Nerve fibers are processes of nerve cells, among which dendrites and axons are distinguished. One of the most important functions of these fibers is the perception of signals of the external and internal environment, their conversion into nerve impulses and the conduction of the latter to dendrites in or along axons from the central nervous system to effector cells.
Nerve fibers (processes of nerve cells) carry out nerve impulses. Nerve fibers are divided into myelin (myelin-coated) and myelin-free. Myelin fibers predominate in the motor nerves, and myelin fibers in the autonomic nervous system.
Fiber structure
The nerve fiber consists of an axial cylinder and its myelin sheath, interrupted at regular intervals (Ranvier intercepts). The myelin sheath is formed as a result of the fact that a lemmocyte (Schwann cell) repeatedly wraps around the axial cylinder, forming a dense lipid layer. Such fibers are called myelin, or pulpy. Nerve fibers that do not have a myelin sheath are called myelin-free, or serene. The axial cylinder has a plasma membrane and axoplasm.
From nerve fibers, nerves or nerve trunks are formed, enclosed in a common connective tissue membrane. The nerve contains both myelin and non-myelin fibers.
Fig. The structure of the nerve fibers
Depending on the function and direction of the nerve impulses, the fibers are divided into afferentconducting signals to the central nervous system, and efferentconducting them from the central nervous system to the executive bodies. Nerve fibers form nerves and numerous signaling pathways within the nervous system itself.
Types of Nerve Fibers
Nerve fibers according to their diameter and rate of excitation are usually divided into three types: A, B, C. Type A fibers, in turn, are divided into subtypes: A-α, A-β, A-γ, A-δ.
Fibers type A covered with myelin sheath. The thickest of them (Aa) have a diameter of 12-22 microns and have the highest speed of excitation - 70-120 m / s. Excitation is carried out along these fibers from the motor nerve centers of the spinal cord to the skeletal muscles and from muscle receptors to the corresponding nerve centers. Other type A fibers have a smaller diameter and a lower speed of excitation (from 5 to 70 m / s). They relate mainly to sensitive fibers that conduct excitation from various receptors (tactile, temperature, etc.) in the central nervous system.
To the fibers type B include myelin preganglionic fibers of the autonomic nervous system. Their diameter is 1-3.5 microns, and the speed of the excitation is 3-18 m / s.
To the fibers type C belong thin (diameter 0.5-2 microns) myelin-free nerve fibers. The speed of the excitation on them is 0.5-3.0 m / s. Fibers of this type are part of the postganglionic fibers of the autonomic nervous system. These fibers also conduct excitation from thermoreceptors and pain receptors.
Conducting excitation along nerve fibers
Features of the excitation in nerve fibers depend on their structure and properties. According to these signs, nerve fibers are divided into groups A, B and C. The fibers of groups A and B are represented by myelinated fibers. They are covered with a myelin sheath, which is formed by tightly adjacent membranes of glial cells, repeatedly wrapped around the axial cylinder of the nerve fiber. In the central nervous system, the myelin sheath is formed by oligodendrocytes, and peripheral nerve myelin is formed by Schwann cells.
Myelin is a multilayer membrane consisting of phospholipids, cholesterol, myelin basic protein and a small amount of other substances. The myelin sheath is interrupted through approximately equal sections (0.5-2 mm), and the nerve fiber membrane remains uncovered by myelin. These sites are called Ranvier intercepts. In the nerve fiber membrane in the area of \u200b\u200binterception, there is a high density of voltage-dependent sodium and potassium channels. The length of the intercepts is 0.3-14 microns. The larger the diameter of the myelinated fiber, the longer its sections are covered with myelin and the smaller the number of Ranvier intercepts per unit length of such fiber.
The fibers of group A are divided into 4 subgroups: a, β, y, δ (Table 1).
Table 1. Properties of various warm-blooded nerve fibers
|
Fiber type |
Fiber diameter, microns |
Speed, m / s |
Function |
The duration of the peak action potential, ms |
Duration of trace depolarization, ms |
Duration of trace hyperpolarization, ms |
|
Proprioception function Skeletal muscle fibers, afferent fibers from muscle receptors |
||||||
|
Tactile function Afferent fibers from touch receptors |
||||||
|
Motor function Afferent fibers from receptors of touch and pressure, afferent fibers to muscle spindles |
||||||
|
Pain, temperature and tactile functions Afferent fibers from some receptors of heat, pressure, pain |
||||||
|
Preganglionic vegetative fibers |
Absent |
|||||
|
Sympathetic function Postganglionic vegetative fibers, afferent fibers from some receptors of heat, pressure, pain |
Aa fiber - the largest in diameter (12-20 microns) - have an excitation speed of 70-120 m / s. They act as afferent fibers that conduct excitation from tactile skin receptors, muscle and tendon receptors, and are also efferent fibers that transmit excitation from spinal a-motor neurons to extrafusal contractile fibers. The information transmitted through them is necessary for the implementation of fast reflex and voluntary movements. Nerve fibers Au excite from spinal y-motor neurons to contractile cells of muscle spindles. Having a diameter of 3-6 μm, Ay fibers conduct excitation at a speed of 15-30 m / s. The information transmitted through these fibers is not used directly to initiate movements, but rather to coordinate them.
From the table. Figure 1 shows that thick myelinated fibers are used in those sensory and motor nerves, with the help of which information should be transmitted most quickly for urgent reactions.
The processes controlled by the autonomic nervous system are carried out at lower speeds than the motor reactions of skeletal muscles. The information necessary for their implementation is perceived by sensory receptors and transmitted to the central nervous system via the thinnest afferent myelinated Aδ-, B- and non-myelinated C-fibers. Efferent fibers of type B and C are part of the nerves of the autonomic nervous system.
The mechanism of excitation along nerve fibers
To date, it has been proven that excitation along the myelin and non-myelin nerve fibers is carried out on the basis of ionic mechanisms of action potential generation. But the mechanism for conducting excitation along the fibers of both types has certain features.
So, during the propagation of excitation along the myelin-free nerve fiber, local currents that arise between its excited and unexcited regions cause depolarization of the membrane and the generation of the action potential. Then local currents arise already between the excited portion of the membrane and the nearest unexcited portion. Repeated repetition of this process contributes to the spread of excitation along the nerve fiber. Since all sections of the fiber membrane are sequentially involved in the excitation process, this mechanism of excitation is called continuous. Continuous action potential occurs in muscle fibers and in type C non-myelin nerve fibers.
The presence in the myelin nerve fibers of areas without this myelin sheath (Ranvier interceptions) determines a specific type of excitation. In these fibers, local electric currents occur between adjacent Ranvier intercepts, separated by a myelin sheath. And the excitement “jumps” through the areas covered with the myelin sheath, from one interception to another. Such a mechanism for the propagation of excitation is called saltatory (spasmodic), or intermittent. The rate of saltatory excitation conduction is much higher than in non-myelin fibers, since not the entire membrane is involved in the excitation process, but only its small sections in the interception region.
"Jumping" of the action potential through the myelin region is possible because its amplitude is 5-6 times higher than the value necessary to initiate the neighboring Ranvier interception. Sometimes the action potential is able to “jump over” even through several interception intervals.
Transport function of nerve fibers
The implementation by the membrane of nerve fibers of one of their main functions - conducting nerve impulses - is inextricably linked with the transformation of electrical potentials into the release of signaling molecules, neurotransmitters, from nerve endings. In many cases, their synthesis is carried out in the core of the body of the nerve cell, and the axons of the nerve cell, which can reach a length of 1 m, deliver neurotransmitters to the nerve endings through special transport mechanisms, called axon transport of substances. With their help, not only neurotransmitters travel along nerve fibers, but also enzymes, plastic and other substances necessary for the growth, maintenance of the structure and function of nerve fibers, synapses and postsynaptic cells.
Axon transport is divided into fast and slow.
Fast axon transport provides the movement of mediators, some intracellular organelles, enzymes in the direction from the body of the neuron to the presynaptic terminals of the axon. Such transport is called antegrade.It is carried out with the participation of actin protein, Ca 2+ ions and microtubules and microfilaments passing along the axon. Its speed is 25-40 cm / day. The energy of cellular metabolism is expended on transport.
Slow axon transport occurs at a speed of 1-2 mm / day in the direction from the body of the neuron to the nerve endings. Slow antegrade transport is the movement of the axoplasm together with the contained organelles, RNA, proteins and biologically active substances from the body of the neuron to its ends. The axon growth rate depends on the speed of their movement, when it restores its length (regenerates) after damage.
Allocate also retrograde axon transport in the direction from the nerve ending to the body of the neuron. Using this type of transport, the enzyme acetylcholinesterase, fragments of destroyed organelles, and some biological substances that regulate protein synthesis in the neuron are transported to the body of the neuron. The speed of transport reaches 30 cm / day. Taking into account the presence of retrograde transport is important because pathogenic agents such as polio, herpes, rabies, and tetanus toxin can penetrate the nervous system.
Axon transport is necessary to maintain the normal structure and function of nerve fibers, the delivery of energy substances, mediators and neuropeptides to presynaptic terminals. It is important for providing trophic effects on innervated tissues and for repairing damaged nerve fibers. If the nerve fiber is crossed, then its peripheral area, deprived of the ability to exchange various substances with the body of the nerve cell via axon transport, degenerates. The central section of the nerve fiber, which remains in contact with the body of the nerve cell, regenerates.
Nerve impulse
Conducting nerve impulses is a specialized function of nerve fibers, i.e. processes of nerve cells.
Nerve fibers are divided into meaty, myelinated,and serene, or non-myelinated. The pulp, sensory, and motor fibers are part of the nerves supplying the sensory organs and skeletal muscle; they are also present in the autonomic nervous system. The soft fibers in vertebrates belong mainly to the sympathetic nervous system.
Nerve fiber structure
Nerves usually consist of both pulpy and soft fibers, and their ratio in different nerves is different. For example, in many cutaneous nerves, serene nerve fibers predominate. So, in the nerves of the autonomic nervous system, for example in the vagus nerve, the number of serene fibers reaches 80-95%. On the contrary, in the nerves innervating skeletal muscles, there is only a relatively small amount of soft fibers.
As shown by electron microscopic studies, the myelin sheath is created as a result of the fact that the myelocyte (Schwann cell) repeatedly wraps around the axial cylinder (Fig. 1), its layers merge, forming a dense fat case - myelin sheath. The myelin sheath is interrupted at intervals of equal length, leaving exposed sections of the membrane about 1 μm wide. These sites are called intercepts ranvier.
Fig. 1. The role of myelocyte (Schwann cell) in the formation of the myelin sheath in the pulmonary nerve fibers: successive stages of the spiral-like twisting of myelocytes around the axon (I); relative position of myelocytes and axons in serene nerve fibers (II)
The length of the interception sites covered with the myelin sheath is approximately proportional to the diameter of the fiber. So, in nerve fibers with a diameter of 10-20 microns, the length of the gap between intercepts is 1-2 mm. In the thinnest fibers (with a diameter of 1-2 microns), these sections have a length of about 0.2 mm.
The serene nerve fibers do not have a myelin sheath, they are isolated from each other only by Schwann cells. In the simplest case, a single myelocyte surrounds one serene fiber. Often, however, in the folds of the myelocyte are several thin, serene fibers.
The myelin sheath has a dual function: the function of an electrical insulator and the trophic function. The insulating properties of the myelin sheath are associated with the fact that myelin as a substance of lipid nature prevents the passage of ions and therefore has a very high resistance. Due to the existence of the myelin sheath, the occurrence of excitation in the pulmonary nerve fibers is possible not all along the axial cylinder, but only in limited areas - Ranvier intercepts. This is important for the propagation of a nerve impulse along the fiber.
The trophic function of the myelin sheath, apparently, consists in the fact that it takes part in the processes of regulation of metabolism and the growth of the axial cylinder.
Excitation in non-myelinated and myelinated nerve fibers
In serene nerve fibers, excitation propagates continuously along the entire membrane, from one excited site to another, located nearby. In contrast, in myelinated fibers, the action potential can only propagate spasmodically, "jumping" through portions of the fiber coated with an insulating myelin sheath. This is called saltatory.
Direct electrophysiological studies by Kago (1924) and then Tasaki (1953) on single myelinated frog nerve fibers showed that the action potentials in these fibers arise only in intercepts, and the areas between intercepts covered with myelin are practically unexcited.
The density of sodium channels in the intercepts is very high: there are about 10,000 sodium channels per 1 μm 2 membrane, which is 200 times higher than their density in the membrane of the giant squid axon. High density of sodium channels is the most important condition for saltatory conduct of excitation. In fig. Figure 2 shows how the "jumping" of a nerve impulse from one interception to another occurs.
At rest, the outer surface of the excitable membrane of all Ranvier intercepts is positively charged. There is no potential difference between adjacent interceptions. At the time of excitation, the surface of the interception membrane FROM becomes charged electronegative with respect to the surface of the adjacent interception membrane D. This leads to a local (local) electric current that flows through the interstitial fluid, membrane and axoplasm through the surrounding fiber in the direction shown by the arrow in the figure. Coming out through interception D current excites it, causing recharging of the membrane. In interception C, the excitation is still ongoing, and for a while it becomes refractory. Therefore interception D only the next interception, etc., can be brought into a state of excitement.
"Jumping" of the action potential through the interception section is possible only because the amplitude of the action potential in each interception is 5-6 times higher than the threshold value necessary to initiate an adjacent interception. Under certain conditions, the action potential can “jump over” not only through one, but also through two interception sites - in particular, if the excitability of the neighboring interception is reduced by any pharmacological agent, for example, novocaine, cocaine, etc.
Fig. 2. The saltatory spread of excitation in the pulp nerve fiber from interception to interception: A - non-myelinated fiber; B - myelinated fiber. The arrows indicate the direction of the current.
The assumption of a spasmodic spread of excitation in nerve fibers was first expressed by B.F. Verigo (1899). This method of conducting has a number of advantages compared to continuous conducting in non-fibrous fibers: firstly, by “jumping” over relatively large sections of the fiber, the excitation can propagate at a much higher speed than when continuously conducting along the non-fibrous fiber of the same diameter; secondly, spasmodic propagation is energetically more economical, since not the entire membrane comes into an activity state, but only its small sections in the interception region, having a width of less than 1 μm. The loss of ions (per unit length of fiber) that accompanies the emergence of an action potential in such limited sections of the membrane is very small, and therefore, the energy costs for the sodium-potassium pump, which are necessary to restore the altered ionic ratios between the internal contents of the nerve fiber and tissue fluid.
The laws of the excitation in the nerves
When studying the conduct of excitation along the nerve, several necessary conditions and rules (laws) of the course of this process were established.
Anatomical and physiological continuity of the fiber. A prerequisite for conducting excitation is the morphological and functional integrity of the membrane. Any strong effect on the fiber - ligature, squeezing, stretching, the action of various chemical agents, excessive action of cold or heat - causes damage to it and termination of excitation.
Bilateral excitation. Excitation is carried out along nerve fibers both in the afferent and in the efferent direction. This feature of nerve fibers was proved by the experiments of A.I. Babukhina (1847) on the electric organ of the Nile catfish. The catfish electric organ consists of separate plates innervated by branches of one axon. A.I. Babukhin removed the middle plates to avoid conducting excitation on the electric organ, and cut one of the nerve branches. Irritating the central end of the severed nerve, he observed a response in all segments of the electric organ. Therefore, excitation along nerve fibers passed in different directions - centripetal and centrifugal.
Bilateral conduct is not only a laboratory phenomenon. Under natural conditions, the action potential of a nerve cell arises in that part of it, where the body passes into its process - the axon (the so-called initial segment). From the initial segment, the action potential spreads bilaterally: in the axon towards the nerve endings and into the body of the cell towards its dendrites.
Isolated holding. In the peripheral nerve, impulses propagate along each fiber in isolation, i.e. without moving from one fiber to another and exerting action only on those cells with which the ends of a given nerve fiber come into contact. This is due to the characteristics of the myelin sheath. With high resistance, it is an insulator that prevents the spread of excitation to neighboring fibers. This is very important due to the fact that any peripheral nerve trunk contains a large number of nerve fibers - motor, sensory and autonomic, which innervate different, sometimes far-spaced and heterogeneous cells and tissues in structure and function. For example, the vagus nerve innervates all the organs of the chest cavity and a significant part of the organs of the abdominal cavity, the sciatic nerve - all the muscles, bone apparatus, blood vessels and skin of the lower limb. If the excitation passed inside the nerve trunk from one fiber to another, then in this case the normal isolated functioning of peripheral organs and tissues would be impossible.
The degeneration of nerve fibers after transection of a nerve. Nerve fibers cannot exist without connection with the body of a nerve cell: nerve transection leads to the death of those fibers that are separated from the body of cells. In warm-blooded animals, just two or three days after transection of a nerve, its peripheral process loses its ability to conduct nerve impulses. Following this, degeneration of nerve fibers begins, and the myelin sheath undergoes fatty degeneration: the meaty sheath loses myelin, which accumulates in the form of drops; the decayed fibers and their myelin dissolve and in place of the nerve fibers there remain cords formed by a lemmocyte (Schwann cell). All these changes were first described by the English physician Waller and named after him Waller degeneration.
Nerve regeneration is very slow. The lemocytes remaining in place of the degenerated nerve fibers begin to grow near the transection site in the direction of the central segment of the nerve. At the same time, the cut ends of the axons of the central segment form the so-called growth flasks - bulges that grow in the direction of the peripheral segment. Some of these branches fall into the old bed of the cut nerve and continue to grow in this bed at a speed of 0.5-4.5 mm per day, until it reaches the corresponding peripheral tissue or organ, where the fibers form the nerve endings. From this time, the normal innervation of an organ or tissue is restored.
In various organs, the restoration of function after nerve transection occurs at different times. In muscles, the first signs of restoration of functions may appear after five to six weeks; final recovery occurs much later, sometimes after a year.
Nerve fiber properties
Nerve fiber has certain physiological properties: excitability, conductivity and lability.
Nerve fiber is characterized by very low fatigue. This is due to the fact that when carrying out one action potential along the nerve fiber, a very small amount of ATP is expended to restore ionic gradients.
Lability and parabiosis of nerve fibers
Nerve fibers possess lability. Lability (instability) is the ability of a nerve fiber to reproduce a certain number of excitation cycles per unit time. A measure of the nerve fiber lability is the maximum number of excitation cycles that it can reproduce per unit time without changing the rhythm of irritation. Nerve fiber can reproduce up to 1000 pulses per second.
Academician N.E. Vvedensky discovered that when a damaging agent (alteration), such as a chemical substance, is exposed to the nerve region, the lability of this region decreases. This is due to the blockade of sodium and potassium membrane permeability. This state of reduced lability N.E. Vvedensky called parabiosis. Parabiosis is divided into three successive phases: egalitarian, paradoxical and inhibitory.
AT equalization phase the same value of the response to the action of strong and weak stimuli is established. Under normal conditions, the magnitude of the response of the muscle fibers innervated by this nerve obeys the law of force: the response is less for weak stimuli, and more for strong stimuli.
Paradoxical phase characterized by the fact that to weak stimuli there is a reaction of a larger magnitude than to strong ones.
AT brake phase fiber lability is reduced to such an extent that stimuli of any strength are not able to cause a response. In this case, the fiber membrane is in a state of prolonged depolarization.
Parabiosis is reversible. In the case of a short-term exposure to a nerve of a damaging substance, after the termination of its action, the nerve leaves the state of parabiosis and goes through the same phases, but in reverse order.
Nerve fatigue
Nerve fatigue was first shown by N.E. Vvedensky (1883), who observed the preservation of the working capacity of the nerve after a continuous 8-hour irritation. Vvedensky conducted an experiment on two neuromuscular preparations of frog legs. Both nerves for a long time were irritated by a rhythmic induction current of the same strength. But on one of the nerves, closer to the muscle, DC electrodes were additionally installed, with which the conduction of excitation to the muscles was blocked. Thus, both nerves were irritated for 8 hours, but excitation passed only to the muscles of one paw. After 8 hours of irritation, when the muscles of the working drug stopped contracting, a block was removed from the nerve of another drug. Thus there was a contraction of his muscles in response to irritation of the nerve. Consequently, the nerve conducting excitation to the blocked paw did not tire, despite prolonged irritation.
Thin fibers get tired faster than thick fibers. The relative fatigue of the nerve fiber is primarily associated with the level of metabolism. Since nerve fibers during activity are excited only at Ranvier intercepts (which makes up a relatively small surface), the amount of energy spent is small. Therefore, the processes of resynthesis easily cover these costs, even if the excitation lasts several hours. In addition, in the natural environment of the functioning of the body, the nerve does not get tired and due to the fact that it carries a load less than its capabilities.
Of all the links of the reflex arc, the nerve has the highest lability. Meanwhile, in the whole organism, the frequency of impulses traveling along the efferent nerve is determined by the lability of the nerve centers, which is small. Therefore, the nerve spends fewer pulses per unit time than it could reproduce. This ensures its relative fatigue.
CONDUCTING A NERVOUS PULSE
nerve impulse, signal transmission in the form of an excitation wave within one neuron and from one cell to another. P. n. and. along nerve conductors occurs with the help of electrotonic potentials and action potentials, which propagate along the fiber in both directions, without passing to neighboring fibers (see Bioelectric potentials, Nerve impulse). The transmission of intercellular signals through synapses is most often done with the help of mediators that cause the appearance of postsynaptic potentials. Nerve conductors can be considered as cables with relatively low axial resistance (axoplasm resistance - ri) and higher sheath resistance (membrane resistance - rm). A nerve impulse propagates along the nerve conductor through the passage of current between the resting and active sections of the nerve (local currents). In the conductor, as the distance from the place of occurrence of excitation increases, a gradual, and in the case of a homogeneous structure of the conductor, exponential decay of the pulse, which decreases by a factor of 2.7 at a distance l (length constant). Since rm and ri are inversely related to the diameter of the conductor, the attenuation of the nerve impulse in thin fibers occurs earlier than in thick ones. The imperfection of the cable properties of nerve conductors is compensated for by the fact that they have excitability. The main condition for excitation is the presence of resting potential in nerves. If a local current through the resting section causes depolarization of the membrane reaching a critical level (threshold), this will lead to the appearance of a propagating action potential (PD). The ratio of the threshold depolarization level and the amplitude of the AP, usually amounting to at least 1: 5, provides high reliability: sections of the conductor with the ability to generate AP can be separated from each other at such a distance, overcoming which the nerve impulse reduces its amplitude by almost 5 times. This attenuated signal will again be amplified to a standard level (amplitude of the AP) and will be able to continue its path along the nerve.
Speed \u200b\u200bP. n. and. depends on the speed with which the membrane capacitance in the area ahead of the pulse is discharged to the level of the threshold for PD generation, which, in turn, is determined by the geometric characteristics of the nerves, changes in their diameter, and the presence of branching nodes. In particular, thin fibers have a higher ri, and a greater surface capacity, and therefore the speed of P. n. and. on them below. At the same time, the thickness of nerve fibers limits the existence of a large number of parallel communication channels. The conflict between the physical properties of nerve conductors and the requirements of the "compactness" of the nervous system was resolved by the appearance of so-called so-called vertebrates during evolution. pulp (myelinated) fibers (see. Nerves). Speed \u200b\u200bP. n. and. in myelinated warm-blooded fibers (despite their small diameter - 4-20 microns) reaches 100-120 m / s. PD generation occurs only in limited areas of their surface - Ranvier intercepts, and in the intercept areas of P. and. and. it is carried out electrotonically (see. Saltation holding). Some medicinal substances, for example anesthetics, greatly slow down until P.'s complete block of n. and. This is used in practical medicine for pain relief.
Lit. see under the articles Excitation, Synapses.
L.G. Magazanik.
Great Soviet Encyclopedia, TSB. 2012
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source of electromagnetic radiation in the visible, infrared and ultraviolet ranges, based on the stimulated emission of atoms and molecules. The word "laser" is composed of the initial ... - COMPTON EFFECT in the Great Soviet Encyclopedia, TSB:
effect, Compton effect, elastic scattering of electromagnetic radiation by free electrons, accompanied by an increase in wavelength; observed when scattering radiation of small wavelengths ... - PHYSICAL KINETICS in the Great Soviet Encyclopedia, TSB:
physical, the theory of nonequilibrium macroscopic processes, that is, processes that occur in systems derived from the state of thermal (thermodynamic) equilibrium. K.F. ...
The conduction of a nerve impulse through the fiber occurs due to the propagation of the depolarization wave through the membrane of the process. Most peripheral nerves in their motor and sensory fibers provide an impulse with a speed of up to 50-60 m / s. The depolarization process itself is rather passive, while the restoration of the resting resting membrane potential and the ability to conduct it is carried out by the functioning of NA / K and Ca pumps. For their work, ATP is necessary, the prerequisite for the formation of which is the presence of segmental blood flow. The cessation of blood supply to the nerve immediately blocks the conduction of a nerve impulse.
According to structural features and functions, nerve fibers are divided into two types: bezmyelinovye and myelin. Bezmyelinovye nerve fibers do not have a myelin sheath. Their diameter is 5-7 microns, the speed of the pulse is 1-2 m / s. Myelin fibers consist of an axial cylinder coated with a myelin sheath formed by Schwann cells. The axial cylinder has a membrane and an oxoplasm. The myelin sheath is 80% lipid and 20% protein. The myelin sheath does not completely cover the axial cylinder, but is interrupted and leaves open sections of the axial cylinder, which are called nodal intercepts (Ranvier intercepts). The length of the sections between intercepts is different and depends on the thickness of the nerve fiber: the thicker it is, the longer the distance between intercepts.
Depending on the speed of the excitation, the nerve fibers are divided into three types: A, B, C. The highest speed of the excitation is possessed by type A fibers, the excitation speed of which reaches 120 m / s, B has a speed of 3 to 14 m / s, C - from 0.5 to 2 m / s.
There are 5 laws of conduction of excitation:
- 1. The nerve must maintain physiological and functional continuity.
- 2. Under natural conditions, the spread of momentum from the cell to the periphery. There is a 2-way impulse conduction.
- 3. Conducting a pulse in isolation, ie myelin-coated fibers do not transmit excitation to neighboring nerve fibers, but only along the nerve.
- 4. The relative tirelessness of the nerve, in contrast to the muscles.
- 5. The speed of the excitation depends on the presence or absence of myelin and the length of the fiber.
- 3. Classification of peripheral nerve damage
Damage happens:
- A) firearms: - direct (bullet, fragmentation)
- -mediated
- -pneumatic damage
- B) non-arched: cut, chipped, bitten, compression, compression-ischemic
Also in the literature, there is a separation of injuries into open (cut, punctured, lacerated, chopped, bruised, crushed wounds) and closed (concussion, bruised, squeezed, stretched, torn and dislocated) injuries of the peripheral nervous system.
Neurotransmitters Are substances that are characterized by the following features:
Accumulate in the presynaptic membrane in sufficient concentration;
Released upon transmission of momentum;
After binding to the postsynaptic membrane, they cause a change in the rate of metabolic processes and the appearance of an electrical impulse;
They have a system for inactivation or a transport system for removing hydrolysis products from the synapse.
Neurotransmitters play an important role in the functioning of nerve tissue, providing synaptic transmission of nerve impulses. Their synthesis occurs in the body of neurons, and accumulation in special vesicles, which gradually move with the participation of systems of neurofilaments and neurotubes to the tips of axons.
Derivatives of amino acids include derivatives of amino acids: taurine, norepinephrine, dopamine, GABA, glycine, acetylcholine, homocysteine \u200b\u200band some others (adrenaline, serotonin, histamine), as well as neuropetides.
Cholinergic synapses
Acetylcholine synthesized from choline and acetyl-CoA. The synthesis of choline requires the amino acids serine and methionine. But, as a rule, a ready-made choline comes from the blood into the nervous tissue. Acetylcholine is involved in the synaptic transmission of a nerve impulse. It accumulates in synaptic vesicles, forming complexes with a negatively charged protein vesiculin (Fig. 22). The transfer of excitation from one cell to another is carried out using a special synaptic mechanism.
Fig. 22. Cholinergic synapse
Synapse is a functional contact of specialized sections of the plasma membranes of two excitable cells. The synapse consists of a presynaptic membrane, a synaptic cleft and a postsynaptic membrane. Membranes at the site of contact have thickenings in the form of plaques - nerve endings. A nerve impulse that has reached a nerve ending is not able to overcome the obstacle that has arisen in front of it - the synaptic cleft. After that, the electrical signal is converted into a chemical one.
The presynaptic membrane contains special channel proteins, similar to the proteins that form the sodium channel in the axon membrane. They also react to the membrane potential, changing their conformation, and form a channel. As a result, Ca 2+ ions pass through the presynaptic membrane along the concentration gradient into the nerve ending. The concentration gradient of Ca 2+ is created by the work of Ca 2+ -dependent ATPase. An increase in the concentration of Ca 2+ inside the nerve ending causes the fusion of the acetylcholine-filled vesicles there. Then, acetylcholine is secreted into the synaptic cleft by exocytosis and attaches to receptor proteins located on the surface of the postsynaptic membrane.
The acetylcholine receptor is a transmembrane oligomeric glycoprotein complex consisting of 6 subunits. The density of receptor proteins in the postsynaptic membrane is very high - about 20,000 molecules per 1 μm 2. The spatial structure of the receptor strictly corresponds to the conformation of the mediator. When interacting with acetylcholine, the receptor protein changes its conformation in such a way that a sodium channel is formed inside it. The cationic selectivity of the channel is ensured by the fact that the channel gates are formed by negatively charged amino acids. T.O. the permeability of the postsynaptic membrane for sodium increases and an impulse (or contraction of muscle fiber) occurs. Depolarization of the postsynaptic membrane causes dissociation of the acetylcholine-protein-receptor complex, and acetylcholine is released into the synaptic cleft. As soon as acetylcholine is in the synaptic cleft, it undergoes rapid hydrolysis in 40 μs under the action of the enzyme acetylcholinesterase for choline and acetyl-CoA.
Irreversible inhibition of acetylcholinesterase causes death. Inhibitors of the enzyme are organophosphorus compounds. Death occurs as a result of respiratory arrest. Reversible acetylcholinesterase inhibitors are used as therapeutic agents, for example, in the treatment of glaucoma and intestinal atony.
Adrenergic synapses(Fig. 23) are found in postganglionic fibers, in the fibers of the sympathetic nervous system, in various parts of the brain. They serve as mediators catecholamines: norepinephrine and dopamine. Catecholamines in the nervous tissue are synthesized according to the general mechanism of tyrosine. A key synthesis enzyme is tyrosine hydroxylase, which is inhibited by end products.
Fig. 23. Adrenergic synapse
Norepinephrine - a mediator in the postganglionic fibers of the sympathetic system and in various parts of the central nervous system.
Dopamine - a mediator of the pathways, the neuron bodies of which are located in the brain. Dopamine is responsible for controlling voluntary movements. Therefore, in violation of dopaminergic transmission, the disease Parkinsonism occurs.
Catecholamines, like acetylcholine, accumulate in synaptic vesicles and also stand out in the synaptic cleft when a nerve impulse arrives. But regulation in the adrenergic receptor is different. There is a special regulatory protein in the presynaptic membrane - achromogranin, which, in response to an increase in the mediator concentration in the synaptic cleft, binds the already released mediator and stops its further exocytosis. There is no enzyme that destroys the mediator in adrenergic synapses. After the momentum transfer, the mediator molecules are pumped by a special transport system by active transport with the participation of ATP back into the presynaptic membrane and are included again in the vesicles. In the presynaptic nerve ending, excess mediator can be inactivated by monoamine oxidase (MAO), as well as catecholamine-O-methyltransferase (COMT) by methylation at the hydroxy group.
Signal transmission in adrenergic synapses occurs with the participation of the adenylate cyclase system. The binding of the mediator to the postsynaptic receptor almost instantly causes an increase in the concentration of cAMP, which leads to rapid phosphorylation of proteins of the postsynaptic membrane. As a result, the generation of nerve impulses of the postsynaptic membrane is inhibited. In some cases, the immediate cause of this is an increase in the permeability of the postsynaptic membrane for potassium, or a decrease in the conductivity for sodium (this condition leads to hyperpolarization).
Taurineformed from the amino acid cysteine. First, sulfur is oxidized in the HS group (the process proceeds in several stages), then decarboxylation occurs. Taurine is an unusual acid in which there is no carboxyl group, and there is a residue of sulfuric acid. Taurine takes part in conducting a nerve impulse in the process of visual perception.
GABA - inhibitory neurotransmitter (about 40% of neurons). GABA increases the permeability of postsynaptic membranes for potassium ions. This leads to a change in membrane potential. GABA inhibits the ban on conducting "unnecessary" information: attention, motor control.
Glycine - auxiliary inhibitory neurotransmitter (less than 1% of neurons). The effects are similar to GABA. Its function is the inhibition of motor neurons.
Glutamic acid - the main exciting mediator (about 40% of neurons). The main function: conducting the main flows of information in the central nervous system (sensory signals, motor commands, memory).
The normal activity of the central nervous system is provided by a delicate balance of glutamic acid and GABA. Violation of this balance (usually in the direction of decreasing inhibition) negatively affects many nervous processes. In case of imbalance, attention deficit hyperactivity disorder of children (ADHD) develops, adult nervousness and anxiety, sleep disturbance, insomnia, and epilepsy increase.
Neuropeptides have in their composition from three to several tens of amino acid residues. They function only in the higher parts of the nervous system. These peptides perform the function of not only neurotransmitters, but also hormones. They transmit information from cell to cell through the circulation system. These include:
Neurohypophysial hormones (vasopressin, liberins, statins) - they are both hormones and meditators;
Gastrointestinal peptides (gastrin, cholecystokinin). Gastrin causes a feeling of hunger, cholecystokinin causes a feeling of fullness, and also stimulates the contraction of the gallbladder and pancreatic function;
Opiate-like peptides (or analgesic peptides). They are formed by reactions of limited proteolysis of the proopiocortin precursor protein. It interacts with the same receptors as opiates (for example, morphine), thereby simulating their effect. Common name - endorphins. They are easily destroyed by proteinases, so their pharmacological effect is negligible;
Peptides of sleep. Their molecular nature has not been established. They cause sleep;
Memory peptides (scotophobin). Accumulated during training to avoid the dark;
Peptides-components of the renin-angiotensin system. Stimulate the center of thirst and the secretion of antidiuretic hormone.
The formation of peptides occurs as a result of limited proteolysis reactions, they are destroyed by the action of proteinases.
Control questions
1. Describe the chemical composition of the brain.
2. What are the features of metabolism in the nervous tissue?
3. List the functions of glutamate in nerve tissue.
4. What is the role of mediators in the transmission of nerve impulses? List the main inhibitory and exciting mediators.
5. What are the differences in the functioning of adrenergic and cholinergic synapses?
6. Give examples of compounds that affect synaptic transmission of nerve impulses.
7. What biochemical changes can be observed in the nervous tissue during mental illness?
8. What are the features of the action of neuropeptides?
Muscle tissue biochemistry
Muscles make up 40-50% of a person’s body weight.
Distinguish three types of muscles:
Striated skeletal muscles (contract at random);
Striated heart muscle (contracting involuntarily);
Smooth muscles (vessels, intestines, uterus) (contract involuntarily).
Striated muscle consists of numerous elongated fibers.
Muscle fiber - multinucleated cell coated with an elastic membrane - sarcolemma. Muscle fiber enters motor nervestransmitting to him a nerve impulse that causes contraction. The length of the fiber in semi-liquid sarcoplasm threadlike formations are located - myofibrils. Sarcomere - a repeating element of myofibrils, limited by the Z-line (Fig. 24). In the middle of the sarcomere is an A-disk, dark in a phase contrast microscope, in the center of which is the M-line, visible by electron microscopy. H-zone occupies the middle part
A-drive. I-discs are light in a phase contrast microscope, and each of them is divided into equal halves by the Z-line. A-disks contain thick myosin and thin actin filaments. Thin filaments begin at the Z-line, pass through the I-disk and are interrupted in the region of the H-zone. Electron microscopy showed that thick filaments are laid in the shape of a hexagon and pass through the entire A-disk. Between thick threads are thin. With muscle contraction, the I-discs practically disappear, and the overlap area between thin and thick threads increases.
Sarcoplasmic reticulum- intracellular membrane system of interconnected flattened vesicles and tubules, which surrounds sarcomeres of myofibrils. On its inner membrane are proteins capable of binding calcium ions.
1. Physiology of nerves and nerve fibers. Types of Nerve Fibers
Physiological properties of nerve fibers:
1) excitability- the ability to come into a state of excitement in response to irritation;
2) conductivity- the ability to transmit nerve excitation in the form of an action potential from the site of irritation along the entire length;
3) refractoriness(stability) - the ability to temporarily sharply reduce the excitability in the process of excitation.
Nerve tissue has the shortest refractory period. Significance of refractoriness - to protect tissue from overexcitation, provides a response to a biologically significant stimulus;
4) lability- the ability to respond to irritation at a certain rate. Lability is characterized by the maximum number of excitation pulses for a certain period of time (1 s) in exact accordance with the rhythm of the applied stimuli.
Nerve fibers are not independent structural elements of the nervous tissue, they are a complex formation, including the following elements:
1) processes of nerve cells - axial cylinders;
2) glial cells;
3) connective tissue (basal) plate.
The main function of nerve fibers is to conduct nerve impulses. The processes of nerve cells conduct the nerve impulses themselves, and glial cells contribute to this conduction. According to structural features and functions, nerve fibers are divided into two types: bezmyelinovye and myelin.
Bezmyelinovye nerve fibers do not have a myelin sheath. Their diameter is 5–7 μm, the velocity of the pulse is 1–2 m / s. Myelin fibers consist of an axial cylinder coated with a myelin sheath formed by Schwann cells. The axial cylinder has a membrane and an oxoplasm. The myelin sheath consists of 80% lipids with high ohmic resistance, and 20% of protein. The myelin sheath does not completely cover the axial cylinder, but is interrupted and leaves open sections of the axial cylinder, which are called nodal intercepts (Ranvier intercepts). The length of the sections between intercepts is different and depends on the thickness of the nerve fiber: the thicker it is, the longer the distance between intercepts. With a diameter of 12–20 μm, the speed of excitation is 70–120 m / s.
Depending on the speed of the excitation, the nerve fibers are divided into three types: A, B, C.
Type A fibers have the highest excitation rate, the excitation rate of which reaches 120 m / s, B has a speed of 3 to 14 m / s, C - from 0.5 to 2 m / s.
The concepts of “nerve fiber” and “nerve” should not be confused. Nerve- a complex formation consisting of a nerve fiber (myelin or bezmyelinovoy), loose fibrous connective tissue that forms the sheath of the nerve.
2. Mechanisms for conducting excitation along the nerve fiber. Laws of conducting excitation along the nerve fiber
The mechanism for conducting excitation along nerve fibers depends on their type. There are two types of nerve fibers: myelin and non-myelin.
The metabolism processes in myelin-free fibers do not provide quick compensation for energy consumption. The spread of excitation will go with a gradual attenuation - with decrement. Decremental excitation behavior is characteristic of a low-organized nervous system. Excitation propagates due to small circular currents that occur inside the fiber or into the surrounding fluid. A potential difference arises between the excited and unexcited sections, which contributes to the occurrence of circular currents. Current will propagate from “+” charge to “-”. At the exit point of the circular current, the permeability of the plasma membrane for Na ions increases, resulting in depolarization of the membrane. A potential difference arises again between the newly excited section and the neighboring unexcited, which leads to the appearance of circular currents. Excitation gradually covers adjacent sections of the axial cylinder and so extends to the end of the axon.
In myelin fibers, due to the perfection of metabolism, excitation passes without decay, without decrement. Due to the large radius of the nerve fiber due to the myelin sheath, an electric current can enter and exit the fiber only in the interception region. When irritation occurs, depolarization occurs in the area of \u200b\u200binterception A, the neighboring interception B is polarized at this time. Between the intercepts, a potential difference occurs, and circular currents appear. Due to the circular currents, other interceptions are excited, while the excitation propagates saltatory, spasmodically from one interception to another. The saltator mode of propagation of excitation is economical, and the speed of propagation of excitation is much higher (70-120 m / s) than along myelin-free nerve fibers (0.5–2 m / s).
There are three laws of nerve fiber stimulation.
The law of anatomical and physiological integrity.
Conducting impulses along the nerve fiber is possible only if its integrity is not broken. If the physiological properties of the nerve fiber are violated by cooling, using various drugs, squeezing, as well as cuts and damage to the anatomical integrity, it will be impossible to conduct a nerve impulse along it.
The law of isolated excitation.
There are a number of features of the spread of excitation in peripheral, pulp and pulmonary nerve fibers.
In peripheral nerve fibers, excitation is transmitted only along the nerve fiber, but is not transmitted to neighboring ones located in the same nerve trunk.
In the pulmonary nerve fibers, the role of the insulator is performed by the myelin sheath. Due to myelin, the resistivity increases and the electric capacity of the shell decreases.
In serene nerve fibers, excitation is transmitted in isolation. This is because the resistance of the fluid that fills the intercellular fissures is significantly lower than the resistance of the membrane of nerve fibers. Therefore, the current arising between the depolarized and non-polarized regions passes through the intercellular fissures and does not enter the neighboring nerve fibers.
The law of two-way excitation.
The nerve fiber conducts nerve impulses in two directions - centripetal and centrifugal.
In a living organism, excitation is carried out in only one direction. Bilateral conduction of nerve fiber in the body is limited by the place where the impulse occurs and by the valve property of synapses, which consists in the possibility of excitation in only one direction.