Why the number of muscle fibers in motor units. The concept of the motor apparatus

motor unit

a group of muscle fibers innervated by a single motor neuron.

1. Small medical encyclopedia. - M.: Medical Encyclopedia. 1991-96 2. First aid. - M.: Great Russian Encyclopedia. 1994 3. Encyclopedic dictionary of medical terms. - M.: Soviet Encyclopedia. - 1982-1984.

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

The basic anatomical unit of a muscle is muscle fiber, or muscle cell. Normally, during muscle activation (voluntary and involuntary), muscle fibers are activated in groups. It is not possible to activate a single muscle cell arbitrarily or during stimulation of nerve fibers. The activation of muscle fibers in groups is due to the anatomical and functional connection of each motor neuron with several muscle fibers. Such an association 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. Scheme of the motor unit of the muscle

(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 - the total potential of the conditional motor unit.

Each motor neuron is connected with muscle fibers in such a way that the territory of the motor unit in space is not isolated from neighboring MUs, but is located in the same volume with them. This principle of location of MUs in a muscle, when there are muscle fibers of several MUs at any point in the volume of the muscle, allows the muscle to contract smoothly, rather than jerkily, which would be the case when different MUs are separated from each other in space. MUs contain a different number of muscle fibers: from 10-20 in small muscles that perform precise and subtle movements, to several hundred in large muscles that perform coarse movements and carry an antigravity load. The first group of muscles can be attributed to the external muscles of the eye, to the second muscle of the thigh. The number of muscle fibers included in the DE is called the innervation number.

According to the functional properties, DUs are slow and fast. Slow motor units are innervated by small alpha motor neurons, are low-threshold, tireless, as they participate in tonic slow movements, providing an anti-gravity function (posture maintenance). Fast MUs are innervated by large alpha motor neurons, are high-threshold, get tired quickly, and participate in fast (phasic) movements. In all muscles, both slow and fast MUs are present, however, in the muscles of the trunk, the proximal limbs, and the soleus muscle involved in the antigravitational function, slow MUs predominate, and in the muscles of the distal limbs involved in performing precise voluntary movements, fast DE. Knowledge of these properties of DU muscles is important in assessing the work of a muscle in various modes of voluntary tension. Needle EMG, which evaluates the parameters of single motor units with minimal effort, allows you to evaluate mainly low-threshold slow MU. High-threshold motor units involved in phasic voluntary movements are available for analysis only at the maximum voluntary effort by the method of interference pattern evaluation and PMU analysis by the decomposition method. In the study of the level of segmental excitability of motor neurons of the spinal cord using the H-reflex technique, the excitability index of two muscles of the lower leg: soleus and gastrocnemius is assessed. The soleus is a tonic muscle, contains more slow MUs, is less corticolized and reflects to a greater extent regulatory influences from the spinal cord. The gastrocnemius muscle is phasic, contains more fast MUs, is more corticolized, and reflects regulatory influences from the brain.

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 by the words of N.A. Bernshtein: “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 active influence on this environment, changing it with results…”. Another manifestation of the significance of movements is that the basis of any professional activity is the work of muscles.

The whole variety of motor activity is carried out with the help of musculoskeletal system. It consists of specialized anatomical formations: muscles, skeleton and central nervous system.

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

Skeleton includes bones and their joints(e.g. joints).

Skeleton serves as a support for internal organs, a place of attachment of muscles, protects internal organs from external mechanical damage. In the bones of the skeleton is the bone marrow - the organ of hematopoiesis. The composition of the bones includes a large amount of minerals (calcium, sodium, magnesium, phosphorus, chlorine are most represented). Bone is a dynamic living tissue with high sensitivity to various regulatory mechanisms, to endo- and exogenous influences. Bone is not only a supporting 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 provide structural adaptation of the bone to changing operating conditions, which is especially significant in connection with regular classes physical education and sports. The basis of the constantly ongoing processes of bone restructuring is the activity of bone cells - osteoblasts and osteoclasts.

muscles due to the ability to contract, they set in motion 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 the working muscles are involved in heat generation. Fine developed muscles are excellent protection internal organs, vessels and nerves.

Bones and muscles, both in mass and in volume, make up a significant part of the whole organism; there are significant gender differences in their ratio. Muscle mass of an adult male - from 35 to 50% (depending on how developed the muscles) of the total body weight, women - about 32-36%. Athletes who specialize in power types sports, muscle mass can reach 50-55%, and for bodybuilders - 60-70% of the 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 muscles internal organs, skin, blood vessels.

Smooth muscles are divided into tonic(not able to develop "fast" contractions, in the sphincters of hollow organs) and phasic-tonic(which are divided into having automation, i.e. the ability to spontaneously generate phase contractions. An example would be the muscles of the gastrointestinal tract and ureters, and do not have this property- the muscular layer of the arteries, seminal ducts, the muscle of the iris of the eye, they contract under the influence of impulses of the vegetative nervous system. The motor innervation of smooth muscles is carried out by the processes of the cells of the autonomic nervous system, the sensitive innervation is carried out by the processes of the cells of the spinal ganglia. As a rule, smooth muscle contraction cannot be voluntarily induced; the cerebral cortex does not participate in the regulation of its contractions. The function of smooth muscles is to maintain prolonged tension, while they spend 5 to 10 times less ATP than would be needed to perform the same task for a skeletal muscle.

Smooth muscles provide the function of hollow organs, whose walls they form. Thanks to smooth muscles, content expulsion from the bladder, intestines, stomach, gallbladder, uterus. Smooth muscles provide sphincter function- create conditions for the storage of certain contents in a hollow organ (urine in 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, 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 purposeful activity, primarily due to voluntary movements (the features of their structure and principles of operation are discussed in more detail below).

Types of muscle fibers

Muscles are made up of muscle fibers that have different strength, speed and duration of contraction, as well as fatigue. The enzymes in them have different activities and are presented in various isomeric forms. There is a noticeable difference in the content of respiratory enzymes - glycolytic and oxidative. According to 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, then the degree of activity of respiratory enzymes varies quite significantly, therefore, along with white and red, there are also intermediate fibers.

Most clearly, muscle fibers differ in the features of the molecular organization of myosin. Among its various isoforms, there are two main ones - "fast" and "slow". When setting up histochemical reactions, they are distinguished by ATPase activity. These properties correlate with the activity of respiratory enzymes. Usually in fast fibers(FF fibers - rapidly shrinking fast twitch fibers), glycolytic processes predominate, they are richer in glycogen, they have less myoglobin, therefore they are also called white. IN slow fibers, designated as S (ST) fibers (slow twitch fibres), on the contrary, the activity of oxidative enzymes is higher, they are richer in myoglobin, and look more red. They turn on at loads in the range of 20-25% of maximum strength and are distinguished by good endurance.

FT fibers, which have a low content of myoglobin compared to red fibers, are characterized by high contractile speed and the ability to develop greater strength. Compared to slow fibers, they can contract twice as fast and develop 10 times the strength. FT fibers, in turn, are subdivided 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 in muscle fibers different types (according to http://medi.ru/doc/g740203.htm).

Energy production in FTO fibers occurs in the same way as in ST fibers, predominantly by oxidative phosphorylation. Because this breakdown process is relatively economical (39 energy phosphate compounds are stored for every molecule of glucose in the breakdown of muscle glycogen for energy), FTO fibers also have a relatively high resistance to fatigue. The accumulation of energy in FTG fibers occurs predominantly by glycolysis, i.e. glucose in the absence of oxygen breaks down to lactate, which is still relatively rich in energy. Due to the fact that this breakdown process is uneconomical (only 3 energetic phosphate compounds are accumulated for each glucose molecule for energy), FTG fibers tire 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 skeletal muscle is motor unit– DE(Fig.2.2.).

Fig 2.2. motor unit

DE includes a motor neuron of the spinal cord 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 certain group of muscle fibers. DU of small muscles that perform fine movements (muscles of the eye, hand) contain a small amount 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 body of the motor neuron of the anterior horn of the spinal cord along the nerve pathways, then either all the muscle fibers of the MU or none react to it. For the biceps, this means the following: with a nerve impulse necessary force, all contractile elements (myofibrils) of all (approximately 1500) muscle fibers of the corresponding MU are shortened.

All DU depending on functional features are divided into 3 groups:

I. Slow tireless. They are formed by "red" muscle fibers, in which there are fewer myofibrils. The rate of contraction and the strength of these fibers are relatively small, but they are not very fatiguing, therefore these fibers are referred to 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 B. Fast, easily fatigued. Muscle fibers contain many myofibrils and are called "white". Contract quickly and develop great strength, but tire quickly. Therefore they are called phase. The motor neurons of these DUs are the largest, have a thick axon with numerous terminal branches. They generate nerve impulses of high frequency. For example, the muscles of the eye.

II A. Fast, fatigue resistant(intermediate).

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

The 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 (for example, big muscles limbs), have a significantly smaller number of MUs, but they include big number fibers (from 600 to 2000).

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

The main properties of muscle fibers (hence, motor units - MU, of which they are included), also determined by the properties of motor neurons, are presented in Table 1.

A motor or motor unit is a group of fibers that are innervated by a single motor neuron. The number of fibers included in one unit may vary depending on the function of the muscle. The smaller movements it provides, the smaller the motor unit and the less effort it takes to excite it.

Motor units: their classification.

In the study of this topic there is important point. There are criteria by which any motor unit can be characterized. Physiology as a science distinguishes two criteria:

the speed of contraction in response to impulse conduction;
fatigue rate.

Accordingly, based on these indicators, three types of motor units can be distinguished.

Slow, not tired. Their motor neurons contain a lot of myoglobin, which has a high affinity for oxygen. Muscles that have a large number of slow motor neurons are called red because of their specific color. They are necessary to maintain a person's posture and keep him in balance.
Fast, tired. Such muscles are able to perform a large number of contractions in a short period of time. Their fibers contain a lot of energy material, from which ATP molecules can be obtained.
Fast, fatigue resistant. These fibers contain few mitochondria, and ATP is formed due to the breakdown of glucose molecules. These muscles are called white because they lack myoglobin.

Units of the first type

The motor unit of the first type or slow tireless, is found most often in large muscles. Such motoneurons have a low threshold of excitation and the speed of the nerve impulse. The central process of the nerve cell in its terminal section branches and innervates not large group fibers. The frequency of discharges to slow motor units is from six to ten impulses per second. The motor neuron can maintain such a rhythm for several tens of minutes.

The strength and speed of contraction of motor units of the first type is one and a half times less than that of other types of motor units. The reason for this is the low rate of ATP formation and the slow release of calcium ions to the outer cell membrane for binding to troponin.

Units of the second type

The motor unit of this type has a large motor neuron with a thick and long axon, which innervates a large bundle of muscle fibers. These nerve cells have the highest excitation threshold and high conduction velocity.

At maximum muscle tension, the frequency of nerve impulses can reach fifty per second. But the motor neuron is not able to maintain such a speed of conduction for a long time, therefore it quickly gets tired. The strength and speed of contraction of the muscle fiber of the second type is higher than that of the previous one, since the number of myofibrils in it is greater. Fiber contains many enzymes that break down glucose, but fewer mitochondria, myoglobin protein and blood vessels.

Units of the third type

The motor unit of the third type refers to fast, but fatigue-resistant muscle fibers. According to its characteristics, it should occupy an intermediate value between the first type of motor units and the second. such muscles are strong, fast and enduring. For energy production, they can use both aerobic and anaerobic pathways.

The ratio of fast and slow fibers is genetically determined and may differ from person to person. That's why someone is good at running on long distances, someone easily overcomes a hundred-meter sprint, while weightlifting is more suitable for someone.

Stretch reflex and motor neuron pool

When any muscle is stretched, the slow fibers are the first to respond. Their neurons fire up to ten pulses per second. If the muscle continues to stretch, then the frequency of the generated impulses will increase to fifty. This will lead to a contraction of the third type of motor units and increase the strength of the muscle tenfold. With further stretching, they will connect motor fibers second type. This will multiply another four or five times.

The motor muscle unit is controlled by a motor neuron. The set of nerve cells that make up one muscle is called the motor neuron pool. In one pool, there can be simultaneously neurons from different, in qualitative and quantitative manifestations, motor units. Because of this, sections of muscle fibers are not included in the work at the same time, but as the tension and speed of nerve impulses increase.

"Principle of magnitude"

The motor unit of a muscle, depending on its type, contracts only when a certain threshold load is reached. The order of excitation of motor units is stereotypical: first, small motor neurons contract, then nerve impulses gradually reach large ones. This pattern was noticed in the middle of the twentieth century by Edwood Henneman. He called it the "principle of magnitude".

Brown and Bronk half a century earlier published their works on the study of the principle of operation of muscle units of different types. They suggested that there are two ways to control the contractions of muscle fibers. The first of them is to increase the frequency of nerve impulses, and the second is to involve as many motor neurons as possible in the process.

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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 MU differs in different muscles. It is less in small muscles that perform fine and smooth regulation of motor function (for example, the 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 eye muscles one MU contains 13-20 muscle fibers, and the MU of the inner head of the gastrocnemius muscle contains 1500-2500.

Fig.4.8. Muscle motor units (MU) and their types.

Muscle fibers of one MU 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, fatigue resistant:
II-B - fast, easy to tire.

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

Human skeletal muscles consist of all three types of MU. Some of them include predominantly slow DUs, others - fast ones, and others - both.

Slow, non-fatiguing motor units (type I)

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Compared to other types of MUs, these MUs have the smallest sizes of motor neurons and, accordingly, the lowest thresholds for their activation, a smaller thickness of the axon and the speed of excitation along it. The axon branches into a small number of terminal branches and innervates a small group of muscle fibers. The motoneurons of slow DUs have a relatively low discharge frequency (6-10 imp/s). They begin to function even with small muscular efforts. Thus, motor neurons DE of the soleus muscle of a person, when comfortably standing, work at a frequency of 4 pulses / s. The stable frequency of their impulsation is 6-8 imp/s. With an increase in the force of muscle contraction, the frequency of discharges of motoneurons of slow DUs increases slightly. Motor neurons of slow DUs are able to maintain a constant frequency of discharges for tens of minutes.

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

Muscle fibers of slow MUs are not fatigued. They have a well-developed capillary network. On one muscle fiber, on average, there are 4-6 capillaries. Due to this, during contraction, they are provided with a sufficient amount of oxygen. Their cytoplasm contains a large number of mitochondria and a high activity of oxidative enzymes. All this determines the essential aerobic endurance of these muscle fibers and allows you to perform work of moderate power for a long time without fatigue.

Fast, easily fatigued DE (type II-B)

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

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

Muscle fibers of fast MUs, unlike slow ones, contain a larger number of contractile elements - myofibrils, therefore, during contraction, they develop greater strength. Due to the high activity of myosin ATPase, they have a higher rate of contraction. Fibers of this type contain more glycolytic enzymes, fewer mitochondria and myoglobin, and are surrounded by a smaller number of 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 MU (type II-A)

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According to its morphofunctional properties, this type of muscle fibers occupies intermediate positions between DE I and II-B types. These are strong, fast-twitch fibers that have great aerobic endurance due to their inherent ability to use both aerobic and anaerobic processes for energy.

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