Motor unit of muscle tissue. Motor units, types and their characteristics

Registration and analysis of bioelectrical activity of muscles is possible only on the basis of knowledge and ideas about the anatomical and functional organization of muscle work. What muscle elements are generators of electrical signals? How is their activation organized in time and space? How are muscle elements connected to the motor neurons (motoneurons) of the spinal cord? What is the trigger for muscle activity? These and other questions arise when you first become acquainted with ENMG and various electromyographic signals.

The basic anatomical unit of a muscle is muscle fiber, or muscle cell. Normally, when a muscle is activated (voluntary and involuntary), muscle fibers are activated in groups. It is not possible to activate a single muscle cell voluntarily or by stimulating nerve fibers. Activation muscle fibers groups is due to the anatomical and functional connection of each motor neuron with several muscle fibers. This combination of a motor neuron and a group of muscle cells is called motor unit(DE) and is an anatomical and functional unit of the neuromotor apparatus. Figure 1 shows a schematic representation of a motor unit.

Rice. 1. Diagram of a muscle motor unit

(According to L.O. Badalyan, I.A. Skvortsov, 1986).

A, B, C – motor neurons of the anterior horns of the spinal cord,

1, 2, 3, 4, 5 – muscle fibers and their corresponding potentials,

I – potentials of individual muscle fibers,

II – total potential of a conditioned motor unit.

Each motor neuron is connected to muscle fibers in such a way that the territory of the motor unit in space is not isolated from neighboring motor units, but is located in the same volume with them. This principle of arrangement of MUs in a muscle, when at any point in the muscle volume there are muscle fibers of several MUs, allows the muscle to contract smoothly, and not jerkily, which would be the case when different MUs are separated from each other in space. DE contain different quantities muscle fibers: from 10-20 in small muscles ah, performing precise and subtle movements, up to several hundred per large muscles performing rough movements and bearing an anti-gravity load. The first group of muscles includes the external muscles of the eye, and the second the muscles of the thigh. The number of muscle fibers included in the motor unit is called the innervation number.

According to their functional properties, MUs can be slow or fast. Slow motor units are innervated by small alpha motor neurons, are low-threshold, non-fatiguing, as they participate in tonic slow movements, providing an anti-gravity function (maintaining posture). Fast motor units are innervated by large alpha motor neurons, are high-threshold, quickly tire, and participate in fast (phasic) movements. All muscles contain both slow and fast MUs, however, in the muscles of the trunk, proximal limbs and soleus muscle, involved in the anti-gravity function, slow MUs predominate, and in the muscles of the distal limbs, involved in performing precise voluntary movements, fast MUs predominate. DE. Knowledge of these properties of MU muscles is important when assessing muscle performance in various modes of voluntary tension. Needle EMG, which assesses the parameters of single motor units with minimal effort, makes it possible to assess mainly low-threshold slow motor units. High-threshold motor units involved in phasic voluntary movements are available for analysis only at maximum voluntary effort using the interference pattern assessment method and MUAP analysis using the decomposition method. In a study of the level of segmental excitability of spinal cord motor neurons, using the H-reflex technique, the excitability index of two lower leg muscles is assessed: the soleus and gastrocnemius. The soleus is a tonic muscle, contains more slow motor units, is less corticolized and reflects to a greater extent regulatory influences from the spinal cord. The gastrocnemius muscle is phasic, contains more fast motor units, is more corticolized and reflects regulatory influences from the brain.

The main morpho-functional element of the neuromuscular apparatus of skeletal muscles is the motor unit (MU). It includes the spinal cord motor neuron with the muscle fibers innervated by its axon. Inside the muscle, this axon forms several terminal branches. Each such branch forms a contact - a neuromuscular synapse on a separate muscle fiber. Nerve impulses coming from a motor neuron cause contractions of a specific group of muscle fibers. Motor units of small muscles that perform fine movements (muscles of the eye, hand) contain a small number of muscle fibers. In large ones there are hundreds of times more of them. All MUs, depending on their functional characteristics, are divided into 3 groups:

I. Slow and tireless. They are formed by “red” muscle fibers, which have fewer myofibrils. The contraction speed and strength of these fibers are relatively small, but they are not easily fatigued. Therefore, they are classified as tonic. The regulation of contractions of such fibers is carried out by a small number of motor neurons, the axons of which have few terminal branches. An example is the soleus muscle.

IIB. Fast, easily tired. Muscle fibers contain many myofibrils and are called "white". They contract quickly and develop great strength, but tire quickly. That's why they are called phase ones. The motor neurons of these motor units are the largest and have a thick axon with numerous terminal branches. They generate high frequency nerve impulses. Muscles of the eye.

IIA. Fast, fatigue resistant. They occupy an intermediate position.

Smooth muscle physiology

Smooth muscles are present in the walls of most digestive organs, blood vessels, excretory ducts of various glands, and the urinary system. They are involuntary and provide peristalsis of the digestive and urinary systems, maintaining vascular tone. Unlike skeletal smooth muscle formed by cells often spindle-shaped and small in size, without transverse striations. The latter is due to the fact that the contractile apparatus does not have an ordered structure. Myofibrils consist of thin filaments of actin that run in different directions and attach to different parts of the sarcolemma. Myosin protofibrils are located next to actin ones. The elements of the sarcoplasmic reticulum do not form a system of tubes. Separate muscle cells They are connected to each other by contacts with low electrical resistance - nexuses, which ensures the spread of excitation throughout the smooth muscle structure. The excitability and conductivity of smooth muscles is lower than that of skeletal muscles.

The membrane potential is 40-60 mV, since the SMC membrane has a relatively high permeability to sodium ions. Moreover, in many smooth muscles the MP is not constant. It periodically decreases and returns to its original level. Such oscillations are called slow waves (SW). When the peak of the slow wave reaches a critical level of depolarization, action potentials begin to be generated on it, accompanied by contractions (Fig.). MV and AP are conducted through smooth muscles at a speed of only 5 to 50 cm/sec. Such smooth muscles are called spontaneously active, i.e. they are automatic. For example, due to such activity, intestinal peristalsis occurs. The pacemakers of intestinal peristalsis are located in the initial sections of the corresponding intestines.

The generation of AP in SMCs is due to the entry of calcium ions into them. The electromechanical coupling mechanisms are also different. Contraction develops due to calcium entering the cell during AP. The connection of calcium with the shortening of myofibrils is mediated by the most important cellular protein - calmodulin.

The contraction curve is also different. The latent period, the period of shortening, and especially relaxation, is much longer than that of skeletal muscles. The contraction lasts several seconds. Smooth muscles, unlike skeletal muscles, are characterized by the phenomenon of plastic tone. This ability is in a state of contraction for a long time without significant energy consumption and fatigue. Thanks to this property, the shape of internal organs and vascular tone are maintained. In addition, smooth muscle cells themselves are stretch receptors. When they are tensioned, PDs begin to be generated, which leads to contraction of the SMC. This phenomenon is called the myogenic mechanism for regulating contractile activity.

From a functional point of view, a muscle consists of motor units. Motor unit(DE) is a structural-functional concept. A separate motor unit includes a motor neuron and a complex of muscle fibers innervated by its axon. Muscle fibers combined into one MU are scattered among other muscle fibers belonging to other MUs and are isolated from the latter. Individual muscles include different amounts of motor units.

Depending on the morphological characteristics of the motor neuron and muscle fibers, motor units are divided into small, medium and large.

Small DE consists of several muscle fibers and a small motor neuron with a thin axon - up to 5 - 7 µm and a small number of axonal branches. MUs of this group are characteristic of small muscles of the hand, forearm, facial and oculomotor muscles. Less commonly, they are found in large muscles of the limbs and trunk.

Large DE consist of large motor neurons with a thick (up to 15 microns) axon, and a significant number (up to several thousand) muscle fibers. They make up the bulk of the MUs of large muscles.

Average, in size, DUs occupy an intermediate position.

What is the relationship between muscle size and the ability to perform movements and motor movements?

In general, the larger the muscle and the less developed the movements in which it participates, the fewer the number of MUs it is represented by and the larger the MUs of its components.

Why are some people strong from birth, while others are resilient?

But here's another one important point. It turns out that there are two types of fibers in each muscle - fast and slow.

Slowly With dyeing fibers are also called red, because they contain a lot of the red muscle pigment myoglobin. These fibers have good endurance.

Fast fibers Compared to red fibers, they have a low myoglobin content, which is why they are called white fibers. They have a high contraction speed and allow you to develop great strength.

Yes, you yourself have seen such fibers in chicken - the legs are red, the breast is white, Wow! This is exactly what it is, only in humans these fibers are mixed and both types are present in one muscle.

Red (slow) fibers use aerobic (oxygen) way to obtain energy, so more capillaries approach them to better supply them with oxygen. Thanks to this method of energy conversion, red fibers are low-fatigue and are able to maintain relatively small but long-lasting tension. Basically, they are important for runners long distances, and in other sports that require endurance. This means that they also have a decisive role for everyone who wants to lose weight.

Fast (white) fibers receive energy for their contraction without the participation of oxygen (anaerobically). This method of obtaining energy (also called glycolysis) allows white fibers to develop greater speed, strength and power. But for the high rate of energy production, white fibers have to pay with rapid fatigue, since glycolysis leads to the formation of lactic acid, and its accumulation causes muscle fatigue and ultimately stops their work. And, of course, throwers, weightlifters, short-distance runners cannot do without white fibers... in general, those who require strength and speed.

Now we will have to confuse you a little, simply because there is no other way. The fact is that there is another, intermediate type of fiber, which is also related to white fibers, but like red fibers, it uses predominantly aerobic energy production and combines the properties of white and red fibers. Let me remind you once again that it refers to white fibers.

The average person has approximately 40% slow (red) and 60% fast (white) fibers. But this is an average value for all skeletal muscles, something like the average temperature in a hospital.

In fact, muscles perform different functions and therefore can differ significantly from each other in fiber composition. Well, for example, muscles that perform a lot of static work (soleus, also known as the gastrocnemius muscle) often have a large number of slow fibers, and muscles that perform mainly dynamic movements (biceps) have a large number fast fibers.

It is interesting that the ratio of fast and slow fibers is constant in us, does not depend on training and is determined at the genetic level. That is why there is a predisposition to certain sports.

Now let's see how it all works.

When does a person lose weight more on a treadmill or on exercise machines?

When gentle effort is required, such as walking or jogging, slow-twitch fibers are recruited. Moreover, due to the great endurance of these fibers, such work can last for a very long time. But as the load increases, the body has to recruit more and more of these fibers, and those that were already working increase the force of contraction. If you increase the load further, fast oxidative fibers (remember the intermediate ones?) will also come into play. When the load reaches 20%-25% of the maximum, for example, during an uphill climb or a final push, the strength of the oxidative fibers becomes insufficient, and this is where fast-glycolytic fibers come into play. As already mentioned, fast-twitch fibers significantly increase the force of muscle contraction, but they also tire quickly, and therefore more and more of them will be involved in the work. As a result, if the load level does not decrease, the movement will soon have to be stopped due to fatigue.

So it turns out that during prolonged exercise at a moderate pace, mainly slow (red) fibers work, and it is thanks to their aerobic method of obtaining energy that fats are burned in our body.

Here is the answer to the question why we lose weight on a treadmill and practically do not lose weight when exercising on exercise machines. It's simple - different muscle fibers are used, and therefore different energy sources.

In general, muscles are the most economical engine in the world. Muscles grow and increase their strength solely due to an increase in the thickness of muscle fibers, but the number of muscle fibers does not increase. Therefore, the lowest runt and Hercules have no advantage over each other in terms of the number of muscle fibers. By the way, the process of increasing the thickness of muscle fibers is called hypertrophy, and decreasing it is called atrophy.

When training to increase strength, muscles gain significantly more volume than during endurance training, because strength depends on the cross-section of muscle fibers, and endurance depends on the additional number of capillaries surrounding these fibers. Accordingly, the more capillaries, the more oxygen in the blood will be delivered to the working mice.

In accordance with the division of muscle fibers and motor neurons into slow and fast, it is customary to distinguish three types of motor units.

Slow, non-fatiguable motor units (MU I) consist of

small motor neurons with a low excitability threshold, high

input resistance. When small neurons are depolarized, a prolonged discharge occurs with little adaptation. Motor neurons with such properties are called tonic. The small diameter of the axon (up to 5-7 microns) also explains the low, compared to thicker ones, the speed of excitation. The muscle fibers included in the MU of this type are red fibers (type I), which have the smallest diameter, their contraction speed is minimal, the maximum tension is weaker than white fibers (type II), they are characterized by low fatigue.

Fast, easily fatigued motor units (type MU II B) are formed from large (up to 100 μm in diameter) motor neurons that have a high excitation threshold, the diameter of their axons is the largest (up to 15 μm), the speed of excitation reaches 120 m/s, high-frequency impulses are short-lived and quickly decrease, because rapid adaptation occurs. Large motor neurons belong to the phasic type neurons. The muscle fibers included in these MUs belong to type II (white fibers). They are capable of developing significant tension, but quickly tire. As a rule, MUs of this type contain a large number of muscle fibers (large MUs). Smooth tetanus in them is observed at a high impulse frequency (about 50 impulses/s), in contrast to MU I, where this is achieved at a frequency of up to 20 impulses/s.

The third type of motor units - type MU II-A belongs to the intermediate type. They contain both fast and slow muscle fibers. Motor neurons are of medium caliber.

Skeletal muscles, depending on their functional characteristics, consist of a different set of motor units. The type of motor unit is formed during ontogenesis and in a mature muscle the ratio of fast and slow motor units no longer changes. As already indicated, in a whole muscle, muscle fibers of one MU are interspersed with fibers of several other MUs. The overlap of MU zones is believed to ensure smooth muscle contraction, even if each individual MU does not reach the state of smooth tetanus.

By doing muscle work increasing power, the activity always first involves slow motor units, which develop a weak, but finely graded tension. To perform significant efforts, large, strong, but quickly fatigued motor units of the second type are connected to the first.

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The muscle fibers of each Motor Unit (MU) are located at a fairly significant distance from each other. The number of muscle fibers included in one motor unit differs in different muscles Oh. It is less in small muscles that carry out fine and smooth regulation of motor function (for example, muscles of the hand, eyes) and more in large ones that do not require such precise control (calf muscle, back muscles). So, in particular, in the eye muscles, one MU contains 13-20 muscle fibers, and the MU of the inner head calf muscle - 1500-2500.

Fig.4.8. Motor units (MU) of muscles and their types.

Muscle fibers of the same motor unit have the same morphofunctional properties.

According to morphofunctional properties, MUs are divided into three main types (Fig. 4.8.):

I - slow, tireless;
II-A - fast, resistant to fatigue:
II-B - fast, easily tired.

1 - slow, weak, tireless muscle fibers.
Low motor neuron activation threshold;
2 - intermediate type DE;
3 - fast, strong, easily fatigued muscles
fibers. High threshold of motor neuron activation.

Human skeletal muscles consist of motor units of all three types. Some of them include predominantly slow MUs, others - fast, and others - both.

Slow, non-fatiguing motor units (type I)

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Compared to other types of motor units, these motor units have the smallest motor neuron sizes and, accordingly, the lowest thresholds for their activation, the thickness of the axon and the speed of excitation along it are smaller. The axon branches into a small number of terminal branches and innervates not large group muscle fibers. Slow motor neurons have a relatively low discharge frequency (6-10 impulses/s). They begin to function even with small muscular efforts. Thus, motor neurons of the human soleus muscle operate at a frequency of 4 impulses/s when standing comfortably. The stable frequency of their impulses is 6-8 pulses/s. With an increase in the force of muscle contraction, the frequency of discharges of slow motor neurons increases slightly. Slow motor neurons are capable of maintaining a constant discharge frequency for tens of minutes.

The muscle fibers of slow MUs develop little force during contraction due to the presence of a smaller number of myofibrils in them, compared to fast fibers. The contraction speed of these fibers is 1.5-2 times less than that of fast fibers. The main reasons for this are the low activity of myosin ATPase and the lower rate of release of calcium ions from the sarcoplasmic reticulum and its binding to troponin during fiber excitation.

The muscle fibers of slow motor units are low-fatigue. They have a well-developed capillary network. On average, there are 4-6 capillaries per muscle fiber. Thanks to this, during contraction they are provided with a sufficient amount of oxygen. Their cytoplasm contains a large number of mitochondria and high activity of oxidative enzymes. All this determines the significant aerobic endurance of these muscle fibers and allows them to perform work of moderate power for a long time without fatigue.

Fast, easily fatigued motor units (type II-B)

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Of all types of motor neurons, motor neurons of this type are the largest; they have a thick axon that branches into a large number of terminal branches and innervates a correspondingly large group of muscle fibers. Compared to others, these motor neurons have the highest excitation threshold, and their axons - higher speed conduction of nerve impulses.

The frequency of motor neuron impulses increases with increasing contraction force, reaching 25-50 impulses/s at maximum muscle tension. These motor neurons are not able to maintain a stable frequency of discharges for a long time, that is, they quickly get tired.

Muscle fibers of fast MUs, unlike slow ones, contain larger number contractile elements - myofibrils, therefore, during contraction they develop greater force. Due to the high activity of myosin ATPase, they have a higher contraction rate. Fibers of this type contain more glycolytic enzymes, less mitochondria and myoglobin, and are surrounded by fewer capillaries compared to slow MUs. These fibers tire quickly. Most of all, they are adapted to perform short-term but powerful work (see Chapter 27).

Fast, fatigue-resistant motor units (type II-A)

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In terms of its morphofunctional properties, this type of muscle fiber occupies an intermediate position between MU types I and II-B. These are strong, fast-twitch fibers that have great aerobic endurance due to their inherent ability to utilize both aerobic and anaerobic processes to produce energy.

U different people the ratio of the number of slow and fast MUs in the same muscle is determined genetically and can differ quite significantly. For example, in the human quadriceps muscle, the percentage of slow fibers can vary from 40 to 98%. The higher the percentage of slow fibers in a muscle, the more suited it is to endurance work. Conversely, individuals with a high percentage of fast, strong fibers are more capable of work that requires great strength and speed of muscle contraction.

Movement is a necessary condition for the development and existence of an organism, its adaptation to the environment. It is movement that is the basis of purposeful behavior, which is revealed in the words of N.A. Bernstein: “The obvious enormous biological significance of the motor activity of organisms is almost the only form of implementation of not only interaction with the environment, but also an active influence on this environment, changing it in ways that are not indifferent to the individual results..." Another manifestation of the significance of movements is that at the basis of any professional activity lies the work of the muscles.

All variety of motor activities is carried out with the help of musculoskeletal system. It is made up of specialized anatomical formations: muscles, skeleton and central nervous system.

In the musculoskeletal system, with a certain degree of convention, a passive part is distinguished - the skeleton and an active part - the muscles.

The skeleton includes bones and their joints(eg joints).

Skeleton serves as a support for internal organs, a place for muscle attachment, and protects internal organs from external mechanical damage. The bones of the skeleton contain bone marrow, a hematopoietic organ. The bones contain a large number of minerals (the most represented are calcium, sodium, magnesium, phosphorus, and chlorine). Bone is a dynamic living tissue with high sensitivity to various regulatory mechanisms, to endo- and exogenous influences. Bone is not only a support organ, but also the most important participant in mineral metabolism (for more details, see the Metabolism section). An integral indicator of the metabolic activity of bone tissue is the processes of active restructuring and renewal of bone structures that continue throughout life. These processes, on the one hand, are an important mechanism for maintaining mineral homeostasis, on the other hand, they ensure structural adaptation of the bone to changing operating conditions, which is especially important in connection with regular classes physical culture and sports. The basis of the constantly ongoing processes of bone remodeling is the activity of bone cells - osteoblasts and osteoclasts.

Muscles Due to the ability to contract, they move individual parts of the body and also ensure the maintenance of a given posture. Muscle contraction is accompanied by the production of a large amount of heat, which means that working muscles are involved in heat generation. Fine developed muscles are excellent protection for internal organs, blood vessels and nerves.

Bones and muscles, both in terms of mass and volume, make up a significant part of the entire body; there are significant gender differences in their ratio. The muscle mass of an adult man is from 35 to 50% (depending on how developed the muscles are) of the total body weight, for women it is approximately 32-36%. Athletes specializing in strength sports have muscle mass can reach 50-55%, and for bodybuilders - 60-70% of total body weight. Bones account for 18% of body weight in men and 16% in women.

There are three types of muscles in humans:

striated skeletal muscles;

striated cardiac muscle;

smooth muscle internal organs, skin, blood vessels.

Smooth muscle are divided into tonic(not able to develop “fast” contractions in the sphincters of hollow organs) and phase-tonic(which are divided into those with automatic, i.e. the ability to spontaneously generate phasic contractions. An example would be the muscles of the gastrointestinal tract and ureters, and not having this property– the muscular layer of arteries, seminal ducts, iris muscle, they contract under the influence of autonomic impulses nervous system. Motor innervation of smooth muscles is carried out by processes of cells of the autonomic nervous system, and sensory innervation is carried out by processes of cells of the spinal ganglia. Typically, a reduction smooth muscle cannot be caused voluntarily; the cerebral cortex does not participate in the regulation of its contractions. The function of smooth muscles is to maintain long-term tension, while they spend 5 to 10 times less ATP than skeletal muscle would need to perform the same task.

Smooth muscles provide the function of hollow organs, the walls of which they form. Thanks to smooth muscles, it is carried out expulsion of contents from the bladder, intestines, stomach, gall bladder, uterus. Smooth muscles provide sphincter function– create conditions for storing certain contents in a hollow organ (urine in the bladder, fetus in the uterus). By changing the lumen of blood vessels, smooth muscles adapt regional blood flow to local needs for oxygen and nutrients, and participate in the regulation of respiration by changing the lumen of the bronchial tree.

Skeletal muscles are an active part of the musculoskeletal system, providing targeted activity, primarily through voluntary movements (the features of their structure and operating principles are discussed in more detail below).

Types of muscle fibers

Muscles are made up of muscle fibers that vary in strength, speed and duration of contraction, and fatigue. The enzymes in them have different activities and are presented in different isomeric forms. There is a noticeable difference in the content of respiratory enzymes - glycolytic and oxidative. Based on the ratio of myofibrils, mitochondria and myoglobin, the so-called white, red And intermediate fibers . According to their functional characteristics, muscle fibers are divided into fast, slow And intermediate . If muscle fibers differ quite sharply in ATPase activity, the degree of activity of respiratory enzymes varies quite significantly, therefore, along with white and red, there are also intermediate fibers.

Muscle fibers most clearly differ in the molecular organization of myosin. Among its various isoforms, there are two main ones - “fast” and “slow”. When performing histochemical reactions, they are distinguished by ATPase activity. The activity of respiratory enzymes also correlates with these properties. Usually in fast fibers(FF fiber - fast-twitch, fast twitch fibers), glycolytic processes predominate, they are richer in glycogen, they have less myoglobin, which is why they are also called white. IN slow fibers, designated as S (ST) fibers (slow twitch fibres), on the contrary, have higher activity of oxidative enzymes, they are richer in myoglobin, and look more red. They turn on at loads within 20-25% of maximum strength and have good endurance.

FT fibers, which have a low myoglobin content compared to red fibers, are characterized by high contractile speed and the ability to develop greater force. Compared to slow-twitch fibers, they can contract twice as fast and produce 10 times more force. FT fibers, in turn, are divided into FTO and FTG fibers. Significant differences between the listed types of muscle fibers are determined by the method of obtaining energy (Fig. 2.1).

Rice. 2.1 Differences in energy supply among muscle fibers different types (from http://medi.ru/doc/g740203.htm).

Energy production in FTO fibers occurs in the same way as in ST fibers, mainly through oxidative phosphorylation. Because this breakdown process is relatively economical (39 energy phosphate compounds are stored for every molecule of glucose when muscle glycogen is broken down for energy), FTO fibers also have a relatively high fatigue resistance. The accumulation of energy in FTG fibers occurs primarily through glycolysis, i.e., glucose in the absence of oxygen breaks down into lactate, which is still relatively energy-rich. Due to the fact that this breakdown process is uneconomical (for each glucose molecule, only 3 energetic phosphate compounds are accumulated for energy), FTG fibers fatigue relatively quickly, but nevertheless they are able to develop great strength and, as a rule, turn on with submaximal and maximum muscle contractions.

Motor units

The main morphofunctional element of the neuromuscular apparatus of skeletal muscles is motor unit– DE(Fig. 2.2.).

Figure 2.2. Motor unit

The motor unit includes a spinal cord motor neuron with muscle fibers innervated by its axon. Inside the muscle, this axon forms several terminal branches. Each such branch forms a contact - a neuromuscular synapse on a separate muscle fiber. Nerve impulses coming from a motor neuron cause contractions of a specific group of muscle fibers. The MU of small muscles that perform fine movements (muscles of the eye, hands) contain a small number of muscle fibers. In large muscles there are hundreds of times more of them.

MUs are activated according to the “all or nothing” law. Thus, if an impulse is sent from the motor neuron body of the anterior horn of the spinal cord along the nerve pathways, then either all muscle fibers of the MU react to it, or none. For the biceps, this means the following: with a nerve impulse With the required force, all contractile elements (myofibrils) of all (approximately 1500) muscle fibers of the corresponding motor unit are shortened.

All MUs, depending on their functional characteristics, are divided into 3 groups:

I. Slow and tireless. They are formed by “red” muscle fibers, which have fewer myofibrils. The contraction speed and strength of these fibers are relatively small, but they are little fatigued, so these fibers are classified as tonic. The regulation of contractions of such fibers is carried out by a small number of motor neurons, the axons of which have few terminal branches. An example is the soleus muscle.

II V. Fast, easily tired. Muscle fibers contain many myofibrils and are called "white". They contract quickly and develop great strength, but tire quickly. That's why they are called phase. The motor neurons of these motor units are the largest and have a thick axon with numerous terminal branches. They generate high frequency nerve impulses. For example, the muscles of the eye.

II A. Fast, fatigue resistant(intermediate).

All muscle fibers of one MU belong to the same type of fibers (FT- or ST-fibers).

Muscles involved in performing very precise and differentiated movements (for example, the muscles of the eyes or fingers) usually consist of a large number of MUs (from 1500 to 3000). Such MUs have a small number of muscle fibers (from 8 to 50). Muscles that perform relatively less precise movements (e.g. big muscles limbs), have a significantly smaller number of motor units, but their composition includes a large number of fibers (from 600 to 2000).

The average person has approximately 40% slow fibers and 60% fast fibers. But this is an average value (across all skeletal muscles), muscles perform different functions. The quantitative and qualitative composition of muscles is heterogeneous; they include a different number of motor units, the ratio of the types of which is also different ( muscle composition). In this regard, the contractile abilities of different muscles are not the same. External muscles of the eye that rotate eyeball, develop maximum tension in one contraction lasting only 7.5 ms, soleus is an anti-gravity muscle lower limb, very slowly develops maximum voltage over 100 ms. Muscles that perform a lot of static work (soleus) often have a large number of slow ST fibers, while muscles that perform primarily dynamic movements (biceps) often have a large number of FT fibers.

The main properties of muscle fibers (and therefore the motor units they contain), also determined by the properties of motor neurons, are presented in Table 1.