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

and endurance, but a limited ability to shorten, and the rest to dynamostatic or dynamic, they are well shortened, but easily fatigued.

The absolute index of the strength of different muscles is the absolute muscular force, i.e. weight of the maximum load, which lifts the muscle divided by the total area of all muscle fibers. In agricultural animals, the absolute strength of skeletal muscles ranges from 5 to 15 kg-force per 1 cm2 of the area of the physiological diameter.

Work of muscles. Evaluating the activity of muscles, they usually take into account only the external work that they perform, i.e. shortening of muscle against load or resistance.

In the simplest case, when the force in the direction is constant, and the movement of the load is rectilinear, work (W) can be estimated as the product of the load mass (P) by the lift height (h):

W = P x h, J (kg / cm).

The power of the muscle is defined as the amount of work per unit time.

Muscle fatigue. Fatigue is a temporary decrease or loss of efficiency of individual organs or the organism as a whole, which comes after the loads. Muscle fatigue occurs with their long reduction (tension) and has a certain biological significance, signaling a partial depletion of resources.

Characteristic signs of the fatigue curve are an increase in the latent period, a decrease in the force (amplitude) of contractions, an extension of the relaxation period, and in extreme terms - rigidity of the muscle (inability to relax). The main causes are: depletion of the energy source - ATP, a decrease in glycogen stores, a lack of oxygen, the depletion of acetylcholine stocks in the axon endings, and the accumulation of metabolic products (lactic and phosphoric acids).

Typical external signs of fatigue (in addition to reducing productivity) are discoordination movements, dyspnea, sweating. All

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this testifies to the fact that the activity of many organs and systems (musculoskeletal system, nervous, respiratory, cardiovascular and other systems) is disrupted, as well as coordination by the central and autonomic nervous systems.

The recovery period after fatigue depends on the degree of fitness of the animal and the severity of the work performed.

3.7. Functional features of smooth muscles

Smooth muscle cells of many internal organs (except for blood vessels and seminal ducts) have spontaneous electrical activity. Tonic muscle contractions are also evident in their isolation and denervation. Consequently, the excitation is not due to the transmission to the muscle of nerve impulses, but is of a myogenic nature. This feature is referred to as the “automatic” smooth muscle.

Excitation occurs in the so-called pacemaker cells (from the English pacemaker - which sets the rhythm), which are similar in structure to other smooth muscle cells, but differ in their electrophysiological properties. They are able to generate and maintain electrical oscillations that are transmitted to other cells, involving them in rhythmic activity.

Emerging pacemaker potentials, activating the calcium channels, depolarize the membrane of the cells to the threshold, resulting in an action potential. Emerging action potentials spread from one muscle cell to another through dense contact plates between their membranes - nexus with low electrical resistance. Thus, the excitation propagates throughout the muscle at a rate of 5-10 cm/s. The muscle reproduces synchronously the activity of pacemakers as a single functional system. In muscles with spontaneous activity of cells, the autonomic nervous system only corrects the activity of pacemakers.

Unlike striated muscles, smooth muscles have a high degree of plasticity, i.e., the ability to maintain the stretched length without

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changing the tension (or the ability to relax both in the shortened and in the stretched state). This prevents an excessive increase in pressure in the hollow organ (for example, in the bladder) as it is filled.

With a strong stretching of the smooth muscle, its contraction occurs as a result of increasing depolarization of pacemaker cells and an increase in the discharge frequency. The contraction caused by stretching promotes self-regulation of the tone of the blood vessels.

The sliding speed of irregularly located actin and myosin filaments of smooth muscle, like the rate of ATP cleavage, is 100-500 times lower than in skeletal muscles. Relaxation also occurs more slowly, since the initial concentration of Ca++ in sarcoplasm is restored longer due to the weak development of the sarcoplasmic reticulum.

All these factors contribute to long lasting contractions of smooth muscles without fatigue and with little energy expenditure.

Due to spontaneous fluctuations in the activity of pacemakers’ rhythmic changes in the tone of smooth muscles with pronounced organ specificity are observed. In the walls of hollow organs (stomach, intestine, gallbladder, bladder), slow (up to 5 s) rhythmic contractions and relaxations of smooth muscles ensure the movement of the contents. In sphincters (from Greek sphingo - to compress) - ring formations at the entrance or exit of hollow organs - very long tonic muscle contractions, on the contrary, prevent the movement of the contents. In the walls of large blood vessels, maintaining a constant muscle tone contributes to the regulation of blood pressure and normal blood supply to organs.

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4.Physiology of nerve fibers

4.1.Features of the structure and classification of nerve fibers

Nerve fibers are the processes of nerve cells covered with glial membranes. The central part of the nerve fiber is the axial cylinder, covered with a shell - axolemma, under which there is an axoplasm containing neurofibrils and a large number of mitochondria.

Nerve fibers are divided into medullated or myelin and nonmedullated or nonmyelinated by particularities of the structure.

For the somatic department of the nervous system innervating the skeletal musculature, which has a high degree of functional load, myelin type of nerve fibers is characteristic, and for the vegetative department innervating the internal organs - the nonmyelinated type.

The myelin sheath is formed by Schwann cells by repeatedly winding them around the axial cylinder and is located not with a solid line, but with segments 0.5-2.0 mm long. The space between the segments (1-2 microns), where the myelin sheath is absent, is called Ranvier intercepts. The outer membrane of Schwann cells covering myelin forms the uppermost membrane of the nerve fiber, which is called the Schwann sheath, or neurilemma. Nonmyelinated fibers are distinguished by the fact that the myelin sheath does not develop in them and their axial cylinders are covered only by the Schwann cover.

By the functional value, nerve fibers are divided into somatic and vegetative.

Somatic nerve fibers - innervate the skeletal muscles, blood vessels and sensory organs.

Vegetative nerve fibers (sympathetic and parasympathetic) - innervate internal organs.

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Nerve fibers are divided into 3 main groups by the speed of excitation and the diameter of the fibers:

Type A - myelin: Aα - the thickest, diameter 10-20 microns and the highest speed of excitation - 60-120 m/s; Aβ - 7-15 microns and 40-90 m/s. Aγ - 4-8 μm and 15-30 m/s, Aδ 3-5 μm and 5-25 m/s, respectively, represent the motor fibers of the somatic nervous system and sensitive fibers of skin receptors (tactile, temperature, pain) and proprioceptors.

Type B - myelin, speed 3-15 m/s, diameter 1-3 μm, represent preganglionic fibers of the sympathetic nervous system.

Type C - nonmyelinated, a small diameter of 0.3-1.0 microns, the speed is low - 0.5-2.0 m/s, most postganglionic fibers of the sympathetic nervous system.

4.2. The structure and types of synapses. Transmission of excitation in the neuromuscular synapse

The transition of excitation from the nerve fiber to the innervated cell (nervous, muscular, secretory) is performed with the participation of synapses.

Synapses (from Greek synapsis - connection, connection) - a special type of intermittent contacts between cells, adapted for oneway transmission of excitation or inhibition from one element to another.

Depending on the location the synapses are divided into:

1)Central (between the brain and spinal cord)

2)Peripheral (between organs and central nervous system)

3)Inter-neuronal (between individual cells - neurons).

By functions synapses are:

1)Excitatory - secrete mediators: adrenaline, norepinephrine, acetylcholine

2)Inhibitory - linked to the amino acid glycine, GABA (gammaaminobutyric acid).

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By the method of transmission of excitation synapses:

1)Chemical (mediator);

2)Electrical (electrical impulse);

3)Electrochemical or mixed.

By the nature of the active agent the synapses are divided into:

1)Cholinergic (acetylcholine);

2)Adrenergic (adrenaline, norepinephrine).

By structure synapses:

1)The simplest - the cell is innervated by only one fiber (neu-

romuscular synapse).

2)complex synapses - the number of terminating axons can be estimated up to several thousand (the brain).

The main components of the synapse are:

1)the presynaptic part (usually a thickened end of the presynaptic axon),

2)the postsynaptic part (the cell site to which the presynaptic ending is suitable)

3)the synaptic cleft that separates them (in synapses with electrical transmission it is absent).

Mechanism of transmission of excitation in the neuromuscular synapse

In the presynaptic membrane, a neurosecret mediator (acetylcholine) is produced. The electrical impulse, entering the presynaptic membrane, causes the mediator to separate into the intersynaptic cleft and irritates the specific receptors of the postsynaptic membrane, which are very sensitive to the mediator. In the postsynaptic membrane, an action potential arises that transfers excitation to the muscle and it shrinks.

One nerve impulse causes a synchronous release of 100-200 portions of the mediator in less than 1 ms. Total stocks of acetylcholine in the end is enough for 2500-5000 pulses. The time from the appearance of a nerve impulse in the presynaptic ending before

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the occurrence of an action potential is called a synaptic delay, it is 0.2-0.5 ms.

To restore the excitability of the postsynaptic membrane, it is necessary to exclude the depolarizing agent, the mediator. This function is performed by the enzyme acetylcholinesterase localized in the synaptic cleft, which hydrolyses acetylcholine to acetate and choline. The permeability of the membrane returns to the initial level, and the membrane repolarizes. This process is very fast: all the acetylcholine released into the gap is split in 20 ms.

4.3. Generation and transmission of excitation in receptors

Receptors (from the Latin recipio - to take) are specific sensitive endings that perceive stimuli from the external or internal environment and convert the stimulus energy into electrochemical signals - nerve impulses.

Being the initial link of complex sensory systems, receptor devices serve as the main source of information for animals.

Through nerve fibers, information encoded in the form of nonspecific pulses enters the neurons of the central nervous system, where it is decoded and analyzed.

By location relative to the source of stimulation distinguished:

1)exteroceptors (distant or contact);

2)interoreceptors (visceroreceptors in internal organs and proprioceptors in muscles).

By type of physical energy (modality) of stimuli, the receptors are divided into:

1)photoreceptors (light),

2)mechanoreceptors (touching),

3)thermoreceptors (thermal, cold),

4)chemoreceptors (chemical substances),

5)noni receptors (painful irritation).

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6) multimodal (reacting to several types of stimuli). By origin, the receptors:

1)primary - the impact is directly perceived by the nerve endings of sensitive neurons (skin, skeletal muscles, internal organs, olfactory organs);

2)secondary - specialized receptor cells are located between the stimulus and the end of the sensitive neuron, forming synapticlike contacts (hearing, taste, vision, balance).

In the mechanism of generation of a nerve impulse in receptors, the energy of the external environment (light, flavor, mechanical, etc.) is translated “into the language” of the receptor cell. In this case, the form of interaction of an adequate stimulus with the receptor membrane can be different (deformation of the membrane in mechanoreceptors, binding of chemical agents by the membrane in chemoreceptors, excitation of photopigment membranes by light photons in photoreceptors, etc.). In all cases this leads to the same result: an increase in the ion permeability of the membrane, the penetration of sodium into the cell, the depolarization of the membrane, and the generation of the so-called receptor potential. In primary receptors, it is transformed into the action potential of the nerve fiber, in the secondary receptors it causes the release of a chemical mediator that depolarizes the membrane of the postsynaptic nerve end, in which the action potential arises.

In principle, the generation and transmission of excitation in receptors is accomplished by the same mechanism and in the same sequence as in the neuromuscular junction. However, the nerve impulses that arise here propagate centripetally and carry information to the analyzing (sensory) centers.

All receptors are characterized by adaptation to the action of the stimulus. The receptor potential decreases during a constant stimulus, which reduces the frequency of generation of action potentials.

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4.4.The laws of excitation in the nerve

1.The law of physiological continuity of excitation. It is possible only with the integrity of the fiber and the normal functional activity of the ion channels of the membrane.

2.The law of bilateral excitation. Excitation is carried out along the nerve fiber in both directions, from the place of irritation.

3.The law of isolated excitation. Pulses that propagate along the same fibers do not pass to other fibers and are directed only to those cells with which the endings of a given nerve fiber contact. It is provided with electrically insulating properties of the myelin sheath, and in the non-myelinated ones - the resistance of the intercellular fluid - axoplasm.

4.The law of the rhythm of excitation. With a single stimulation, the nerve corresponds to a single excitation pulse, as the frequency of stimulation increases, and the excitation frequency is compared with it.

4.5.Pessimum and optimum of strength and frequency

of stimulation. Parabiosis

Outstanding Russian physiologist N.E. Vvedenskiy (1886) established that the transition of excitation to inhibition, and vice versa, depends on the frequency and strength of the stimulus and on the level of lability of the irritated tissue. Increasing the frequency and strength of stimulation to a certain limit causes an increase in the height of the tetanic contraction of the skeletal muscle. The most favorable frequency of nerve impulses entering the skeletal muscle causes the greatest height of the tetanus. This frequency is called the optimal frequency, or the optimum of frequency. The optimum frequency corresponds to a frequency at which each

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subsequent stimulation catches the skeletal muscle in the state of greatest excitability observed in the exaltation phase. Conversely, if each subsequent stimulation catches the skeletal muscle in the phase of absolute refractoriness, then the tetanic contraction of the muscle decreases dramatically or does not occur. This excessively high frequency is the worst, pessimal, or pessimum of frequency. Optimum of frequency corresponds to a high level of lability of the nerve and muscle, and pessimum of frequency - a low level of nerve lability, even lower than the lability of the muscle.

The most favorable stimulus, causing maximum tetanic contraction of the skeletal muscle, is called the optimum of force. A further increase in the strength of the stimulation reduces the height of the contraction of the muscles. With an excessively large force of irritation, the muscle does not contract. This worst force of irritation is called a pessimal or pessimum of force - also a result of changes in excitability and lability, caused by previous irritations.

Parabiosis (parabiosis, Greek para - about + biosis - life) is a state of excitable tissue that occurs under the influence of strong stimuli and is characterized by a temporary loss of ability to function of the nerve (tissue) due to impaired conduction and excitability.

The term “parabiosis” was introduced in 1901 by N.E. Vvedenskiy, who first studied and described this condition on a neuromuscular drug. The nerve in a small area was damaged (altered) by chemical substances (cocaine, chloroform, phenol, potassium chloride), strong Faradic current, mechanical factor, then applied electric stimulation. The contraction of the muscle testified to the excitation of the nerve.

In this case, 3 phases were identified:

I phase (primum) - is characterized by a decrease in excitability and increased lability of the tissue;

II phase (optimum) - the greatest activity, while the excitability reaches a maximum, and the lability begins to decrease;

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