form of bundles, there are many thread-like formations with a thickness of 0.5 to several microns - myofibrils, which have , like the entire fiber as a whole, is cross-striated. Each myofibril is divided into several hundred sections 2.5-3 microns long, called sarcomeres. Each sarcomere, in turn, consists of alternating sections - disks, which have unequal optical density and give the myofibrils and muscle fiber as a whole a characteristic transverse striation, clearly detectable when observed under a phase-contrast microscope. Darker disks have the ability to be birefringent and are called anisotropic, or disks A. Lighter disks do not have this ability and are called isotropic, or disks I. The middle part of disk A is occupied by a zone of weaker birefringence - zone H. Disk I is divided into 2 equal parts by a dark Z-plate delimiting one sarcomere from another. Each sarcomere has two types of filaments consisting of muscle proteins: thick myosin and thin actin. Smooth muscle fibers have a slightly different structure. They are spindle-shaped mononuclear cells, lacking transverse striations. Their length usually reaches 50-250 microns (in the uterus - up to 500 microns), width - 4-8 microns; the myofilaments in them are usually not united into separate myofibrils, but are located along the length of the fiber in the form of many single actin filaments. There is no ordered system of myosin filaments in smooth muscle cells. In the smooth muscles of mollusks, the most important role in the implementation of the obturator function is apparently played by paramyosin fibers (tropomyosin A).

The chemical composition of muscles varies depending on the type and functional state of the muscle and a number of other factors. The main substances that make up human striated muscles and their content (in% of wet weight) are presented below:

• Water 72-80
• Dense substances 20-28

Including:

• Squirrels 16,5-20,9
• Glycogen 0,3-3,0
• Phosphatides 0,4-1,0
• Cholesterol 0,06-0,2
• Creatine + creatine phosphate 0,2-0,55
• Creatinine 0,003-0,005
• ATP 0,25-0,4
• Carnosine 0,2-0,3
• Carnitine 0,02-0,05
• Anzerin 0,09-0,15
• Free amino acids 0,1-0,7
• Lactic acid 0,01-0,02
• Ash 1,0-1,5

On average, about 75% of muscle wet weight is water. Proteins account for the bulk of dense substances. There are myofibrillar (contractile) proteins - myosin, actin and their complex - actomyosin, tropomyosin and a number of so-called minor proteins (a and b-actinins, troponin, etc.), and sarcoplasmic - globulins X, myogens, respiratory pigments, in particular myoglobin , nucleoproteins and enzymes involved in metabolic processes in muscles. Of the other compounds, the most important are extractives, which take part in metabolism and the contractile function of muscles: ATP, phosphocreatine, carnosine, anserine, etc.; phospholipids, which play an important role in the formation of cellular microstructures and metabolic processes; nitrogen-free substances: glycogen and its breakdown products (glucose, lactic acid, etc.), neutral fats, cholesterol, etc.; minerals- salts K, Na, Ca, Mg. Smooth muscles differ significantly in chemical composition from striated ones (lower content of contractual proteins - actomyosin, high-energy compounds, dipeptides, etc.).

Functional features of striated muscles. Striated muscles are richly supplied with various nerves, with the help of which the regulation of muscle activity is carried out by the nerve centers. The most important of them are: motor nerves, which conduct impulses to the muscles, causing their excitation and contraction; sensory nerves, through which information about its condition is transmitted from the muscle to the nerve centers, and, finally, adaptive-trophic fibers of the sympathetic nervous system, affecting metabolism and slowing down the development of muscle fatigue.

Each branch of the motor nerve, which innervates a whole group of muscle fibers that form the so-called motor unit, reaches a separate muscle fiber. All muscle fibers that make up such a unit contract when excited almost simultaneously. Under the influence of a nerve impulse, a mediator, acetylcholine, is released at the endings of the motor nerve, which interacts with the cholinergic receptor of the postsynaptic membrane (synapses). As a result of this, the permeability of the membrane for Na and K ions increases, which, in turn, causes its depolarization (the appearance of a postsynaptic potential). After this, an excitation wave (electronegativity wave) appears in adjacent areas of the muscle fiber membrane, which propagates along the skeletal muscle fiber, usually at a speed of several meters per second. As a result of excitation, the muscle changes its elastic properties. If the attachment points of the muscle are not fixed motionless, it shortens (contracts). In this case, the muscle produces a certain mechanical work. If the attachment points of the muscle are immobile, tension develops in it. Between the occurrence of excitation and the appearance of a contraction wave or tension wave, some time passes, called the latent period. Muscle contraction is accompanied by the release of heat, which continues for a certain time even after relaxation.

In human muscles, the existence of “slow” muscle fibers has been established (these include “red” ones, containing the respiratory pigment myoglobin) and “fast” (“white” ones, which do not have myoglobin), differing in the speed of the contraction wave and its duration. In “slow” fibers, the duration of the contraction wave is approximately 5 times longer, and the conduction speed is 2 times less than in “fast” fibers. Almost all skeletal muscles are of the mixed type, i.e. contain both “fast” and “slow” fibers. Depending on the nature of the irritation, either a single - phasic - contraction of muscle fibers occurs, or a long-term - tetanic one. Tetanus occurs when a series of irritations enters a muscle with such a frequency that each subsequent irritation still finds the muscle in a state of contraction, resulting in a summation of contractile waves. NOT. Vvedensky established that an increase in the frequency of stimulation causes an increase in tetanus, but only up to a certain limit, which he called the “optimum”. Further increase in stimulation reduces tetanic contraction (pessimum). The development of tetanus has great importance when contracting “slow” muscle fibers. In muscles with a predominance of “fast” fibers, maximum contraction is usually the result of the summation of contractions of all motor units, into which nerve impulses, as a rule, do not arrive simultaneously, asynchronously.

In striated muscles, the existence of so-called purely tonic fibers has also been established. Tonic fibers are involved in maintaining “fatigue-free” muscle tone. A tonic contraction is a slowly developing continuous contraction that can be maintained for a long time without significant energy expenditure and is expressed in “tireless” resistance to external forces tending to stretch the muscular organ. Tonic fibers react to a nerve impulse with a wave of contraction only locally (at the site of irritation). However, due to the large number of terminal motor plaques, the tonic fiber can be excited and contracted as a whole. The contraction of such fibers develops so slowly that even at very low frequencies of stimulation, individual waves of contraction overlap each other and merge into a long-lasting shortening. Long-term resistance of tonic fibers, as well as slow phase fibers, to tensile forces is ensured not only by elastic tension, but also by an increase in the viscosity of muscle proteins.

To characterize the contractile function of muscles, the concept is used "absolute strength", which is a quantity proportional muscle cross section, directed perpendicular to its fibers, and is expressed in kg/cm2. For example, the absolute strength of the human biceps muscle is 11.4, and the gastrocnemius muscle is 5.9 kg/cm2.

Systematic intensive work of muscles (training) increases their mass, strength and performance. However, excessive work leads to the development of fatigue, i.e. to a decrease in muscle performance. Muscle inactivity leads to muscle atrophy.

Functional features of smooth muscles

The smooth muscles of internal organs differ significantly from skeletal muscles in the nature of innervation, excitation and contraction. Waves of excitation and contraction occur in smooth muscles at a very slow pace. The development of a state of “tireless” smooth muscle tone is associated, as in tonic skeletal fibers, with the slowness of contractile waves, merging with each other even with rare rhythmic stimulation. Smooth muscles are also characterized by the ability to automatize, i.e. to activities not related to the entry of nerve impulses into the muscle from the central nervous system. It has been established that not only the nerve cells present in smooth muscles, but also the smooth muscle cells themselves have the ability to rhythmically spontaneously excite and contract.

The ability of smooth muscles to change length without increasing tension (filling hollow organs, such as the bladder, stomach, etc.) is essential for the body.

Human skeletal muscles

Human skeletal muscles, varying in shape, size, and position, make up over 40% of his body weight. When contracting, the muscle shortens, which can reach 60% of its length; the longer the muscle (the longest muscle in the body, the sartorius, reaches 50 cm), the greater the range of movement. Contraction of the dome-shaped muscle (for example, the diaphragm) causes its flattening, while contraction of the ring-shaped muscles (sphincters) is accompanied by a narrowing or closing of the opening. Muscles of the radial direction, on the contrary, cause expansion of the holes when contracting. If the muscles are located between the bony protrusions and the skin, their contraction causes a change in the skin texture.

All skeletal, or somatic (from the Greek soma - body), muscles, according to topographic-anatomical principles, can be divided into the muscles of the head, among which there are facial and masticatory muscles that affect the lower jaw, muscles of the neck, torso and limbs. The muscles of the trunk cover the chest and make up the walls of the abdominal cavity, as a result of which they are divided into the muscles of the chest, abdomen and back. The dismemberment of the skeleton of the limbs serves as the basis for identifying the corresponding muscle groups: for the upper limb - these are the muscles of the shoulder girdle, upper arm, forearm and hand; for the lower limb - muscles of the pelvic girdle, thigh, lower leg, foot.

A person has about 500 muscles associated with the skeleton. Among them, some are large (for example, the quadriceps femoris muscle), others are small (for example, the short back muscles). Collaboration muscle movement is performed on the principle of synergy, although individual functional muscle groups work as antagonists when performing certain movements. So, in front of the shoulder there are the biceps and brachialis muscles, which perform flexion of the forearm at the elbow joint, and at the back is the triceps brachii muscle, the contraction of which causes the opposite movement - extension of the forearm.

Simple and complex movements occur in spherical joints. For example, in the hip joint, hip flexion is caused by the iliopsoas muscle, and extension by the gluteus maximus. The thigh is abducted by contraction of the gluteus medius and minimus muscles, and is adducted by the five muscles of the medial thigh. The muscles that cause inward and outward rotation of the hip are also located around the circumference of the hip joint.

The most powerful muscles are located on the torso. These are the back muscles - the erector torso, the abdominal muscles, which make up a special formation in humans - the abdominal press. Due to the vertical position of the body, the muscles of the lower limb of a person have become stronger, since, in addition to participating in locomotion, they provide support for the body. In the process of evolution, the muscles of the upper limb, on the contrary, have become more dexterous, guaranteeing the performance of fast and accurate movements.

Based on the analysis of spatial position and functional activity of muscles modern science also uses the following association: a group of muscles that carries out movements of the torso, head and neck; a muscle group that carries out movements of the shoulder girdle and free upper limb; muscles of the lower limb. Within these groups, smaller ensembles are distinguished.

Muscle pathology

Violations of the contractile function of muscles and their ability to develop and maintain tone are observed with hypertension, myocardial infarction, muscular dystrophy, atony of the uterus, intestines, bladder, various forms paralysis (for example, after suffering polio), etc. Pathological changes in the functions of muscle organs can occur due to disturbances in nervous or humoral regulation, damage to individual muscles or their parts (for example, during myocardial infarction) and, finally, at the cellular and subcellular levels. In this case, there may be a metabolic disorder (primarily the enzyme system for the regeneration of high-energy compounds - mainly ATP) or a change in the protein contractile substrate. These changes may be due to insufficient formation of muscle proteins due to impaired synthesis of the corresponding information, or matrix, RNA, i.e. congenital defects in the DNA structure of the chromosomal apparatus of cells. The last group of diseases is thus classified as hereditary diseases.

Sarcoplasmic proteins of skeletal and smooth muscles are of interest not only from the point of view of their possible participation in the development of the viscous aftereffect. Many of them have enzymatic activity and are involved in cellular metabolism. When muscle organs are damaged, for example, with myocardial infarction or impaired permeability of the surface membranes of muscle fibers, enzymes (creatine kinase, lactate dehydrogenase, aldolase, aminotransferases, etc.) can be released into the blood. Thus, determining the activity of these enzymes in blood plasma in a number of diseases (myocardial infarction, myopathies, etc.) is of serious clinical interest.