Fast, fatigue-resistant motor units. motor unit
According to morphofunctional properties, motor units are divided into 3 types:
1. Slow tireless DE. Motoneurons have the lowest activation threshold, are able to maintain a stable frequency of discharges for tens of minutes (i.e., tireless). Axons have a small thickness, low speed of excitation, innervate a small group muscle fibers. Muscle fibers develop a small force during contraction due to the presence of the smallest amount of contractile proteins in them - myofibrils. These are the so-called "red fibers" (the color is due good development capillary network and a small number of myofibrils). The rate of contraction of these fibers is 1.5 - 2 times less than fast ones. They are tireless thanks to a well-developed capillary network, a large number of mitochondria and a high activity of oxidative enzymes.
2. Fast, easy to tire DE. They have the largest motor neuron, which has the highest threshold of excitation, are not able to maintain a stable discharge frequency for a long time (fatigue). Axons are thick, with a high speed of conduction of nerve impulses, innervating many muscle fibers. Muscle fibers contain big number myofibrils, therefore, when contracted, they develop great strength. Due to the high activity of enzymes, the rate of contraction is high. These fibers get tired quickly, because. contain fewer, compared with slow, mitochondria and are surrounded by fewer capillaries.
3. Fast, fatigue resistant. Strong, fast twitch fibers with great endurance due to the ability to use aerobic and anaerobic energy processes. Fibers of types 2 and 3 are called "white fibers" due to the high content of myofibrils and the low content of myoglobin.
Comparison of slow and fast muscle fibers
Human skeletal muscle consists of 3 types of fibers, but their ratio can vary significantly depending on the function of the muscle, as well as congenital and acquired individuality. The more white fibers in the muscles, the better man adapted to work that requires great speed and strength. The predominance of red fibers provides endurance during long-term work.
The structure of the skeletal muscle
Skeletal muscle consists of many muscle fibers that are bundled in a common connective tissue sheath and are attached to tendons associated with the skeleton. Each muscle fiber is a thin (from 10 to 100 microns) elongated (from 5 to 400 mm) multinuclear formation - symplast.
The muscle fiber membrane is similar in structure to the nervous one, but it gives regular T-shaped invaginations. Inside the muscle fiber, parallel to the membrane, there is a branched closed system of tubules. - sarcoplasmic reticulum- intracellular depot Ca 2+ . The T-system and adjacent SR are the apparatus for transmitting excitation from the muscle fiber membrane to contractile structures (myofibrils). In the sarcoplasm of the muscle fiber, one can see transverse alternating light and dark areas - respectively, J- (isotropic) and A- (anisotropic) disks. In neighboring myofibrils, the discs of the same name are located at the same level, which gives the fiber transverse striation. A complex of one dark and two halves of light discs adjacent to it, bounded by transverse Z-plates, is called sarcomere.
Each myofibril consists of many parallel thick (myosin) And thin (actin) protein filaments myofilaments. Over the fiber cross section, thick and thin filaments are arranged in a highly organized manner at the nodes of the hexagonal lattice. Each thick thread is surrounded by six thin ones, each of the thin threads is partially surrounded by three adjacent thick ones. Myosin filaments have transverse protrusions extending from them with heads consisting of approximately 150 myosin molecules. An actin filament consists of two chains of actin molecules twisted one around the other (like twisted strings of beads). Molecules are located on actin filaments. troponin, and in the grooves between the two actin filaments there are filaments tropomyosin.
Mechanism of muscle fiber contraction
In 1954, G. Huxley and N. Hanson found that actin and myosin filaments do not change their length during shortening or lengthening of the sarcomere and deduced thread slip theory: muscle contraction occurs when several centers of the myosin head of the cross bridge are sequentially connected to certain areas on the actin filaments.
In resting muscle fibers, tropomyosin molecules at rest are arranged in such a way that they prevent the attachment of myosin cross-bridges to actin filaments (the muscle is relaxed).
Arising in the region of the axo-somatic synapse, AP propagates through the system of T-tubules deep into the fiber, causing depolarization of the sarcoplasmic reticulum cisterns (Ca 2+ depot). Upon activation of the SR membrane, Ca channels open and Ca 2+ is released along the concentration gradient.
With an increase in the concentration of Ca 2+ ions in the myoplasm, it combines with troponin, the latter conforms and pushes back the tropomyosin thread, opening up the possibility for the myosin head to connect with actin. The connection of the head leads to a sharp "bending" of the bridge and movement of the actin filament by 1 step (20 nm or 1% of the actin length) to the middle of the sarcomere, followed by rupture of the bridge.
In the absence of re-excitation, the concentration of Ca 2+ decreases due to the work of the Ca-pump. Therefore, Ca 2+ is detached from troponin and tropomyosin again blocks actin. At the same time, the energy of 1 ATP molecule is spent on one working movement of one bridge, and the energy of another one is spent on returning 2 Ca 2+ ions to the tanks.
All this leads to relaxation of the muscle until the next flow of nerve impulses arrives, when the process described above is repeated.
The set of processes that determine the distribution of AP deep into the muscle fiber, the release of Ca 2+ ions from the sarcoplasmic reticulum, the interaction of contractile proteins and the shortening of the muscle fiber is called electromechanical interface.
Muscle mechanics. Physical properties and modes of muscle contractions
Physical properties skeletal muscle
1. Extensibility- the ability of a muscle to change its length under the action of a tensile force.
2. Elasticity- the ability of the muscle to take its original length after the cessation of the tensile or deforming force.
3. Force muscles. It is determined by the maximum load that the muscle is able to lift. Specific force - the maximum load that the muscle is able to lift, divided by the number of square centimeters of its physiological cross section.
4. Muscle ability do work. The work of the muscle is determined by the product of the value of the lifted load by the height of the lift. The work of the muscle gradually increases with an increase in the load, but up to a certain limit, after which an increase in the load leads to a decrease in work, since the height of the load is reduced. Therefore, the maximum muscle work is performed at average loads ( law of average loads).
Modes of muscle contractions
There are isotonic, isometric and mixed modes of muscle contraction.
At isotonic contraction of the muscle changes its length, and the tension remains constant. Such a contraction occurs when the muscle does not move the load. Under natural conditions, contractions of the muscles of the tongue are close to the isotonic type of contractions.
At isometric contraction, the length of the muscle fibers remains constant, the tension of the muscle changes. Such a muscle contraction can be obtained when trying to lift an unbearable load.
In the whole organism, muscle contractions are never purely isotonic or isometric, they always have a mixed character, that is, there is a change in both length and muscle tension. This reduction mode is called auxotonic if muscle tension predominates, or auxometric if shortening prevails.
The main morpho-functional element of the neuromuscular apparatus of skeletal muscles is the motor unit (MU). It includes the 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. The motor units of small muscles that perform fine movements (muscles of the eye, hand) contain a small amount of muscle fibers. In large ones, there are hundreds of times more. 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 strength of these fibers are relatively small, but they are not very fatiguable. Therefore, they 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.
IIB. 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. Muscles of the eye.
IIA. Fast, fatigue resistant. They occupy an intermediate position.
Physiology of Smooth Muscles
Smooth muscles are found 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 muscles are formed by cells more often spindle-shaped and small in size, which do not have transverse striation. The latter is due to the fact that the contractile apparatus does not have an ordered structure. Myofibrils are made up 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. The elements of the sarcoplasmic reticulum do not form a system of tubules. Separate muscle cells are interconnected 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 ones.
The membrane potential is 40-60 mV, since the SMC membrane has a relatively high permeability for sodium ions. Moreover, in many smooth muscles, MP is not constant. It periodically decreases and again returns to its original level. Such oscillations are called slow waves (SW). When the top of the slow wave reaches a critical level of depolarization, action potentials begin to be generated on it, accompanied by contractions (Fig.). MV and PD 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 mechanisms of electromechanical coupling are also different. The contraction develops due to calcium entering the cell during PD. The most important cellular protein, calmodulin, mediates the relationship of calcium with the shortening of myofibrils.
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 a few seconds. Smooth muscles, unlike skeletal muscles, are characterized by the phenomenon of plastic tone. This ability is in a state of reduction for a long time without significant energy consumption and fatigue. Thanks to this property, the shape of the internal organs and vascular tone are maintained. In addition, smooth muscle cells themselves are stretch receptors. When they are stretched, APs begin to be generated, which leads to a reduction in the SMC. This phenomenon is called the myogenic mechanism of regulation of contractile activity.
motor unit includes a motor neuron along with the group of muscle fibers innervated by it. IN different muscles motor units include different numbers of muscle fibers. Yes, in oculomotor muscles 1 neuron has about 10 muscle fibers, and in the large muscles of the body - more than 1000 fibers. Small motor units provide fast and precise movements. There are 3 types motor units: fast, tired; slow, untiring; fast, tireless. In any muscle there are all types of fibers, but in different proportions. There are more fast muscle fibers in the muscles of sprinters, and more slow muscle fibers in the athletes. Fast fibers are less well supplied with blood, therefore they are capable of short-term work. Slow fibers are abundantly supplied with blood and can work for a long time without fatigue. The bodies of motor neurons of slow motor units are small and have a low excitability threshold, i.e., they can be activated even by weak signals. The motor neuron bodies of fast motor units are larger, but less excitable, and fire when more strength is needed.
The mechanism of excitation transmission in the central synapses, excitatory mediators, the formation of an excitatory postsynaptic potential (EPSP). Significance of chemo-regulated and voltage-gated ion channels.
The mechanism of transmission of excitation in the synapse. Mediators are chemical mediators that transmit information in a synapse from one neuron to another. The release of the mediator from the presynaptic ending is possible only if the presynaptic membrane is depolarized by the impulses that have arrived at the nerve ending. In the presynaptic membrane there are channels for calcium ions, which are closed in the absence of excitation. Calcium ions play a decisive role in the release of the mediator. When the presynaptic membrane is depolarized by excitation that comes here, calcium channels open, calcium from the synaptic cleft enters the presynaptic ending, ensures the fusion of mediator vesicles with the presynaptic membrane and release of the mediator into the synaptic cleft. The mediator released into the synaptic cleft moves to the postsynaptic membrane, where it binds to specific receptors that simultaneously act as ion channels. The resulting “mediator-receptor” complex increases the permeability of the postsynaptic membrane for certain ions, as a result, the potential difference across the postsynaptic membrane changes and a postsynaptic potential is formed. Depending on the nature of the mediator and the nature of the receptors that bind it, the postsynaptic membrane can be depolarized, which is typical for excitatory synapses, or hyperpolarized, which is typical for inhibitory synapses. Excitatory postsynaptic potential (EPSP) is formed on the postsynaptic membrane in response to the action of excitatory mediators. These mediators include: acetylcholine, norepinephrine, dopamine, serotonin. The mediator interacts with the receptors of the postsynaptic membrane like a key with a lock, that is, for each mediator there is a certain type of receptor. As a result of the interaction of the mediator with the receptors of the postsynaptic membrane, sodium channels open (possibly the participation of calcium channels). Sodium enters the cell through the postsynaptic membrane and depolarizes it. The resulting potential difference across the postsynaptic membrane is called the excitatory postsynaptic potential. If its value is sufficient, then action potentials are formed in the extrasynaptic part of the neuron membrane. The cessation of the action of the mediator is due to its removal from the synaptic cleft either due to the reverse "capture" of the structures of the presynaptic ending, or its destruction by special enzymes of the postsynaptic membrane. In synapses, the process of inhibition can develop, which will be discussed later.
14. Inhibition in the CNS and its physiological role. Teachings of I. M. Sechenov about central inhibition. Brake mediators. Mechanisms of pre- and postsynaptic inhibition.
For the first time, I. M. Sechenov spoke about inhibition as a process in the central nervous system (1863). Irritating the area of the thalamus of a frog with salt crystals, Sechenov noted a slowing down of the motor reaction. He came to the conclusion that the process of inhibition develops in the central nervous system and, accordingly, there are inhibitory centers. This type of braking was called by Sechenov central. Postsynaptic inhibition develops if an inhibitory neuron forms synapses either on the dendrites or on the body of the excitatory neuron. Synapses have the same structural elements: pre-, postsynaptic membrane, synaptic cleft and neurotransmitters. Only in this case, inhibitory mediators are involved: GABA, glycine, acytylcholine, etc. The mediators cause a change in the permeability of the postsynaptic membrane not for sodium, but either for chlorine or potassium through the activation of the corresponding receptors and the opening of chemodependent ion channels. If channels for Cl - ions open, it passes through the postsynaptic membrane inward and hyperpolarizes it. As a result, the value of the membrane potential increases, and excitability decreases. If channels for K + are activated in the inhibitory synapse, then it exits along the gradient to the surface of the postsynaptic membrane, which also becomes hyperpolarized. The magnitude of hyperpolarization is called inhibitory postsynaptic potential (IPSP), and the type of inhibition is called postsynaptic. presynaptic inhibition observed in axo-axonal synapses. Here, the axon of the inhibitory neuron synapses on the axon of the excitatory neuron, before it even synapses with another neuron. Therefore, inhibition is called presynaptic. This type of inhibition blocks the passage of excitation along the axon and is important for filtering information in sensory neurons. The role of inhibition in the central nervous system. Inhibition provides: orderliness of excitation propagation; consistency in the interaction of centers; protective, protective role from overexcitation. Examples prove the importance of inhibition: with tetanus or strychnine poisoning, inhibitory synapses are blocked in the nervous system, so excitation becomes disordered, resulting in muscle cramps and death. Inhibition is a process of excitation of specialized neurons, leading to inhibition of the development and spread of excitation. It is important to remember that inhibition is a local, local non-spreading process, in contrast to excitation.
motor units
The strength and work of the muscle fiber. motor units.
The amount of contraction (muscle strength) depends on the morphological properties and physiological state muscles:
1. Initial muscle length (rest length). The strength of muscle contraction depends on the initial length of the muscle or the resting length. The more the muscle is stretched at rest, the stronger the contraction (Frank-Starling law).
2. Muscle diameter or cross section. There are two diameters:
a) anatomical diameter - the cross section of the muscles.
b) physiological diameter - a perpendicular section of each muscle fiber. The larger the physiological cross section, the greater the strength of the muscle.
Muscle strength is measured by the weight of the maximum load lifted to a height or maximum tension, ĸᴏᴛᴏᴩᴏᴇ it is able to develop under conditions of isometric contraction. It is measured in kilograms or newtons. The technique for measuring muscle strength is commonly called dynamometry.
There are two types of muscle strength:
1. Absolute strength - the ratio of maximum strength to physiological diameter.
2. Relative strength - the ratio of maximum strength to the anatomical diameter.
When a muscle contracts, it can do work. The work of the muscle is measured by the product of the lifted load by the amount of shortening.
Muscle work is characterized by power. Muscle power is determined by the amount of work per unit of time and is measured in watts.
The greatest work and power is achieved at medium loads.
A motor neuron with a group of muscle fibers innervated by it constitutes a motor unit. The axon of motor neurons can branch and innervate a group of muscle fibers. So, one axon can innervate from 10 to 3000 muscle fibers.
Motor units are distinguished by structure and function.
By structure, motor units are divided into:
1. Small motor units that have a small motor neuron and a thin axon that can innervate 10-12 muscle fibers. For example, the muscles of the face, the muscles of the fingers.
2. Large motor units are represented by a large motor neuron body, a thick axon, which is capable of innervating more than 1000 muscle fibers. For example, the quadriceps muscle.
According to their functional value, motor units are divided into:
1. Slow motor units. Οʜᴎ include small motor units, are easily excitable, are characterized by a low speed of propagation of excitation, are included in the work first, but at the same time they are practically not fatiguing.
2. Fast motor units. Οʜᴎ consist of large motor units, are poorly excitable, have a high speed of excitation. They have high strength and speed of response. For example, the muscles of a boxer.
These features of motor units are due to a number of properties.
Muscle fibers that make up motor units have similar properties and differences. So, slow muscle fibers have:
1. Rich capillary network.
3. Contains a lot of myoglobin (ᴛ.ᴇ. able to bind a large amount of oxygen).
4. They are high in fat.
Due to these features, these muscle fibers have high endurance, are capable of contractions that are small in strength, but long in time.
Distinctive features fast muscle fibers
2. They have greater speed and force of contraction.
In connection with these features, fast muscle fibers are quickly fatigued, but have great strength and high response speed.
Motor units - concept and types. Classification and features of the category "Motor units" 2017, 2018.
A motor unit is a group of muscle fibers innervated by a single motor neuron.
Big Medical Dictionary. 2000 .
See what a "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
motor unit- - a group of muscle fibers innervated by one motor neuron; neuromotor unit... Glossary of terms for the physiology of farm animals
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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