Fast and slow motor units. Types of motor units

The combination of a motor neuron and the muscle fibers it innervates is called motor (neuromotor unit. The number of muscle fibers in a motor unit varies over a wide range different muscles. Motor units are small in muscles adapted for fast movements, from several muscle fibers to several dozen of them (muscles of the fingers, eyes, tongue). On the contrary, in the muscles that carry out slow movements (maintenance of the trunk muscles), the motor units are large and include hundreds and thousands of muscle fibers.

When a muscle contracts in natural (natural) conditions, its electrical activity (electromyogram - EMG) can be recorded using needle or skin electrodes. In an absolutely relaxed muscle, there is almost no electrical activity. With a small voltage, for example, when maintaining a posture, the motor units are discharged at a low frequency (5-10 imp/s), with a high voltage, the impulse frequency rises to an average of 20-30 imp/s. EMG makes it possible to judge the functional ability of neuromotor units. From a functional point of view, motor units are divided into slow and fast.

slow motor units include slow motor neurons and slow muscle fibers (red). Slow motor neurons are usually low threshold, as they are usually small motor neurons. A stable level of impulses in slow motor neurons is observed even with very weak static muscle contractions, while maintaining a posture. Slow motor neurons are able to maintain a long discharge without a noticeable decrease in the frequency of impulses for a long time. Therefore they are called fatigued or non-fatiguing motoneurons. Surrounded by slow muscle fibers, there is a rich capillary network that allows you to receive a large amount of oxygen from the blood. An increased content of myoglobin facilitates the transport of oxygen in muscle cells to the mitochondria. Myoglobin causes the red color of these fibers. In addition, the fibers contain a large number of mitochondria and oxidation substrates - fats. All this determines the use by slow muscle fibers of a more efficient aerobic oxidative pathway of energy production and determines their high endurance.

fast motor units are composed of fast motor neurons and fast muscle fibers. Fast high-threshold motor neurons are activated only to provide relatively strong static and dynamic muscle contractions, as well as at the beginning of any contractions, in order to increase the rate of muscle tension increase or to impart the necessary acceleration to the moving part of the body. The greater the speed and strength of movements, i.e., the greater the power of the contractile act, the greater the participation of fast motor units. Fast motor neurons are tired - they are not capable of maintaining a high-frequency discharge for a long time.

Fast muscle fibers (white muscle fibers) are thicker, contain more myofibrils, and have more strength than slow fibers. These fibers are surrounded by fewer capillaries, and there are fewer mitochondria, myoglobin, and fats in the cells. The activity of oxidative enzymes in fast fibers is lower than in slow fibers, however, the activity of glycolytic enzymes and glycogen stores are higher. These fibers do not have great endurance and are more adapted for powerful, but relatively short-term contractions. The activity of fast fibers is important for performing short-term high-intensity work, such as sprinting.

There are also tonic muscle fibers, they have 7-10 synapses, usually belonging to several motor neurons. PKP of these muscle fibers does not cause the generation of AP in them, but directly triggers muscle contraction.

The rate of contraction of muscle fibers is directly dependent on the activity of myosin-ATPase, an enzyme that breaks down ATP and thereby promotes the formation of cross bridges and the interaction of actin and myosin myofilaments. The higher activity of this enzyme in fast muscle fibers also provides a higher speed of their contraction compared to slow fibers.

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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 smaller in small muscles that carry out fine and smooth regulation. 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.

By morpho functional properties DU 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 big number terminal branches and innervates not large group muscle fibres. 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 morphological and functional 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.

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, makes it possible to evaluate mainly low-threshold slow MUs. 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.

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.

See what "Motor unit" is in other dictionaries:

ENGINE UNIT- Basic unit of action nervously muscular system; it includes a separate efferent nerve fiber from a single motor neuron along with the muscle fiber it innervates... Dictionary in psychology

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unit motor- Functional unit of the neuromotor apparatus. It is a peripheral motor neuron, its processes and a group of muscle fibers innervated by it. At the same time, the axon of the motor neuron, which goes to the muscle that provides fine movements, is innervated by 5–12 ... Encyclopedic Dictionary of Psychology and Pedagogy

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Fast	Slow
Neuron
Large motor neurons	small motor neurons
Less excitability	More excitability
Axon diameter is greater	Smaller axon diameter
The speed of excitation is more	The rate of excitation is less
frequency more	Frequency less
Muscle fibers
Actomyosin ATPase activity is higher	Less actomyosin ATPase activity
The packing density of actomyosin filaments is higher	The packing density of actomyosin filaments is less
More pronounced sarcoplasmic reticulum (calcium depot)	Less pronounced sarcoplasmic reticulum (calcium depot)
The latent period after PD is less	The latent period after the receipt of PD is longer
The density of the calcium pump is higher	The density of the calcium pump is less
Contracts and relaxes faster	Contracts and relaxes more slowly
Higher activity of glycolysis enzymes	Higher activity of oxidation enzymes
Faster ATP recovery	ATP recovery is slower but more economical
1 mol of glucose -2-3 mols of ATP	1 mole of glucose 36-58 moles of ATP
Under-oxidized substrates are formed, "acidification" - rapid fatigue	fatigue is less pronounced
	Higher capillary density - more oxygenation, more myoglobin
motor unit
Less excitable, greater strength and speed of contraction, greater fatigue, low endurance	More excitable, less strength, contraction speed, low fatigue, high endurance
sprinters
IN external muscle hips slow fibers from 13 to 96%	Triceps brachii 33%, biceps 49%, tibialis anterior 46%, soleus 84%

Neurophysiological bases of the method of electromyography.

Electromyography is a method of studying the neuromuscular system by recording the electrical potentials of the muscles. Although for the first time an electromyogram (EMG) was recorded using a telephone device by N. E. Vvedensky back in 1884, and in 1907 it was possible to make a graphic recording of human EMG, the intensive development of electromyography as a clinical diagnostic technique began in the 30s and 40s years of the XX century A certain delay in progress in this area compared, for example, with the development of electroencephalography, is explained by the high requirements for the quality of registration and the accuracy of reproducing the true parameters of electrical potentials in electromyography. The creation of high-quality amplifiers that give linear characteristics in the high-frequency range, and the development of cathodic recording methods that provide undistorted reproduction of the high-frequency components of the electrical potential up to a range of 20,000 Hz, have led to significant progress in the field of clinical application of electromyography.

With intracellular registration, the action potential looks like a positive peak, consisting of a rapid depolarization, lasting about 1 ms, a rapid repolarization, which is a return of the potential almost to the resting level, lasting about 2 ms; this is followed by slow repolarization, a slight trace hyperpolarization, and a return of the potential to the resting level. In clinical electromyography during extracellular registration with a macroelectrode, the muscle fiber action potential is represented by a negative peak lasting 1-3 ms.

EMG recording and recording technique

The principles of the EMG recording and recording technique do not differ from the techniques of electroencephalography, electrocardiography and other electrographic methods. The system consists of electrodes that remove muscle potentials, an amplifier of these potentials, and a recording device. In electromyography, two types of electrodes are used - surface and needle. Surface electrodes are metal plates or disks with an area of about 0.2 - 1 cm 2, usually mounted in pairs in fixing blocks, ensuring the constancy of the distances between the discharge electrodes, which is important for assessing the amplitude of the recorded activity. Such electrodes are applied to the skin over the region of the motor point of the muscle. Before applying the electrode, the skin is wiped with alcohol and moistened with an isotonic sodium chloride solution. The electrode is fixed over the muscle using rubber bands, cuffs or adhesive tape. If a long-term study is required, a special electrode paste used in electroencephalography is applied to the area of skin-electrode contact. The large size and remoteness of the surface electrode from the muscle tissue make it possible to record with its help only the total muscle activity, which is the interference of the action potentials of many hundreds and even thousands of muscle fibers. At high amplifications and strong muscle contractions, the surface electrode also registers the activity of neighboring muscles. All this makes it impossible to study the parameters of individual muscle potentials using surface electrodes. In the resulting registration, only approximately estimate the frequency, periodicity and amplitude of EMG. The advantage of surface electrodes are atraumatic, no risk of infection, ease of handling of the electrodes. The painlessness of the study does not impose restrictions on the number of muscles examined at a time, making this method preferable when examining children, as well as for physiological control in sports medicine or when examining massive and strong movements.

Needle electrodes are concentric, bipolar and monopolar. In the first version, the electrode is represented by a hollow needle with a diameter of about 0.5 mm, inside which a wire rod made of platinum or stainless steel separated from it by an insulation layer passes. The potential difference is measured between the body of the needle and the tip of the central rod. Sometimes, to increase the locality of abduction, the needle is also isolated from the outside, and only its elliptical surface along the cut plane is left uninsulated. The area of the discharge surface of the axial rod of a standard concentric electrode is 0.07 mm 2 The parameters of EMG potentials given in modern publications refer to electrodes of this type and size. With a significant increase in the contact area of the discharge electrode, the parameters of the potentials can change significantly. The same applies to changes in the design of the electrode (bipolar, monopolar, multielectrode). The bipolar electrode contains inside the needle two identical rods isolated from each other, between the bare tips, which, separated by tenths of a millimeter from each other, measure the potential difference. Finally, for monopolar leads, electrodes are used, which are a needle, isolated throughout, except for the pointed end, which is bare for 1-2 mm. Needle electrodes are used to study the PD parameters of individual MUs and muscle fibers. Leading with a needle electrode is the main one in clinical myography, focused on the diagnosis of primary muscular and neuromuscular diseases. Recording individual APs in MU and muscle fibers allows you to accurately assess the duration, amplitude, shape and phase of the potential

Lead types

Regardless of the type of electrodes, there are two ways of diverting electrical activity - mono- and bipolar. In electromyography, such a lead is called monopolar when one electrode is located directly near the area of \u200b\u200bthe muscles under study, and the second in the area remote from it (skin over the bone, earlobe, etc.). The advantage of monopolar derivation is the ability to determine the shape of the potential of the structure under study and the true phase of the potential deviation. The disadvantage is that with a large distance between the electrodes, potentials from other parts of the muscle or even from other muscles interfere with the recording. A bipolar lead is a lead in which both electrodes are at a fairly close and equal distance from the muscle area under study. This is the abduction with the help of bipolar or concentric needle electrodes and with the help of a pair of surface electrodes fixed in one block. The bipolar lead registers activity from distant potential sources to a small extent, especially when needle electrodes are used. The effect on the potential difference of the activity coming from the source to both electrodes leads to a distortion of the shape of the potential and the inability to determine the true phase of the potential. Nevertheless, the high degree of locality makes this method preferred in clinical practice. Since the lead by surface electrodes registers in any case the interference activity of many mutually overlapping PDs, the use of such a monopolar lead does not make sense.

In addition to the electrodes, the potential difference of which is fed to the input of the EMG amplifier, a surface ground electrode is installed on the skin of the test subject, which is connected to the corresponding terminal on the electrode panel of the electromyograph. The potential difference from the electrodes is fed to the input of the voltage amplifier. The amplifier is equipped with a stepped gain switch that allows you to adjust the gain level depending on the amplitude of the recorded activity. Enhanced electrical activity is output not only to the oscilloscope, but also to the loudspeaker, which makes it possible to evaluate electrical potentials by ear.

General principles of EMG analysis and electromyographic semiotics.

The analysis of the electromyographic curve includes, at the first stage, the differentiation of the actual electrical potentials of the muscles from possible artifacts, and then, at the main stage, the assessment of the EMG itself. A preliminary operational assessment is carried out on the oscilloscope screen and acoustic phenomena that occur when the amplified EMG is output to the loudspeaker; the final analysis with a quantitative characteristic of EMG and a clinical conclusion is made by recording on paper or film.

Artifact potentials in EMG are called potentials that are not actually related to the activity of muscle elements. In superficial recording, artifacts can be caused by the movement of the electrode due to its loose fixation on the skin, which leads to the appearance of irregularly shaped high-amplitude potential jumps. With a needle lead, similar potential changes can occur when you touch the electrode, connecting wires, with massive movements of the muscle under study. The most common type of interference is 50 Hz pickup from industrial current operating devices. It is easily recognized by its characteristic sinusoidal shape and constant frequency and amplitude. Its occurrence may be associated with a large electrode resistance, which requires appropriate processing of the needle electrode. With surface electrodes, the elimination of pickup can be achieved by more thorough cleaning of the skin with alcohol, using electrode paste.

EMG analysis includes an assessment of the shape, amplitude and duration of the action potentials of individual muscle fibers and MUs and a characterization of the interference activity that occurs during voluntary muscle contraction. The form of a separate fluctuation of muscle potential can be mono-, di-. three or polyphasic. As in electroencephalography, such an oscillation is called monophasic, in which the curve deviates in one direction from the isoelectric line and returns to its original level. An oscillation is called diphasic, in which the curve, after making a deviation in one direction from the isoelectric line, crosses it and oscillates in the opposite phase; a three-phase oscillation makes, respectively, three deviations in opposite directions from the isoelectric line. Polyphasic is an oscillation containing four or more phases.

Stimulation methods in electromyography

In addition to studying the electrical activity of muscles at rest, during reflex and voluntary contractions, modern complex methods of clinical electromyography include the study of electrical responses of nerves and muscles to electrical stimulation. The equipment and methods for recording stimulation-induced electrical activity are the same as in conventional electromyography. Electrical stimulators are used to stimulate nerves and muscles. Muscles are stimulated with skin electrodes at motor points, nerves are stimulated according to their projection zones on the skin. Stimulating electrodes are made in the form of metal discs with a diameter of 6-8 mm, mounted in a metal holder and wetted with an isotonic sodium chloride solution. Stimulation methods in the diagnosis of neuromuscular diseases solve the following main tasks: 1) study of direct muscle excitability; 2) study of neuromuscular transmission; 3) study of the state of motor neurons and their axons; 4) study of the state of sensitive fibers of peripheral nerves. With the help of electromyography, it is possible to determine whether a change in electrical activity is associated with a lesion of a motor neuron or synaptic and suprasegmental structures.

Electromyographic data are widely used to clarify the topical diagnosis and objectify pathological or recovery processes. The high sensitivity of this method, which makes it possible to detect subclinical lesions of the nervous system, makes it especially valuable. Electromyography is widely used not only in neurological practice, but also in the study of damage to other systems, when secondary disorders of motor function occur (cardiovascular, metabolic, endocrine diseases).

With voluntary relaxation of the muscles, only very weak (up to 10-15 μV) and frequent fluctuations of the biopotential are captured. Reflex changes in muscle tone are characterized by a slight increase in the amplitudes of frequent, rapid and rhythmically variable oscillations of biopotentials (up to 50 μV). With voluntary muscle contractions, interference electromyograms are recorded (with frequent high-voltage biopotentials up to 2000 μV).

Damage to the cells of the anterior horn of the spinal cord causes a change in EMG depending on the severity of the damage, the nature of the course of the disease and its stage. With paresis, slowed, rhythmic fluctuations are observed with an increase in duration up to 15-20 ms. Damage to the anterior root or peripheral nerve causes a decrease in the amplitude and frequency of biopotentials, a change in the shape of the EMG curve. Flaccid paralysis is manifested by "bioelectric silence".

EMG of one of the muscles of the human arm is normal. . Electromyogram in lesions of the anterior horns of the spinal cord.

Questions for independent extracurricular work of students:

The composition of the motor unit. The concept of a motor pool.

Classification of motor units.

Comparative characteristics of fast and slow motor units.

Regulation of the force of contraction of an integral muscle. Principles of "involvement" of motor units, fractionation of the motor pool, a common final path.

Electromyography method, principle of the method, medical significance of the EMG method.

In the notebook of practical work, prepare a brief description of the EMG method (the principle of the method, the necessary equipment, types of electrodes and features of their use, the medical significance of the method).