Физиология и этология животных теоретический курс (возбудимые ткани) = Physiology and ethology of Animals theoretical course (excitable tissues). Учеб

3.Physiology of muscles

3.1.Types and properties of muscles.

Many processes of vital activity (animal movement, breathing, circulation, digestion, excretion) are carried out due to the activity of various muscle groups.

1.Animals distinguish the following types of muscles:

2.Transversely striated skeletal (voluntary),

3.Transversely striated cardiac (involuntary),

4.Smooth muscles of internal organs, vessels and skin (invol-

untary).

Specialized contractile formations - myoepithelial cells, pupil muscles and ciliary body of the eye

In addition to the properties of excitability and conductivity, muscles have contractility, i.e. the ability to shorten or change the degree of stress upon excitation. The function of reduction is possible due to the presence in the muscle tissue of special contractible structures.

3.2. Structure and composition of muscles

Skeletal muscles. The muscle consists of primary bundles, each of which contains 20-60 fibers each. The bundles are separated from one another by a connective tissue membrane - a perimisium, and the fibers by an endomysium. In the muscle of animals, there are from several hundred to several hundred thousand fibers with a diameter of 20 to 100 μm and a length of up to 12-16 cm. The fiber is covered with a true cell membrane - the sarcolemma, under which the nuclei are located. The fiber consists of myofibrils (their 1000-2000 and more, diameter 0.5-2.0 microns), stretching from end to end. Between the myofibrils, rows are located mitochondria, to supply the muscle with energy.

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Fibers have a specific transverse striation, which is due to the alternation of optically more and less dense patches. Under the light microscope, the myofibrils are represented by alternating dark discs A-isotropic (have a birefringence), their length is constant, and the light discs I-isotropic (almost do not have birefringence), the length of which depends on the stage of contraction of the muscle fiber. In the middle of the isotropic disc is the Z-band (supporting collagen cells), in the middle of the anisotropic disc - a less pronounced M-band.

Myofibrilla consists of parallel filaments, or protofibrils (filaments, 2000-2500) of different thicknesses and chemical composition. The protofibrils have a 5-15 nm spore and a length of 1.0-1.5 μm. Thick protofibrils contain molecules of the myosin (55-60%) and form anisotropic disks. Thin protofibrils consist of actin (2127%) and form isotropic disks. Together they form an actomyosin complex.

Threads of actin are attached to strip Z, crossing it in both directions; they occupy not only the region of the I-disk, but also enter the gaps between the threads of the myosin in the region of the A-disk. In these areas, the threads of actin and myosin are connected among themselves by transverse bridges that depart from myosin. Each myosin fiber is surrounded by six actin filaments.

The structural and functional contractile unit of the myofibril is the sarcomere, a repeating region of the fibril limited by two strips of Z. It consists of half of the isotropic, whole anisotropic and half of the other isotropic disks. The value of the sarcomere in muscles of homothermous is about 2 μm.

A smooth endoplasmic reticulum of muscular fibers, or sarcoplasmic reticulum, forms a single system of tubules and cisterns and participates in the propagation of excitation inside the muscle fibers, as well as in the processes of muscle contraction and relaxation.

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Other contractile proteins include tropomyosin (4-6%) and a troponin complex (4-6%), contained in fine filaments. The muscle also contains myoglobin, glycolytic enzymes and other soluble proteins that do not perform contractile function (5-10%).

Smooth muscles. The main structural elements of smooth muscle tissue are myocytes - muscular cells spindle-shaped and stellateshaped 60-200 microns long and 4-8 microns in diameter. The largest length of cells (up to 500 μm) is observed in the uterus during pregnancy. The nucleus is in the middle of the cells. Its shape is ellipsoidal, with the contraction of the cell it twists into a corkscrew. Around the nucleus, mitochondria and other trophic components are concentrated. There is no transverse striation of the fibers. There are only longitudinally oriented, irregularly distributed myosin and actin protofibrils 1-2 μm in length. In the protoplasm of cells there are in large quantities bubbles containing Ca++.

In the walls of hollow organs smooth muscle cells are connected by special intercellular contacts (desmosomes). Such formations, in which cells closely adjoin, but cytoplasmic and membrane continuity between them are absent, are called “ functional syncytium”. In syncytium cells, excitation can freely propagate from one cell to another.

3.3. The mechanism of muscle contraction

Under normal conditions, skeletal muscles are excited by impulses that travel along the fibers of motor neurons (motoneurons) located in the anterior horns of the spinal cord or in the nuclei of the cranial nerves.

Motorneuron, its long process (axon) and a group of muscle fibers innervated by this axon, constitute a motor, or neuromotor, unit.

The thinner specialized the muscle is, the less the number of muscle fibers enters the neuromotor unit. Small motor units include only 3-5 fibers (for example, in the muscles of the eyeball,

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small muscles of the facial part of the head), large motor units - up to several thousand fibers (in large muscles of the trunk and extremities). The neuromotor unit works as a whole: the impulses emanating from the motoneuron, activate the muscle fibers.

The excitability of muscle fibers is lower than the excitability of nerve fibers innervating the muscles. This is because the resting potential of muscle fibers is higher (about -90 mV) of the resting potential of nerve fibers (-70 mV). Consequently, for the emergence of an action potential in the muscle fiber, it is necessary to depolarize the membrane by a larger amount than in the nerve fiber.

The duration of the action potential in the muscle fiber is 5 ms (in the nervous fiber - 0.5-2.0 ms), the excitation rate is up to 5 ms (in myelinated nerve fibers - up to 120 ms).

Reduction is a change in the mechanical state of the myofibrillar apparatus of muscle fibers under the influence of nerve impulses. Externally, the contraction is manifested in the change in the length of the muscle or in the degree of its tension, or at the same time both.

According to the accepted “theory of slip”, the basis of contraction is the interaction between actin and myosin filaments of myofibrils due to the formation of transverse bridges between them. As a result, there is a “retraction” of fine actin myofilaments between myosin.

During sliding, the actin and myosin filaments themselves do not shorten, the length of A-disks also remains the same, while the I-disks and H-bands become narrower . The length of the threads does not change when the muscle is stretched, only the degree of their overlapping decreases.

The molecular mechanism of contraction is based on the socalled electromechanical conjugation process, and the Ca++ ions contained in the sarcoplasmic reticulum play a key role in the interaction of myosin and actin myofilaments.

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A decrease in the level of Ca++ in the sarcoplasm suppresses the ATP activity of actomyosin; while the transverse bridges of myosin are detached from actin. There is relaxation, lengthening of the muscle, which is a passive process.

Thus, muscle contraction and relaxation is a series of processes unfolding in the following sequence: stimulus → occurrence of action potential → electromechanical co-tension (T-tube excitation, release of Ca++ and its effect on the troponin-tropomyosin-actin system) → formation of transverse bridges and “slipping” of actin filaments along myosin lines → shortening of myofibrils → decrease in Ca++ ion concentration due to calcium pump → spatial variation proteins of the contractile system → relaxation of myofibrils.

3.4. The energy of muscle contraction

Muscles are organs that convert chemical energy into mechanical and thermal energy. The coefficient of efficiency is about 30%, that is, about 2/3 of the energy is lost as heat.

A direct source of free energy for muscle contraction is ATP, which undergoes hydrolytic cleavage to ADP and inorganic phosphate during the contractile act.

ATP + H2O ↔ ADP + Pinorg. + H+ (energy for contraction).

The reserves of ATP in the muscle are sufficient for 10-12 reductions in continuous operation. Therefore, the consumption of ATP should be restored. One of the stable sources of the necessary high-energy phosphate groups is creatine phosphate. The transfer of phosphate groups from phosphocreatine to ADP is carried out by the enzyme creatine kinase. In turn, the energy for the resynthesis of creatine phosphate is provided by oxidation.

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Creatine kinase

↓

Creatine phosphate + ADP → ATP + Creatine Creatine + Pinorg. + Energy from oxidation → Creatine phosphate

Although this pathway is effective, the reserves of creatine phosphate in the muscle during continuous operation are rapidly depleted, and, consequently, the reserves of ATP decrease, the concentration of ADP and inorganic phosphorus increases. All this leads to stimulation in the working muscle of glycolysis, a cycle of tricarboxylic acids and oxidative phosphorylation. As the initial “fuel” skeletal muscles use free fatty acids and ketone bodies or glucose.

Resting muscles consume only small amounts of glucose; the main substratum of energy metabolism is fatty acids and ketone bodies coming from the liver.

With moderate exercise, in addition to fatty acids and ketone bodies, the consumption of glucose delivered by blood from the liver increases in muscles.

With intense muscle load, a large expenditure of ATP is not covered by the delivery of conventional substrates and oxygen by blood. The preferred energy substrate is the reserve muscle poly- saccharide-glycogen.

Up to a certain limit of power, glycogenolysis is carried out mainly by aerobic means.

C6H12O6 + 6O2 + 38ADP + 38 PhN ↔ 6CO2 + 6H2O + 38ATP (Including 2 ATP in the process of glycolysis)

At even higher loads, the rate of anaerobic glycogenolysis sharply increases, the final product of which is lactic acid. Energetically this process is about 15 times less efficient.

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Accumulation in the muscles of lactic acid, which slowly diffuses into the blood, causes a decrease in pH and an increase in the temperature of the muscles. This reduces the efficiency of metabolism in the muscles and by the feedback mechanism inhibits the activity of glycolytic enzymes.

Entering into the blood lactic acid is transferred to the liver, where it turns into glucose and glycogen. The resulting glycogen either remains in the liver, or serves as a source of blood glucose used to synthesize muscle glycogen. Thus, the energy for muscle contraction is largely provided by the processes of oxidative phosphorylation in the liver.

Muscle glycogen → Lactic acid of blood

↑↓ ↓

Glucose of blood ← Glycogen of liver СО2

The ability of the muscle to develop maximum activity under anaerobic conditions leads to the appearance of so-called oxygen debt, which must be repaid in the recovery period (the animal continues to breathe heavily and after a load).

3.5. Modes and types of muscle contractions

Excitation and contraction of muscles in natural motor acts is caused by nervous impulses coming from the central nervous system.

To study the properties of muscles in an experiment, the neuromuscular apparatus of a frog is usually used as an object, and the electric current is used as an irritant.

Irritation of the motor nerve, which causes muscle contraction, is called indirect stimulation, and direct stimulation of the muscle itself is a direct irritation.

The recording of muscle contractions when stimulated on a device by a myograph is called myography. The removal of the bio-

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potentials of the muscle itself when it is excited (with the help of electrodes) is called electromyography.

Modes of muscle contraction are determined by the frequency and strength of stimulation.

For stimuli of no more than 10 Hz, the muscle responds with single contractions, since the liberated Ca++ in the intervals between the stimuli completely flows back into the sarcoplasmic reticulum. The contraction does not come immediately after the stimulation, but after a certain period of time, called the latent period, followed by a phase of contraction (shortening) and a relaxation phase of the muscle. In skeletal muscle in the latent period, the property of refractoriness is observed, and in the phase of contraction - exaltation.

When applying repeated stimuli to the muscle of constant force, duration and frequency (below the tetanizing one), the “ladder phenomenon” is observed, i.e. increase in the amplitude of contractions by the first few stimuli.

When the rhythmic stimuli of the high frequency are applied to the muscle, summation occurs and a strong and prolonged muscle contraction occurs. Such a state of sustained contraction is called tetanic contraction or tetanus. It can be denticulate or incomplete (with a stimulation frequency of 10-20 Hz), when the repeated stimulus acts on the muscle at the moment when it begins to relax, and is smooth or full (at a frequency of more than 20 Hz), when a second stimulus acts on the muscle before the beginning of its relaxation. The amplitude of tetanic contraction is 2-4 times higher than the amplitude of a single contraction with the same strength of irritation.

In tetanic contractions, muscle fibers become fatigued more than with single contractions.

Impulses from motoneurons in rest are involved in maintaining the so-called muscle tone. By tonus is understood the state of natural constant muscle tension at low energy costs.

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Types of muscle contractions. There are isotonic, isometric and auxotonic types of muscle contractions.

1.Isotonic (from the Greek isos - equal, tonos - tension) - this is an abbreviation in which the shortening of muscle fibers occurs, but their tension does not change. In the experiment, an isotonic contraction is obtained by electrically stimulating an isolated muscle burdened with a small load (Fig. 9, A).

2.Isometric (from Greek isos - equal, meros - measure) is a contraction, in which the length of the fibers does not decrease, but their tension increases. In the experiment, this reduction is obtained by stimulation of an isolated muscle, both ends of which are immovably fixed, or muscles weighed down by excessive weight.

3.The mixed type of muscle contraction, in which both the length and the tension change, is called auxotonic. In the natural conditions of muscle activity, there is practically no isotonic or purely isometric contraction. Change their length muscles can be unhindered only in the absence of resistance. In the body, they are attached to the bone levers and for the shortening must overcome the resistance. At the same time, it is hardly possible to have absolutely rigid fixation of the joints, excluding the shortening of the muscles when the muscles contract. Therefore, when animals perform complex motor acts, all working muscles are contracted auxotonically, with either isotonic or isometric contractions predominating.

3.6. Work and fatigability of muscles

The main indicators that characterize the activity of muscles are their strength and efficiency. The effect will depend on the structure of the working muscles (length, number and location

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of the fibers), the magnitude of their stretching, and the degree of muscle fatigue.

Strength of muscles. Strength is a measure of mechanical effect on the muscle from the side of other bodies, is expressed in newtons or kg-force. With isotonic contraction in the experiment, the force is determined by the mass of the maximum load that the muscle can lift (dynamic force), with isometric - the maximum voltage that it can develop (static force).

A single muscle fiber develops a tension of 100-200 mg-force. The thicker the fiber, the greater the tension. The increase in muscle strength during training is due precisely to hypertrophy of muscle fibers, and not by an increase in their number.

When determining the strength of whole muscles, the decisive factor is not their thickness, but the “physiological” cross-sectional area of individual fibers.

The anatomical diameter is the cross-sectional area, perpendicular to the muscle length (the line connecting its beginning and end) and passing through the abdomen in its widest part. The indicator characterizes the size of the muscle.

The physiological diameter is the total cross-sectional area of all muscle fibers that make up the muscle. This indicator characterizes the strength of the muscle.

The physiological cross-section coincides with the anatomical only in muscles with longitudinally arranged fibers. In muscles with an oblique arrangement of fibers (half-pinnate and pinnate), the “physiological diameter” always exceeds the anatomical diameter. Therefore, the strength of muscles with oblique fibers is always greater than that of muscles of the same thickness, but with longitudinal fibers.

The most powerful are the multi-pinnate muscles, then one-, bi-pinnate, semi-pinnate, spindle-shaped and longitudinally fibrous. The first two groups in the anatomical and functional classification are referred to as statodynamic, they have great strength

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