Smooth muscles form walls (muscle layer) internal organs and blood vessels. There is no transverse striation in myofibrils of smooth muscles. This is due to the chaotic location contractile proteins... The smooth muscle fibers are relatively shorter.

Smooth muscles less excitable than striated. Excitation in smooth muscles can be transmitted from one fiber to another, in contrast to nerve fibers and fibers of striated muscles.

The contraction of smooth muscles occurs more slowly and lastingly.

The refractory period in smooth muscles is longer than in skeletal ones.

An important property of smooth muscle is its large plastic , i.e. the ability to maintain the length given by stretching without changing the stress.

A feature of smooth muscles is their capacity for automatic activity , which is provided by nerve elements embedded in the walls of smooth muscle organs.

An adequate stimulus for smooth muscles is their rapid and strong stretching, which has great importance for the functioning of many smooth muscle organs (ureter, intestines and other hollow organs)

A feature of smooth muscles is also their high sensitivity to some biologically active substances (acetylcholine, adrenaline, norepinephrine, serotonin, etc.).

Smooth muscles are innervated by sympathetic and parasympathetic autonomic nerves, which, as a rule, have the opposite effect on their functional state.

Motor units, their classification. Physical properties of muscles. Muscle strength and work. The Law of Power (ADD OUT)

Each motor nerve fiber is a process of a nerve cell - motor neuron, located in the anterior horn spinal cord or in the motor nucleus of the cranial nerve. In the muscle, the motor fiber branches and innervates not one, but the whole group muscle fibers... A motor neuron, together with a group of muscle fibers innervated by it, is called motor unit.

Motor neurons are large and small.

Small motor neurons have thin axons and innervate a small number (tens) of muscle fibers, forming small motor units. Large motor neurons have thick axons that innervate a large number of muscle fibers (up to several thousand), forming large motor units.
Small motor units are mainly composed of small muscles(fingers, face, etc.), but they are also part of large muscles. Small motor units provide fast and subtle movements (such as finger movements). Large motor units are predominantly part of the large muscles of the trunk and limbs. These muscles perform relatively less subtle and slower movements than, for example, the movements of the fingers. Small motoneurons (low threshold) are excited more easily and faster than large (high threshold) ones.

Smooth muscles form the walls (muscle layer) of the internal organs and blood vessels.

The microscopic structure of smooth and striated muscles is different.

The physiological properties of smooth muscles in connection with the peculiarities of their structure and the level of metabolic processes differ significantly from the physiological properties of striated muscles.

Smooth muscles are less excitable than striated muscles. Excitation in smooth muscles can be transmitted from one fiber to another, in contrast to nerve fibers and fibers of striated muscles. Excitation through smooth muscles spreads at a low speed - 2-15 cm / s.

The contraction of smooth muscles occurs more slowly and lastingly. So, the contraction of the smooth muscles of the intestine of a rabbit can last 5 seconds, relaxation proceeds even more slowly.

Due to the duration of the contractile act, smooth muscle, even under the influence of rare stimuli, can go into a state of prolonged contraction, which resembles the tetanus of skeletal muscles. Long-term tonic contractions are also characteristic of smooth muscles.

The refractory period in smooth muscles is longer than in skeletal ones.

An important property of smooth muscle is its great plasticity, i.e. the ability to maintain the length created by stretching without changing the stress. This property is essential, since some organs of the abdominal cavity (uterus, bladder, gall bladder) sometimes stretch significantly.

Characteristic feature smooth muscle is their ability to automation, which is provided by nerve elements embedded in the walls of smooth muscle organs.

An adequate stimulus for smooth muscles is their rapid and strong stretching, which is of great importance for the functioning of many smooth muscle organs (ureter, intestines and other hollow organs).

A feature of smooth muscles is also their high sensitivity to certain biologically active substances (acetylcholine, adrenaline, norepinephrine, serotonin, and others).

Smooth muscles are innervated by sympathetic and parasympathetic autonomic nerves, which, as a rule, have the opposite effect on their functional state.

THEN. Duration of a single muscle contraction- 0.1s. Approximately the shortening and relaxation phase for skeletal muscle the same - 0.05s. LP is longer than PD.

In smooth muscles, the duration is from a few seconds to several minutes. The relaxation phase is longer. LP is shorter than PD.

SECTION: EXCITABLE TISSUE PHYSIOLOGY

LESSON # 1

TOPIC: BIOELECTRIC PHENOMENA IN THE BODY.

POTENTIAL OF REST, POTENTIAL OF ACTION.

THE LAWS OF EXCITATION

The duration of the lesson is 2 hours.

Plan and organization of the lesson.

1. Preparatory stage of the lesson:

a) organizational measures - 5 min.

b) checking and correcting the initial level of knowledge, by analyzing the material orally or using a textbook - 20 min.

2. The main stage of the lesson:

a) practical work - 45 minutes.

b) record of the research protocol - 15 min.

c) analysis of research results - 10 min.

3. The final stage of the lesson:

a) control of the final level of assimilation teaching material test control or solving situational problems - 20 min.

3. Learning objectives of the lesson.

KNOW:

1. The concepts of excitability and irritability.

2. The role, significance and function of the plasma membrane of cells.

3. The doctrine of the potential dependent sodium, potassium, chloride, calcium channels.

4. The doctrine of the uneven distribution of ions in excitable tissues, the transmembrane electrochemical gradient and equilibrium potential.

5. Membrane-ionic mechanisms of origin, physical characteristics and the physiological role of resting potential.

6. The mechanism of the action potential, as a manifestation of spreading excitement. Dynamics of ion currents upon excitation.

7. The ionic nature of the local response and the physiological characteristics that distinguish the local response from propagating excitation.

8. Change in excitability in different phases of action potential generation. Explanation of lability.

9. The law of electrotonic potential: the processes occurring under the cathode and anode, with the extracellular action of direct current on excitable tissues.

10. Laws of "force", "all or nothing", "force-time". Accommodation of channels of cell membranes.

11. The concept of rheobase, chronaxy.

12. History of the study of electrical phenomena in excitable tissues.

BE ABLE TO:

1. Draw schemes for the development of rest potential and action potential.

2. Draw curves of the action potential and changes in the excitability of the cell during its generation.

4. Draw a diagram of the equivalent electrical model of the plasma membrane.

5. Prepare the frog neuromuscular preparation.

6. Work with measuring instruments.

4. Methodology for conducting the lesson:

1. Preparatory stage of the lesson.

At the beginning of the lesson, its purpose and objectives should be formulated, which students should know and be able to do at the end of the lesson. In accordance with this, it is necessary to explain to students that knowledge of the material on this topic will be required to understand the significance of the role of the plasma membrane in the mechanisms of functioning of all cells of the body, and they are especially important in the study of the physiological properties and characteristics of nervous, muscle and secretory tissues. Knowledge of the structural features and modes of transport through the plasma membrane will allow students to explain the origin and maintenance of the basic constants of cells, the mechanisms of action of hormones, mediators and medicinal substances, the development of excitation and inhibition processes in the cells of the body and the performance of other specific functions. All the knowledge gained will be necessary in the study of other sections of physiology, in training at subsequent theoretical and clinical departments. Students should be drawn to the fact that at present the main research in the world in the field of physiology is carried out at the cellular, membrane or molecular level, that without knowledge of these sections it is impossible to explain and understand the causes of various diseases and to carry out the necessary therapy.

The main part preparatory phase classes should be devoted to monitoring the initial level of students' knowledge by means of oral or test questioning.

2. The main stage of the lesson.

This stage of the lesson should be devoted to the analysis and correction of the initial level of students' knowledge, taking into account the control carried out. For this purpose, it is recommended to conduct an oral analysis of the material on the main issues of the lesson and invite students to write and draw basic formulas, graphs and diagrams. In the process of parsing the educational material, it is necessary to find out all the questions of the lesson, write down the basic concepts and formulations, sketch diagrams, graphs and formulas in the reports. At the same time, students can use any educational literature: textbooks, reference books, atlases, electronic textbooks and other sources of information.

Practical part: Conducting laboratory work in accordance with the work program.

3. The final stage of the lesson.

At this stage of the lesson, the final level of students' knowledge is monitored, for which it is recommended to use either test control of knowledge or solving situational problems.

At the end of the lesson, the teacher checks and signs the students' protocols for the implementation of laboratory work, sets the task for self-preparation to the next lesson.

Laboratory works.

1. Preparation of the frog neuromuscular preparation.

To study the physiological properties of muscles and nerves, a neuromuscular preparation prepared from the hind legs of a frog is often used. The gastrocnemius muscle and sciatic nerve that innervates her.

Progress... Having dissected the nerve to knee joint, cut the limb above and below the knee joint and receive a neuromuscular preparation. To prepare the drug isolated muscle the nerve is cut off from the neuromuscular preparation.

2. Nerve conduction and its violation.

One of the main physiological properties of excitable tissues is excitability, which is different for different tissues. The threshold of irritation serves to characterize the level of excitability, i.e. the minimum strength of the stimulus, under the action of which a response occurs. Under experimental conditions, to determine the excitability of a muscle, a direct method of its stimulation is used, i.e. irritation applied directly to the muscle. Excitability of the nerve is examined by irritation of the nerve that innervates this muscle, i.e. by the method of indirect muscle irritation.

Progress... A neuromuscular preparation is being prepared. By applying to the nerve single stimuli with a constant duration, for example 0.5 ms, they gradually increase the amplitude and find that minimal stimulus force that causes a barely noticeable muscle contraction - this will be the irritation threshold for the nerve.

To determine the threshold of muscle irritation, direct stimulation is applied to it through the conducting circuit of the myograph connected to the stimulator. The irritation threshold is found in the same way as for indirect irritation.

Recommendations for the design of the work. Draw a diagram of the installation for direct and indirect stimulation of the muscle, write down the results of the experiment and give a comparative assessment of the excitability of the nerve and muscle. Draw conclusions about the difference in the values ​​of excitability of the nerve and muscle.

3. Experiments by Galvani.

Galvani's first experience.

Progress... Prepare a preparation of two hind legs of the frog and hang it on a tripod. Take bimetallic tweezers, one branch of which is made of copper and the other of iron. A copper branch is brought under the nerve plexuses, and the other is applied to the muscles of the paws. Contraction of the muscles of the legs is observed.

Galvani's second experiment (cutting without metal).

Galvani's second experiment was the first to prove the existence of "animal electricity" in the tissues, which arises between the damaged and undamaged surfaces of the muscle. If these two sections are connected by a nerve of a neuromuscular preparation, then a resting current occurs , which irritates the nerve and causes the muscle to contract.

Progress... The sciatic nerve is spread so that it simultaneously touches the damaged and undamaged surface of the thigh muscles. In this case, the muscles of the lower leg contract.

4. The experience of Matteuchi.

The Matteuchi experience.

Nerve irritation by skeletal muscle currents (secondary tetanus). Matteuchi showed in 1840 that muscle contraction of a neuromuscular preparation can occur if the nerve of this preparation is thrown over the contracting muscles of another neuromuscular preparation. Based on this, it was concluded that currents arise in the muscle when it is excited, which can become an irritant for another neuromuscular drug. These currents were called action currents.

Progress... The nerve of one neuromuscular preparation (with a piece of the spine) is placed with a glass hook on the electrodes that are connected to the stimulator. The nerve of the second neuromuscular preparation is thrust onto the muscles of this preparation in the longitudinal direction. The nerve of the first neuromuscular preparation is subjected to rhythmic stimulation. Tetanic contraction of both legs is observed.

5. Dependence of the strength of the response on the strength of the stimulus.

Skeletal muscle responds to stimuli of the threshold force with a minimum threshold contraction. If the strength of the stimulus is gradually increased, then the amplitude of contractions of the skeletal muscle will also gradually increase from threshold to submaximal and maximum contractions, after which an increase in the strength of the stimulus does not cause a further increase in the amplitude of contraction. This reaction of skeletal muscle is due to its structure. It consists of many muscle fibers with different excitability and, therefore, their involvement in the reaction proceeds gradually: the muscle fibers with the highest excitability react to the threshold strength of the stimulus, i.e. having the lowest threshold of irritation. As the strength of the stimulus increases, fibers with less excitability are gradually involved in the contractile process. With the maximum strength of the stimulus, all muscle fibers that make up the given muscle contract, and therefore the amplitude of muscle contractions no longer increases, despite the increase in the strength of the stimulus.

Progress... Preparing the drug calf muscle frogs. Find the threshold of irritation for the muscle, which is determined by its minimum contraction. Further, increasing the strength of the stimulus, the muscle contraction is recorded on a kymograph.

Make a conclusion about the relationship between the magnitude of irritation and the strength of muscle contraction.

6. The action of stimuli of various nature.

Problem number 1.

With a deterioration in the blood supply to the myocardium in the intercellular fluid, the concentration of potassium ions increases. How and why will this affect the generation of AP in myocardial cells?

Sample answer.

With an increase in the concentration of potassium ions in the intercellular fluid, hyperpolarization of the membranes of myocardial fibers occurs. The value of their critical level of depolarization approaches zero, as a result of which the generation of the action potential (AP) will become impossible.

Problem number 2.

How and why the AP cell amplitude will change

a) with an increase in the concentration of potassium ions in the cytoplasm

b) with an increase in the concentration of sodium ions in the intercellular fluid

c) with an increase in the permeability of the cell membrane for potassium ions?

Sample answer.

The AP amplitude will decrease with an increase in the concentration of potassium ions in the cytoplasm and an increase in the permeability of the cell membrane for these ions, and will increase with an increase in the concentration of sodium ions in the intercellular fluid.

Problem number 3.

What practical significance is the consequence of the "force-time" law, according to which, with an extremely short time of action of a superstrong stimulus, excitation will not arise in the tissue?

Sample answer.

This property (a law, a consequence of the "force-time" law) is the biophysical basis of the UHF-therapy method. Such electricity due to its ultra-high frequency, it does not have time to cause a change in the state of proteins of channels and pumps of cell membranes, therefore, the generation of AP in muscle cells and nerve endings does not occur. However, due to the presence of electrical resistance of tissues, they are heated.

Problem number 4.

Under the influence of local anesthetic, the number of inactivated sodium channels in the cell membrane increased. How and why will this affect the parameters of APs arising in the cell?

Sample answer.

With an increase in the number of inactivated sodium channels of the cell membrane, its conductivity for sodium ions will decrease. As a result, the diffusion flux of positively charged sodium ions entering the cell during the ascending phase of PD will decrease. This will lead to a decrease in the steepness of this phase and to a decrease in the AP amplitude.

Problem number 5.

Under the influence of pharmacological factors, the number of potassium channels in the cell membrane increased, which can be activated during the generation of PD in the cell. How and why will this affect the parameters of the cell's PD?

Sample answer.

If, during the generation of PD, the number of activated potassium channels of the cell membrane increases, then the diffusion flux of positively charged potassium ions, which leaves the cell, will increase, mainly during the descending phase of PD. This will lead to a decrease in the duration of this phase, and, consequently, the entire AP as a whole. In addition, the AP amplitude may also decrease slightly.

LESSON number 2

TOPIC: MUSCLE PHYSIOLOGY

Smooth muscles are part of the internal organs. Thanks to contraction, they provide the motor (motor) function of their organs (alimentary canal, genitourinary system, blood vessels, etc.). Unlike skeletal muscle, smooth muscle is involuntary.
Morpho-functional structure of smooth (not striated) muscles. The main structural unit of smooth muscle is a muscle cell, which is spindle-shaped and covered from the outside by a plasma membrane. Under an electron microscope, numerous depressions - caveolae - can be seen in the membrane, which significantly increase the total surface of the muscle cell. The sarcolemma of the unimplemented muscle cell includes the plasma membrane, together with the basement membrane that covers it from the outside, and the adjacent collagen fibers. The main intracellular elements:
nucleus, mitochondria, lysosomes, microtubules, sarcoplasmic reticulum and contractile proteins.
Muscle cells form muscle bundles and muscle layers. The intercellular space (100 nm or more) is filled with elastic and collagen fibers, capillaries, fibroblasts, etc. In some areas, the membranes of neighboring cells lie very tightly (the gap between the cells is 2-3 nm). It is assumed that these areas (nexus) serve for intercellular communication, transmission of excitement. It has been proven that some smooth muscles contain a large number of nexus (the sphincter of the pupil, circular muscles of the small intestine, etc.), while others have few or none (vas deferens, longitudinal muscles of the intestines). There is also an intermediate, or desmopodibny, connection between non-darkened muscle cells (through a thickening of the membrane and with the help of cell processes). Obviously, these connections are important for the mechanical connection of cells and the transfer of mechanical force by cells.
Due to the chaotic distribution of myosin and actin protofibrils, smooth muscle cells are not striated like skeletal and cardiac ones. Unlike skeletal muscles, there is no T-system in smooth muscles, and the sarcoplasmic reticulum is only 2-7% of the volume of myoplasm and has no connection with the external environment of the cell.
Physiological properties of smooth muscles. Smooth muscle cells, like striated ones, contract due to the sliding of actin protofibrils between myosin cells, however, the sliding speed and ATP hydrolysis, and hence the rate of contraction, is 100-1000 times less than in striated muscles. Thanks to this, smooth muscles are well adapted for prolonged sliding with little energy consumption and without fatigue.
Smooth muscles, taking into account the ability to generate AP in response to threshold or supra-horny stimulation, are conventionally divided into phasic and tonic. Phase muscles generate full-fledged AP, tonic - only local, although they also have a mechanism for generating full-fledged potentials. failure to tonic muscles to PD is explained by the high potassium permeability of the membrane, which prevents the development of regenerative depolarization.
The magnitude of the membrane potential of smooth muscle cells of non-brained muscles varies from -50 to -60 mV. As in other muscles, including in nerve cells, K +, Na +, Cl- are mainly involved in its formation. In the smooth muscle cells of the alimentary canal, uterus, and some vessels, the membrane potential is unstable; spontaneous oscillations are observed in the form of slow depolarization waves, at the top of which PD discharges may appear. The duration of PD of smooth muscles ranges from 20-25 ms to 1 s or more (for example, in muscles Bladder), i.e. she
longer than the duration of skeletal muscle AP. Ca2 + plays an important role in the mechanism of AP of smooth muscles next to Na +.
Spontaneous myogenic activity. Unlike skeletal muscles, smooth muscles of the stomach, intestines, uterus, ureters have spontaneous myogenic activity, i.e. develop spontaneous tetanygodibny contractions. They are stored under conditions of isolation of these muscles and during pharmacological shutdown of the intrafusal plexus. So, PD occurs in the smooth muscle itself, and is not due to the transmission of nerve impulses to the muscles.
This spontaneous activity is of myogenic origin and occurs in muscle cells that act as a pacemaker. In these cells, the local potential reaches a critical level and is transformed into AP. But for membrane repolarization, a new local potential spontaneously arises, which causes another AP, etc. AP, spreading through the nexus to neighboring muscle cells at a speed of 0.05-0.1 m / s, covers the entire muscle, causing it to contract. For example, peristaltic contractions of the stomach occur with a frequency of 3 times in 1 min, segmental and pendulum movements of the colon - 20 times in 1 min in the upper sections and 5-10 in 1 min - in the lower ones. Thus, smooth muscle fibers of these internal organs have automaticity, which is manifested by their ability to rhythmically contract in the absence of external stimuli.
What is the reason for the emergence of potential in the smooth muscle cells of the pacemaker? Obviously, it occurs due to a decrease in potassium and an increase in sodium and (or) calcium permeability of the membrane. Regarding the regular occurrence of slow waves of depolarization, most pronounced in the muscles of the gastrointestinal tract, there is no reliable data on their ionic origin. It is possible that a decrease in the initial inactivating component of the potassium current during depolarization of muscle cells due to inactivation of the corresponding ionic potassium channels plays a certain role. Due to this, the occurrence of repeated G1D becomes possible.
Elasticity and extensibility of smooth muscles. Unlike skeletal muscles, they are smooth when stretched as plastic, elastic structures. Due to plasticity, smooth muscle can be completely relaxed in both contracted and extended states. For example, the plasticity of the smooth muscles of the wall of the stomach or bladder as these organs fill up prevents an increase in intracavitary pressure. Excessive stretching often leads to stimulation of contraction, which is due to depolarization of pacemaker cells, which occurs when the muscle is stretched, and is accompanied by an increase in the frequency of AP, and as a result, an increase in contraction. The contraction, which activates the stretching process, plays an important role in self-regulation of the basal tone of the blood vessels.
The mechanism of smooth muscle contraction. A prerequisite the appearance of a contraction of smooth muscles, like skeletal muscles, and an increase in the concentration of Ca2 + in myoplasms (up to 10v-5 M). It is believed that the contraction process is activated predominantly by extracellular Ca2 + entering muscle cells through voltage-gated Ca2 + channels.
The peculiarity of neuromuscular transmission in smooth muscles is that the innervation is carried out by autonomic nervous system and it can have both exciting and inhibitory effects. By type, there are cholinergic (acetylcholine mediator) and adrenergic (norepinephrine mediator) mediators. The former are usually found in the muscles of the digestive system, the latter in the muscles of the blood vessels.
The same mediator in some synapses can be excitatory, and in others - inhibitory (depending on the properties of cytoreceptors). Adrenergic receptors are divided into a- and B-. Norepinephrine, acting on a-adrenergic receptors, constricts blood vessels and inhibits the motility of the digestive tract, and acting on B-adrenergic receptors, stimulates the activity of the heart and expands the blood vessels of some organs, relaxes the muscles of the bronchi. Described neuromuscular. transmission in smooth muscles for help and other mediators.
In response to the action of an excitatory mediator, depolarization of smooth muscle cells occurs, which manifests itself in the form of an excitatory synaptic potential (ERP). When it reaches a critical level, PD occurs. This happens when several impulses come up to the nerve endings one after the other. The emergence of ZSGI is a consequence of an increase in the permeability of the postsynaptic membrane for Na +, Ca2 + and SI ".
The inhibitory mediator causes hyperpolarization of the postsynaptic membrane, which is manifested in the inhibitory synaptic potential (SHP). Hyperpolarization is based on an increase in membrane permeability, mainly for K +. The role of an inhibitory mediator in smooth muscles excited by acetylcholine (for example, the muscles of the intestine, bronchi) is played by norepinephrine, and in smooth muscles, for which norepinephrine is an excitatory mediator (for example, the muscles of the bladder), acetylcholine.
Clinical and physiological aspect. In some diseases, when the innervation of skeletal muscles is disturbed, their passive stretching or displacement is accompanied by a reflex increase in their tone, i.e. resistance to stretching (spasticity or stiffness).
In case of impaired blood circulation, as well as under the influence of certain metabolic products (lactic and phosphoric acids), toxic substances, alcohol, fatigue, a decrease in muscle temperature (for example, during prolonged swimming in cold water) after prolonged active contraction of the muscle, contracture may occur. The more the muscle function is impaired, the more pronounced the contracture aftereffect (for example, contracture chewing muscles with pathology maxillofacial area). What is the origin of contracture? It is believed that the contracture arose due to a decrease in the concentration of ATP in the muscle, which led to the formation of a permanent connection between the transverse bridges and actin protofibrils. In this case, the muscle loses flexibility and becomes hard. The contracture heals and the muscle relaxes when the ATP concentration reaches normal levels.
In diseases such as myotonia, muscle cell membranes are excited so easily that even a slight irritation (for example, the introduction of a needle electrode in electromyography) causes a discharge of muscle impulses. Spontaneous AP (fibrillation potentials) are also recorded at the first stage after muscle denervation (until inactivity leads to muscle atrophy).
Tonic contractions of some smooth muscles, especially the muscles of the vascular walls (basal or myogenic, tone) are activated mainly by extracellular Ca 2 +. Physiologically active substances and mediators can cause a decrease in smooth muscle tone by closing chemosensitive Ca2 + channels (through activation of chemoreceptors) or hyperpolarization, which causes suppression of spontaneous AP and closure of voltage-gated Ca2 + channels.

Important smooth muscle property is its great plasticity, that is, the ability to maintain the length given by stretching without changing the stress. The difference between skeletal muscle, which has little plasticity, and smooth muscle, with well-defined plasticity, is easily detected by first slowly stretching them and then removing the tensile weight. shortened immediately after removal of the load. In contrast, a smooth muscle, after removing the load, remains stretched until, under the influence of any irritation, its active contraction occurs.

The property of plasticity is very important for the normal activity of the smooth muscles of the walls of hollow organs, for example, the bladder: due to the plasticity of the smooth muscles of the walls of the bladder, the pressure inside it changes relatively little with different degrees of filling.

Excitability and arousal

Smooth muscles less excitable than skeletal: their thresholds of irritation are higher, and chronaxia is longer. The action potentials of most smooth muscle fibers have a small amplitude (about 60 mV instead of 120 in skeletal muscle fibers) and a long duration - up to 1-3 seconds. On rice. 151 the action potential of a single fiber of the uterine muscle is shown.

The refractory period lasts for the entire period of the action potential, i.e. 1-3 seconds. The rate of conduction of excitation varies in different fibers from a few millimeters to several centimeters per second.

There are many different types of smooth muscle in the body of animals and humans. Most of the hollow organs of the body are lined with smooth muscles that have a sentimental type of structure. The individual fibers of such muscles are very closely adjacent to each other and it seems that morphologically they constitute a single whole.

However, electron microscopic studies have shown that membrane and protoplasmic continuity between individual fibers of muscle syncytium does not exist: they are separated from each other by thin (200-500 Å) gaps. The concept of "syncytial structure" is currently physiological rather than morphological.

Syncytium is a functional entity that ensures that action potentials and slow waves of depolarization can freely propagate from one fiber to another. Nerve endings are located only on a small number of syncytium fibers. However, due to the unhindered propagation of excitation from one fiber to another, the involvement of the entire muscle in the reaction can occur if a nerve impulse arrives at a small number of muscle fibers.

Contraction of smooth muscle

With a large force of a single stimulus, smooth muscle contraction may occur. The latent period of a single contraction of this muscle is much longer than that of skeletal muscle, reaching, for example, in the intestinal musculature of a rabbit, 0.25-1 second. The duration of the contraction itself is also long ( rice. 152): in the stomach of a rabbit it reaches 5 seconds, and in the stomach of a frog - 1 minute or more. Relaxation proceeds especially slowly after contraction. The wave of contraction spreads over smooth muscles very slowly too, it travels only about 3 cm per second. But this slowness of the contractile activity of smooth muscles is combined with their great strength. So, the muscles of the stomach of birds are capable of lifting 1 kg per 1 cm2 of its cross-section.

Smooth muscle tone

Due to the slowness of contraction, smooth muscle, even with rare rhythmic irritations (10-12 irritations per minute are enough for the frog's stomach), easily passes into a prolonged state of persistent contraction, reminiscent of the tetanus of skeletal muscles. However, the energy expenditure with such a persistent contraction of the smooth muscle is very small, which distinguishes this contraction from the tetanus of the striated muscle.

The reasons why smooth muscles contract and relax much more slowly than skeletal muscles have not yet been fully elucidated. It is known that smooth muscle myofibrils, like skeletal muscle, consist of myosin and actin. However, there is no transverse striation in smooth muscles, there is no membrane Z, and they are much richer in sarcoplasm. Apparently, these features of the structure of smooth muscle waves are responsible for the slow rate of the contractile process. This corresponds to a relatively low level of smooth muscle metabolism.

Smooth muscle automation

A characteristic feature of smooth muscles, which distinguishes them from skeletal muscles, is the ability for spontaneous automatic activity. Spontaneous contractions can be observed when examining the smooth muscles of the stomach, intestines, gallbladder, ureters and a number of other smooth muscle organs.

Smooth muscle automatism is of myogenic origin. It is inherent in the muscle fibers themselves and is regulated by nerve elements that are located in the walls of smooth muscle organs. The myogenic nature of automation was proved by experiments on strips of muscles of the intestinal wall, freed by careful dissection from the nerve plexuses adjacent to it. Such strips, placed in a warm Ringer-Locke solution, which is saturated with oxygen, are capable of automatic contractions. Subsequent histological examination revealed the absence of nerve cells in these muscle strips.

In smooth muscle fibers, the following spontaneous fluctuations of the membrane potential are distinguished: 1) slow waves of depolarization with a cycle duration of the order of several minutes and an amplitude of about 20 mV; 2) small fast fluctuations of the potential preceding the emergence of action potentials; 3) action potentials.

The smooth muscle reacts to all external influences by changing the frequency of spontaneous rhythmics, which results in muscle contractions and relaxation. The effect of irritation of the smooth muscles of the intestine depends on the ratio between the frequency of stimulation and the natural frequency of spontaneous rhythm: at low tone - at rare spontaneous action potentials - the applied irritation increases the tone; at a high tone, relaxation occurs in response to irritation, since an excessive increase in impulses leads to that each next impulse falls into the refractory phase from the previous one.

Physical and physiological properties skeletal, cardiac and smooth muscles

According to morphological characteristics, three muscle groups are distinguished:

1) striated muscles (skeletal muscles);

2) smooth muscles;

3) heart muscle (or myocardium).

Functions of striated muscles:

1) motor (dynamic and static);

2) ensuring breathing;

3) mimic;

4) receptor;

5) depositing;

6) thermoregulatory.

Smooth muscle functions:

1) maintaining pressure in the hollow organs;

2) regulation of pressure in blood vessels;

3) emptying the hollow organs and promoting their contents.

Heart muscle function- pumping, ensuring the movement of blood through the vessels.

Physiological properties of skeletal muscle:

1) excitability (lower than in the nerve fiber, which is explained by the low value of the membrane potential);

2) low conductivity, about 10-13 m / s;

3) refractoriness (it takes a longer period of time than that of a nerve fiber);

4) lability;

5) contractility (the ability to shorten or develop tension). There are two types of abbreviations:

a) isotonic contraction(length changes, tone does not change);

b) isometric contraction (tone changes without changing fiber length). Distinguish between single and titanic contractions. Single contractions occur under the action of a single stimulus, and titanic ones arise in response to a series of nerve impulses;

6) elasticity (the ability to develop tension when stretched).

Smooth muscles have the same physiological properties as skeletal muscles, but they also have their own characteristics:

1) unstable membrane potential, which maintains muscles in a state of constant partial contraction - tone;

2) spontaneous automatic activity;

3) contraction in response to stretching;

4) plasticity (reduction of stretching with increasing stretching);

5) high sensitivity to chemicals.

Physiological feature of the heart muscle is her automatism... Excitation occurs periodically under the influence of processes occurring in the muscle itself. The ability to automate is possessed by certain atypical muscle areas of the myocardium, poor in myofibrils and rich in sarcoplasm.

Structural organization of skeletal muscle. Skeletal muscle is made up of many muscle fibers that have points of attachment to bones and are parallel to each other. Each muscle fiber (myocyte) includes many subunits - myofibrils, which are built from blocks repeating in the longitudinal direction (sarcomeres). The sarcomere is a functional unit of the contractile apparatus of the skeletal muscle. Myofibrils in the muscle fiber lie in such a way that the location of the sarcomeres in them coincides. This creates a cross-striation pattern.

Motor unit. The functional unit of skeletal muscle is motor unit (DE)... DE - a set of muscle fibers that are innervated by the processes of one motor neuron. Excitation and contraction of the fibers that make up one DE occurs simultaneously (when the corresponding motoneuron is excited). Individual MUs can be excited and contracted independently of each other.

The DE includes:

1. nerve cell- mainly motor neurons, whose bodies lie in the anterior horns of the spinal cord;

2. motor neuron axon- myelin fibers;

3. muscle fiber group- depending on the type of activity, the amount of fibers is different. If fine work is 2-4, if rough - up to several thousand.