The structure of the reflex arc diagram. Conditioned reflexes

Each of us at least once in our lives tested the knee jerk. In many cases, the doctor sees and receives a response from the knee - extension of the limb. But there are situations when the knee jerk is absent. In order to understand the reason for the absence, you need to understand what kind of reflex it is and how it works.

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Anatomical features

The knee jerk is a response of the body that occurs when the femoral muscle is slightly stretched. Muscle contraction occurs as a result of a slight blow to the patella, under which the tendon is located. Under an external factor, the tendons stretch and actuate the extensor muscle. This reflex is very important for diagnosing many diseases. But it is impossible to perform this procedure without a reflex arc.

The activity of the body depends on the reaction to irritating receptors that come from the central nervous system. It is this structural basis of the reflex that is the reflex arc. Reflex arc - the path of the incoming signal from the receptor to the corresponding organ that reacted to it. In another way, it is also called the nervous arc. This name is explained by the fact that the knee reflex is performed due to nerve impulses that come through a certain path.

The arc is located in the cells of the spinal cord, which, after excitation, are able to transmit an impulse to the muscles. The scheme with the designations of the reflex arc is not difficult, and it is possible to understand the functioning of the process with the help of a photo. The neural arch consists of the following components:

• Links (central, efferent, afferent);
• Receptors;
• Effector (an organ that can change during a reflex).

There are two types of reflex arcs: simple and complex. Simple or monosynaptic reflex arcs consist of 2 neurons (efferent and afferent) and a synapse. They have the following features:

• Short duration of the reflex;
• Very close effector and receptor;
• The arc is two-neuron;
• Muscles have a single muscle contraction;
• Group A neurons.

Complex or polysynaptic arcs contain three neurons (effector, receptor, or a pair of intercalary neurons). Features of the complex neural arch:

• The arc is three-neuron;
• Nerve fibers of groups B and C;
• The receptor and effector are not close;
• Muscle contraction according to the type of tetanus.

Role and functions in the body

In simple words, the neural arch is the path along which the impulse that originated from the receptor to the organ or muscle passes. According to this factor, the reflex arc is designed to transmit nerve impulses. The impulse transmission scheme is based on the fact that a signal is transmitted from the receptor to sensitive neurons. Further, the excitatory reaction is transmitted to the cells of the gray matter of the spinal cord. As a result, the motor cells contract, and the leg may twitch or rise.

The blow acts as an external irritant on the nervous system. Thanks to the connection between the spinal cord, sensory system, motor neurons, the process takes place. To visually present the description and understand the path of the nerve impulse will help the drawing, which depicts the neural arch.

Arc receptors receive signals from the stimulus, and as a result of feedback are excited on them. The links perform the transmission of an impulse to a specific organ. They are: central, efferent and afferent. An effector is an organ that responds to the action of a receptor.

According to these components of the arc, it will perform the following functions:

• Transmits a signal to the muscle of the calf region;
• From neurons it sends an impulse to motor muscles;
• Depending on the stimulus, it generates a neural impulse that transmits to the effector (organ);
• Affects the movement of the limb, muscle contraction of the leg.

How to define it?

In order to correctly determine the presence of a knee jerk, you need to perform the following steps:

1. The patient is placed in such a position on a chair so that he can freely cross his legs or that the limbs do not touch the floor.
2. The doctor then strikes the kneecap with a neurological hammer, causing it to respond. These measures will help the specialist determine the reflex arc of the knee.

But another diagnostic method is possible to determine the neural arch of the knee joint. The patient lies on his back, while bending his legs at an angle so that they clearly and firmly rest their feet on the surface of the couch. Strike with a hammer on the tendon. This method contributes to the assessment and analysis of the patellar (knee) reflex arc.

Absence and reduction of the arc

The roots of the gray matter can come into contact with other neurons. After that, they come into contact with the central neurons, forming the links of the pathway. In this case, the reflex arc may fail, as a result of the attachment of neurons to the spinal reflex. Rapid excitations of the nervous system can be transmitted to the cerebral cortex and provoke new reflexes. As a result, irritation may return to the peripheral neuron, resulting in a complete absence of the knee jerk (areflexia).

The reflex can decrease through intoxication of the body, infection, epileptic seizure. The arch of the knee is at rest due to the pathology of the nervous system, the patient's personal characteristics. Pathological changes in the nervous system, manifested in the knee jerk, may have the following character: hyporeflexia, hyperreflexia and areflexia.

Hyporeflexia

• The irritant reaction in this pathology will decrease. A characteristic feature of this phenomenon is that the knee reacts poorly to the stimulus. A deviation occurs due to a violation of the conductivity and integrity of the reflex arc during the transmission of an impulse through neurons.
• The absence of a reflex may indicate a disease of the centers of the brain. Loss of body weight, infection leads to the depletion of neurons and improper functioning of cells. The reaction disappears after the application of a tourniquet, anesthesia.

hyperreflexia

• The slightest impact on the limb leads to an increased knee jerk. Very often observed in the spinal cord. Since these structures block impulses, in response to irritation.
• Occurs in individuals of a neurotic type, with neuritis, plexitis, sciatica. In addition, pathological movements, with a rapid contraction of the muscles of the stretched tendon, act as an increase in the reflex. They often affect the foot and kneecap.

Areflexia

• It is a special type of pathology of the knee reflex, manifested as a result of the presence of a serious disease of the central nervous system. With such a pathological process, there is generally no irritable reaction to the imitating factor.
• Areflexia occurs in the case of neuritis, poliomyelitis, polyneuritis, tabes. Damage to the conductive neuron or motor neuron, sensory fibers is observed. The reflex functions associated with damage to the nerve sections of the brain and spinal cord decrease, and muscle reflexes fade.

A highly qualified specialist will be able to determine the deviation from the norm and the degree of pathology using the methods of research, examination, and additional measures.

Video "Inspection of the knee reflex"

How to conduct a neurological examination by a specialist you can see in the following video.

The reflex arc consists of:

- receptors - perceiving irritation.

- sensitive (centripetal, afferent) nerve fiber that transmits excitation to the center

- the nerve center, where the switching of excitation from sensory neurons to motor neurons occurs

- motor (centrifugal, efferent) nerve fiber that carries excitation from the central nervous system to the working organ

- effector - a working organ that performs an effect, a reaction in response to receptor irritation.

Receptors and receptive fields

Receptor- cell perceiving irritation.

receptive field- this is the anatomical region, when irritated, this reflex is caused.

The receptive fields of primary sensory receptors are organized in the most simple way. For example, the tactile or nociceptive receptive field of the skin surface is a branching of a single sensory fiber.

Receptors located in different parts of the receptive field have different sensitivity to adequate stimulation. A highly sensitive zone is usually located in the center of the receptive field, and sensitivity decreases closer to the periphery of the receptive field.

The receptive fields of secondary sensory receptors are organized in a similar way. The difference is that the branches of the afferent fiber do not end freely, but have synaptic contacts with sensitive receptor cells. Gustatory, vestibular, acoustic receptive fields are organized in this way.

overlapping receptive fields. One and the same area of ​​the sensory surface (for example, skin or retina) is innervated by several sensory nerve fibers, which, with their branchings, overlap the receptive fields of individual afferent nerves.

Due to the overlap of receptive fields, the total sensory surface of the body increases.

Classification of reflexes.

By type of education:

Conditional (acquired) - respond to the name, saliva from the dog into the light.

Unconditional (congenital) - blinking swallowing, knee.

By location receptors:

Exteroceptive (skin, visual, auditory, olfactory)

Interoceptive (from receptors of internal organs)

Proprioceptive (from receptors in muscles, tendons, joints)

For effectors:

Somatic, or motor, (skeletal muscle reflexes);

Vegetative internal organs - digestive, cardiovascular, excretory, secretory, etc.

By biological origin:

Defensive, or protective (response to tactile pain division)

Digestive (irritating receptors in the oral cavity.)

Sexual (hormones in the blood)

Approximate (turn of the head, body)

Motor

Posotonic (supporting body postures)

By the number of synapses:

Monosynaptic, the arcs of which consist of afferent and efferent neurons (for example, knee).

Polysynaptic, the arcs of which also contain 1 or more intermediate neurons and have 2 or more synaptic switches. (somatic and vegetative references).

Disynaptic (2 synapses, 3 neurons).

By the nature of the response:

Motor \ motor (muscle contractions)

Secretory (secretory gland secretion)

Vasomotor (expansion and narrowing of blood vessels)

Cardiac (change. The work of the heart muscle.)

According to duration:

phasic (fast) hand withdrawal

tonic (slow) posture maintenance

According to the location of the nerve center:

Spinal (SM neurons are involved) - pulling the Hand away from the hot segments 2-4, knee jerk.

reflexes in the brain

Bulbar (medulla oblongata) - closing of the eyelids when touching. to the cornea.

Mesencephalic (middle m) - vision landmark.

Diencephalic (midbrain) - sense of smell

Cortical (bark BP GM) - conditional. ref.

Properties of nerve centers.

1. Unilateral propagation of excitation.

Excitation is transmitted from the afferent to the efferent neuron (reason: the structure of the synapse).

Slowing down the transfer of excitation.

Conditioner The presence of many synapses also depends on the strength of the irritant (summation) and on the physical state. CNS (fatigue).

3.Summation summation of effects, below threshold stimuli.

Temporary: ref. From prev. Imp-sa has not yet passed, but a trace. Already arrived.

Spatial: mixing several. Backwater They are owl conditioned. Images. Ref.

Facilitation and occlusion center.

The center of relief - occurs under the action of the optimal stimulus (max response) - appeared. Relief Center.

Under the action of min irr. (reduced otv. Rektsiya) there was an occlusion.

Assimilation and transformation of the rhythm of excitation.

Transformation - a change in the frequency of a nerve impulse when passing through the nerve center. The frequency can be increased or decreased.

Assimilation (dance, daily routine)

Consequence

The delay in the end of the response after the cessation of the stimulus. Associated with the circulatory nerve. Imp. Closed Circuits of neurons.

short term (fractions of a second)

long (seconds)

Rhythmic activity of nerve centers.

An increase or decrease in the frequency of nerve impulses associated with the properties of the synapse and the integrative duration of neurons.

8. Plasticity of nerve centers.

The ability to rebuild the functionality of a property for more effective regulation of functions, the implementation of new reflexes that were not previously characteristic of this center, or the restoration of func. At the heart of the layer of synps is a change in the mall-th str-ry.

Changes in excitability under the influence of chemicals.

High sensitivity to real difference.

Fatigue of the nerve centers.

Associated with high synapse fatigue. Reduce feelings. Receptors.

General principles of the coordination activity of the central nervous system.

Braking- special ner. percent manifested in the reduction or complete disappearance of resp. reactions.

Convergence principle

Convergence is the convergence of impulses coming through different afferent pathways in any one central neuron or nerve center.

2 . The principle of convergence is closely related to the principle common final path open Sherrinkton. Many different stimuli can excite the same motor neuron and the same motor response. This principle is due to the unequal number of afferent and efferent pathways.

Principle of divergence

This is the contact of one neuron with many others.

Irradiation and concentration of excitation.

The spread of the excitation process to other nerve centers is called irradiation (electoral- in one direction , generalized- extensive).

After some time, irradiation is replaced by the phenomenon of excitation concentration in the same initial point of the CNS.

The process of irradiation plays a positive (formation of new conditioned reflexes) and negative (violation of the subtle relationships that have developed between the processes of excitation and inhibition, which leads to a disorder of motor activity) roles.

The principle of reciprocity (slows down)

Excitation of some cells causes inhibition of others through the intercalary neuron.

Dominant principle

Ukhtomsky formulated the principle of dominance as a working principle of the activity of nerve centers. term dominant denotes the dominant focus of excitation of the central nervous system, which determines the current activity of the body.

Principles of the dominant focus :

Increased excitability of nerve centers;

Persistence of excitation of excitation over time;

The ability to summation of extraneous stimuli;

Inertia (the ability to maintain arousal for a long time after the end of the action of irritation); the ability to cause conjugate inhibitions.

simple reflex arc consists of at least two neurons, one of which is associated with some sensitive surface (for example, skin), and the other, with the help of its neurite, ends in a muscle (or gland). When a sensitive surface is stimulated, the excitation goes along the neuron associated with it in a centripetal direction (centripetally) to reflex center where the junction (synapse) of both neurons is located. Here, the excitation passes to another neuron and goes already centrifugally (centrifugally) to muscle or gland. As a result, there is a contraction of the muscle or a change in the secretion of the gland. Often a simple reflex arc includes a third intercalary neuron, which serves as a relay station with sensory pathway to motor.

In addition to a simple (three-term) reflex arc, there are complex multineuron reflex arcs passing through different levels of the brain, including its cortex. In higher animals and humans, against the background of simple and complex reflexes, also with the help of neurons, temporary reflex connections of a higher order are formed, known as the name of conditioned reflexes(I.P. Pavlov).

Thus, the entire nervous system can be imagined as functionally consisting of three kinds of elements.

1. receptor (receiver) transforming the energy of external irritation into a nervous process; it is associated with an afferent (centripetal, or receptor) neuron, which propagates the initiated excitation (nerve impulse) to the center; analysis begins with this phenomenon (I. P. Pavlov).

2. Conductor (conductor), an intercalary, or associative, neuron that closes, i.e., switches excitation from a centripetal neuron to a centrifugal one. This phenomenon is a synthesis, which represents, “obviously, the phenomenon of nervous closure” (IP Pavlov). Therefore, IP Pavlov calls this neuron a contactor, a circuit breaker.

3. Efferent (centrifugal) neuron, carrying out a response (motor or secretory) due to the conduction of nervous excitation from the center to the periphery, to the effector. Effector- this is the nerve ending of an efferent neuron that transmits a nerve impulse to the working organ (muscle, gland). Therefore, this neuron is also called effector. Receptors are excited from three sensitive surfaces, or receptor fields, of the body: 1) from the outer, skin, surface of the body (exteroceptive field) through the genetically related sense organs that receive irritation from the external environment; 2) from the inner surface of the body (interoceptive field), which receives irritation mainly from chemicals entering the cavities of the viscera, and 3) from the thickness of the walls of the body itself (proprioceptive field), which contain bones, muscles and other organs that produce irritations perceived by special receptors. The receptors from these fields are connected with afferent neurons, which reach the center and switch there, through a sometimes very complex system of conductors, to various efferent conductors; the latter, connecting with the working organs, give one or another effect.

A reflex arc is a chain of neurons from a peripheral receptor through the central nervous system to a peripheral effector. The elements of the reflex arc are a peripheral receptor, an afferent pathway, one or more interneurons, an efferent pathway, and an effector.

All receptors are involved in certain reflexes, so that their afferent fibers serve as the afferent path of the corresponding reflex arc. The number of interneurons is always greater than one, except for the monosynaptic stretch reflex. The efferent pathway is represented by either motor axons or postganglionic fibers of the autonomic nervous system, and the effectors are skeletal muscles and smooth muscles, the heart, and glands.

The time from the beginning of the stimulus to the response of the effector is called the reflex time. In most cases, it is determined mainly by the conduction time in the afferent and efferent pathways and in the central part of the reflex arc, to which should be added the time of transformation of the stimulus in the receptor into a propagating impulse, the time of transmission through synapses in the central nervous system (synaptic delay), the time of transmission from efferent pathway to effector and effector activation time.

Reflex arcs are divided into several types

1. Monosynaptic reflex arcs - only one synapse, located in the central nervous system, participates in such an arc. Such reflexes are quite common in all vertebrates and are involved in the regulation of muscle tone and posture (for example, the knee jerk). In these arcs, neurons do not reach the brain, and reflex acts are carried out without its participation, since they are stereotyped and do not require thought or conscious decision. They are economical in terms of the number of central neurons involved and dispense with the intervention of the brain.

2. Polysynaptic spinal reflex arcs - they involve at least two synapses located in the central nervous system, since a third neuron is included in the arc - an intercalary, or intermediate neuron. Here there are synapses between the sensory neuron and the interneuron and between the intercalary and motor neurons. Such reflex arcs allow the body to carry out automatic involuntary reactions necessary to adapt to changes in the external environment (for example, the pupillary reflex or maintaining balance when moving) and to changes in the body itself (regulation of respiratory rate, blood pressure, etc.).

3. Polysynaptic reflex arcs involving both the spinal cord and the brain - in this type of reflex arcs there is a synapse in the spinal cord between the sensory neuron and the neuron that sends impulses to the brain.

Reflexes can be classified according to various criteria. So, depending on the level of arc closing, i.e. according to the location of the reflex center, the reflexes are divided into spinal (the reflex closes in the spinal cord), bulbar (the reflex center is the medulla oblongata), mesencephalic (the reflex arc is closed in the midbrain), diencephalic and cortical reflex centers are located in the telencephalon and cortex of large hemispheres, respectively.

According to the effector feature, they are somatic, when the efferent path of the reflex provides motor innervation of the skeletal muscles, and vegetative, when internal organs are the effectors.

Depending on the type of irritated receptors, reflexes are divided into exteroceptive (if the receptor perceives information from the external environment), proprioceptive (the reflex arc starts from the receptors of the musculoskeletal apparatus) and interoceptive (from the receptors of internal organs).

Interoceptive reflexes, in turn, are divided into viscero-visceral (a reflex arc connects two internal organs), viscero-muscular (receptors are located on the muscular-tendon apparatus, the effector is an internal organ) and viscero-cutaneous (receptors are localized in the skin, working organs - viscera).

According to Pavlov, reflexes are divided into conditioned (developed during life, specific for each individual) and unconditioned (congenital, species-specific: food, sexual, defensive-motor, homeostatic, etc.).

Regardless of the type of reflex, its reflex arc contains a receptor, an afferent pathway, a nerve center, an efferent pathway, a working organ, and feedback. The exception is axon reflexes, the reflex arc of which is located within one neuron: sensory processes generate centripetal impulses, which, passing through the body of the neuron, propagate along the axon to the central nervous system, and along the branch of the axon, the impulses reach the effector. Such reflexes are attributed to the functioning of the metasympathetic nervous system; through them, for example, mechanisms for regulating vascular tone and the activity of skin glands are carried out.

The function of perceiving irritation and converting it into excitation energy is performed by receptors of reflex arcs. The receptor energy of excitation has the character of a local response, which is important in the gradation of excitation by strength.

Based on the structure and origin of the receptors, they can be divided into primary sensory, secondary sensory and free nerve endings. In the former, the neuron itself acts as a receptor (it develops from the neuroepithelium); there are no intermediary structures between the stimulus and the first afferent neuron. The local response of the primary sensory receptors - the receptor potential - is also a generator potential, i.e. inducing an action potential across the membrane of the afferent fiber. Primary sensory receptors include visual, olfactory, chemo- and baroreceptors of the cardiovascular system.

Secondary-sensing cells are special structures of non-nervous origin that interact with the dendrites of pseudo-unipolar sensory cells with the help of synaptic neuroreceptor contacts. The receptor potential arising under the action of a stimulus in secondary-sensing cells is not a generator and does not cause the appearance of an action potential on the membrane of the afferent fiber. The excitatory postsynaptic potential arises only through the mechanism of release of the mediator by the receptor cell. The gradation of the strength of the stimulus is carried out through the excretion of various amounts of the mediator (the more the mediator is released, the stronger the stimulus).

Secondary sensory cells include auditory, vestibular, carotid, tactile and other receptors. Sometimes, due to the peculiarities of functioning, this group includes photoreceptors, which, from an anatomical point of view and due to their origin from the neuroepithelium, are secondary-sensing.

Free nerve endings are branching dendrites of pseudo-unipolar sensory cells and are localized in almost all tissues of the human body.

According to the energy nature of the stimulus to which the receptor responds, they are divided into mechanoreceptors (tactile, baroreceptors, volumoreceptors, auditory, vestibular; they, as a rule, perceive mechanical irritation with the help of cell outgrowths), chemoreceptors (olfactory), chemoreceptors of blood vessels, the central nervous system , photoreceptors (perceive irritation through rod- and cone-shaped outgrowths of the cell), thermoreceptors (react to the “warm-cold” change - Rufini bodies and Krause flasks of mucous membranes) and nociceptors (non-encapsulated pain endings).

The post-receptor formation of reflex arcs is an afferent pathway formed by a pseudo-unipolar sensory neuron, the body of which lies in the spinal ganglion, and the axons form the posterior roots of the spinal cord. The function of the afferent pathway is to conduct information to the central link, moreover, at this stage information is encoded. For these purposes, in the body of vertebrates, a binary code is used, made up of bursts (volleys) of impulses and gaps between them. There are two main types of coding: frequency and spatial.

The first is the formation of a different number of impulses in a burst, a different number of bursts, their duration and the duration of the breaks between them, depending on the strength of the stimulation applied to the receptor. Spatial coding carries out the gradation of the strength of the stimulus, involving a different number of nerve fibers, along which excitation is simultaneously carried out.

The composition of the afferent pathway includes mainly A-b, A-c and A-d fibers.

Having passed through the fibers, the nerve impulse enters the reflex center, which in the anatomical sense is a collection of neurons located at a certain level of the central nervous system and taking part in the formation of this reflex. The function of the reflex center is to analyze and synthesize information, as well as to switch information from the afferent to the efferent path.

Depending on the department of the nervous system (somatic and autonomic), reflexes, the center of which is located in the spinal cord, differ in the localization of intercalary neurons. So, for the somatic nervous system, the reflex center is located in the intermediate zone between the anterior and posterior horns of the spinal cord. The reflex center of the autonomic nervous system (the bodies of intercalary neurons) lies in the posterior horns. The somatic and autonomic parts of the nervous system also differ in the localization of efferent neurons. The bodies of the motor neurons of the somatic nervous system lie in the anterior horns of the spinal cord, the bodies of the preganglionic neurons of the autonomous system lie at the level of the middle horns.

The axons of both cell types form the efferent path of the reflex arc. In the somatic nervous system, it is continuous, it is made up of fibers of the A-b type. The only exceptions are A-g fibers, which conduct excitation from the cells of the spinal cord to the intrafusal fibers of the muscle spindles. The efferent path of the autonomic nervous system is interrupted in the autonomic ganglion, located either intramurally (parasympathetic part) or near the spinal cord (separately or in the sympathetic trunk - sympathetic part). The preganglionic fiber belongs to the B-fibers, the postganglionic fiber belongs to the C group.

The working organ for the somatic part of the nervous system is a striated skeletal muscle, in the vegetative arc the effector is a gland or a muscle (smooth or striated cardiac). Between the efferent pathway and the working organ there is a chemical myoneural or neurosecretory synapse.

The reflex arc closes in a ring due to reverse afferentation - the flow of impulses from the effector receptors back to the reflex center. Feedback function - signaling to the central nervous system about the performed action. If it is not performed enough, the nerve center is excited - the reflex continues. Also, due to the reverse afferentation, the control of peripheral activity of the central nervous system is carried out.

Distinguish between negative and positive feedback. The first, when performing a certain function, launches a mechanism that inhibits this function. Positive feedback consists of further stimulation of a function that is already being performed or inhibition of a function that is already depressed. Positive reverse afferentation is rare, as it brings the biological system into an unstable position.

Simple (monosynaptic) reflex arcs consist of only two neurons (afferent and efferent) and differ only in proprioceptive reflexes. The remaining arcs include all of the above components.

Physiological properties and functional significance of nerve fibers

Nerve fibers have the highest excitability, the highest rate of conduction of excitation, the shortest refractory period, and high lability. This is ensured by a high level of metabolic processes and a low membrane potential.

Function: conduction of nerve impulses from receptors to the central nervous system and vice versa.

Structural features and types of nerve fibers

The nerve fiber - the axon - is covered with a cell membrane.

There are 2 types of nerve fibers:

Unmyelinated nerve fibers - one layer of Schwann cells, between them - slit-like spaces. The cell membrane is in contact with the environment throughout. When irritation is applied, excitation occurs at the site of action of the stimulus. Unmyelinated nerve fibers have electrogenic properties (the ability to generate nerve impulses) throughout.

Myelinated nerve fibers - covered with layers of Schwann cells, which in places form nodes of Ranvier (areas without myelin) every 1 mm. The duration of the interception of Ranvier is 1 µm. The myelin sheath performs trophic and insulating functions (high resistance). The areas covered with myelin do not have electrogenic properties. They have the interceptions of Ranvier. Excitation occurs in the interception of Ranvier closest to the site of action of the stimulus. In the intercepts of Ranvier, there is a high density of Na-channels, therefore, in each interception of Ranvier, an increase in nerve impulses occurs.

Interceptions of Ranvier act as repeaters (generate and amplify nerve impulses).

The mechanism of conduction of excitation along the nerve fiber

1885 - L. German - circular currents arise between the excited and unexcited sections of the nerve fiber.

Under the action of an irritant, there is a potential difference between the outer and inner surfaces of the tissue (areas that carry different charges). Between these areas, an electric current arises (the movement of Na + ions). Inside the nerve fiber, a current arises from the positive pole to the negative pole, i.e., the current is directed from the excited area to the unexcited one. This current exits through the unexcited region and causes it to recharge. On the outer surface of the nerve fiber, the current flows from the unexcited area to the excited area. This current does not change the state of the excited area, since it is in a state of refractoriness.

Evidence of the presence of circular currents: the nerve fiber is placed in a NaCl solution and the speed of excitation is recorded. Then the nerve fiber is placed in oil (resistance increases) - the conduction speed decreases by 30%. After that, the nerve fiber is left in the air - the rate of excitation is reduced by 50%.

Features of the conduction of excitation along myelinated and unmyelinated nerve fibers:

myelin fibers - have a sheath with high resistance, electrogenic properties only in the nodes of Ranvier. Under the action of the stimulus, excitation occurs in the nearest intercept of Ranvier. Neighbor intercept in polarization state. The resulting current causes depolarization of the adjacent intercept. The nodes of Ranvier have a high density of Na-channels, therefore, in each next node, a slightly larger (in amplitude) action potential arises, due to this, the excitation propagates without a decrement and can jump over several nodes. This is Tasaki's saltatory theory. The proof of the theory is that drugs were injected into the nerve fiber that block several intercepts, but the conduction of excitation was recorded after that. This is a highly reliable and profitable method, since minor damage is eliminated, the speed of excitation is increased, and energy costs are reduced;

non-myelinated fibers - the surface has electrogenic properties throughout. Therefore, small circular currents occur at a distance of a few micrometers. The excitation has the form of a constantly traveling wave.

This method is less profitable: high energy costs (for the operation of the Na-K pump), a lower rate of excitation.

Classification of nerve fibers

Nerve fibers are classified according to:

the duration of the action potential;

structure (diameter) of the fiber;

speed of excitation.

The following groups of nerve fibers are distinguished:

group A (alpha, beta, gamma, delta) - the shortest action potential, the thickest myelin sheath, the highest rate of excitation;

group B - the myelin sheath is less pronounced;

Group C - no myelin sheath.

Morphological differences between dendrites and axons

1. An individual neuron has several dendrites, an axon is always one.

2. Dendrites are always shorter than the axon. If the size of the dendrites does not exceed 1.5-2 mm, then the axons can reach 1 m or more.

3. Dendrites smoothly move away from the cell body and gradually have a constant diameter over a considerable distance.

4. Dendrites usually branch at an acute angle, and the branches are directed away from the cell. Axons give off collaterals most often at right angles; the orientation of collaterals is not directly related to the position of the cell body.

5. The pattern of dendritic branching in cells of the same type is more constant than the branching of the axon of these cells.

6. The dendrites of mature neurons are covered with dendritic spines, which are absent on the soma and the initial part of the dendritic trunks. Axons do not have spines.

7. Dendrites never have a pulpy shell. Axons are often surrounded by myelin.

8. Dendrites have a more regular spatial organization of microtubules, axons are dominated by neurofilaments and microtubules are less ordered

9. In dendrites, especially in their proximal parts, there are endoplasmic reticulum and ribosomes, which are not in axons.

10. The surface of dendrites in most cases is in contact with synoptic plaques and has active zones with postsynaptic specialization.

The structure of the dendrites

If there is a relatively large literature on the geometry of dendrites, the length of their branches, and orientation, then there is only scattered information about the internal structure, about the structure of the individual components of their cytoplasm. This information became possible only with the introduction of electron microscopic studies into neurohistology.

The main characteristic features of the dendrite, which distinguish it on electron microscopic sections:

1) lack of myelin sheath,

the presence of the correct system of microtubules,

3) the presence of active zones of synapses on them with a clearly expressed electron density of the cytoplasm of the dendrite,

4) departure from the common trunk of the dendrite of the spines,

5) specially organized zones of branch nodes,

6) inclusion of ribosomes,

7) the presence of granular and non-granular endoplasmic reticulum in the proximal areas.

The most notable feature of the dendritic cytoplasm is the presence of numerous microtubules. They are well identified both in transverse sections and in longitudinal sections. Starting from the proximal section of the dendrite, microtubules run parallel to the long axis of the dendrite to its distal branches. Microtubules follow in the dendrite parallel to each other, without connecting or intersecting with each other. In cross sections, it can be seen that the distances between the individual tubules are constant. Individual dendritic tubules extend over fairly long distances, often following curves that may be along the course of the dendrites. The number of tubules is relatively constant per unit area of ​​the dendrite cross section and is approximately 100 per 1 µm. This number is typical for any dendrites taken from different parts of the central and peripheral nervous system in different animal species.

The function of microtubules is the transport of substances along the processes of nerve cells.

When microtubules are destroyed, the transport of substances in the dendrite can be disrupted, and, thus, the final sections of the processes are deprived of the influx of nutrients and energy from the cell body. Dendrites, in order to maintain the structure of synaptic contacts under extreme conditions and thereby ensure the function of interneuronal interaction, make up for the deficiency of nutrients at the expense of structures adjacent to them (synaptic plaques, myelin multilayer sheath of soft fibers, and fragments of glial cells).

If the action of the pathogenic factor is eliminated in a timely manner, the dendrites restore the structure and the correct spatial organization of microtubules, thereby restoring the substance transport system, which is inherent in the normal brain. If the strength and duration of the pathogenic factor are significant, then the phenomena of endocytosis, instead of their adaptive function, can become fatal for dendrites, since phagocytosed fragments cannot be utilized and, accumulating in the cytoplasm of dendrites, will lead to its irreversible damage.

Violation in the organization of microtubules leads to a sharp change in the behavior of animals. In animals in which microtubules in dendrites were destroyed in the experiment, disorganization of complex forms of behavior was observed, while simple conditioned reflexes were preserved. In humans, this can lead to serious disturbances in higher nervous activity.

The fact that dendrites are the most sensitive locus to the action of a pathological agent in mental illness is evidenced by some works by American scientists. It turned out that in senile dementia (cyanotic dementia) and Alzheimer's disease, brain preparations processed by the Golgi method do not reveal the processes of nerve cells. The trunks of the dendrites seem to be burnt and charred. The non-detection of these processes on histological preparations of the brain is probably also associated with a violation of the system of microtubules and neurofilaments in these processes.

Found in dendrites. They follow parallel to the long axis of the dendrite, they can lie separately or be collected in bundles, but they are not strictly located in the cytoplasm. Probably, together with microtubules, they can be the equivalent of neurofibrils.

All CNS dendrites are characterized by an increase in surface due to multiple dichotomous division. In this case, special expansion sites or branch nodes are formed in the division zones.

Normal analysis shows that at the branch node, to which two dendritic branches approach, each carrying its own signal, the following operations can be carried out. Through the branch node into the common trunk and further to the body of the neuron pass:

or a signal from one branch,

or just from another

or the result of the interaction of two signals,

or the signals cancel each other out.

The cytoplasm of the branch node contains almost all the components that are characteristic of the body of a nerve cell, and the sections differ sharply in their structure from the cytoplasm of the common dendritic trunk and branches obtained during division. The branch nodes contain an increased number of mitochondria, a granular and smooth reticulum, clusters of single ribosomes and ribosomes assembled into rosettes are visible. These components (granular and smooth reticulum, ribosomes) are directly involved in protein synthesis. The accumulation of mitochondria in these places indicates the intensity of oxidative processes.

Functions of dendrites

I would like to note that the main difficulties that a researcher encounters when studying the function of dendrites is the lack of information about the properties of the dendrite membrane (as opposed to the membrane of the neuron body) due to the impossibility of introducing a microelectrode into the dendrite.

Assessing the overall geometry of the dendrites, the distribution of synapses, and the special structure of the cytoplasm in the places of dendritic branching, one can speak of special neuron loci with their own function. The simplest thing that could be attributed to dendritic sites at branching sites is a trophic function.

From the foregoing, it follows that the cytoplasm of dendrites contains many ultrastructural components capable of providing their important functions. There are certain loci in the dendrite, where its work has its own characteristics.

The main purpose of the numerous dendritic branches of a nerve cell is to provide interconnection with other neurons. In the cerebral cortex of mammals, a large proportion of axodendrial connections fall on contacts with special specialized outgrowths of dendrites - dendritic spines. Dendritic spines are phylogenetically the youngest formations in the nervous system. In ontogeny, they mature much later than other nervous structures and represent the most plastic apparatus of the nerve cell.

As a rule, the dendritic spine has a characteristic shape in the cerebral cortex of mammals. (Fig. 2). A relatively narrow stalk departs from the main dendritic trunk, which ends with an extension - the head. It is likely that this form of the dendritic appendage (the presence of a head) is associated, on the one hand, with an increase in the area of ​​synaptic contact with the axon ending, and on the other hand, it serves to accommodate specialized organelles inside the spine, in particular, the spiny apparatus, which is present only in the dendritic spines of the mammalian cerebral cortex. In this regard, an analogy with the shape of the synaptic axon ending, when a thin preterminal fiber forms an extension, seems appropriate. This expansion (synaptic plaque) forms extensive contact with the innervated substrate and contains inside a large set of ultrastructural components (synaptic vesicles, mitochondria, neurofilaments, glycogen granules).

There is a hypothesis (which, in particular, is shared and developed by the Nobel laureate F. Crick) that the geometry of the spines can change depending on the functional state of the brain. In this case, the narrow neck of the spine can expand, and the spine itself flattens, resulting in an increase in the efficiency of the axo-spine contact.

If the shape and size of dendritic spines in the cerebral cortex of mammals can vary somewhat, then the most constant in them is the presence of a specific spine apparatus. It is a complex of interconnected tubules (cistern) located, as a rule, in the head of the spine. Probably, this organelle is associated with very important functions inherent in the phylogenetically youngest brain formations, since the spiny apparatus is found mainly in the cerebral cortex, and only in higher animals.

Despite everything, the spine is a derivative of the dendrite, it lacks neurofilaments and dendritic tubules, its cytoplasm contains a coarsely or finely granular matrix. Another characteristic feature of spinules in the cerebral cortex is the obligatory presence of synaptic contacts with axon endings on them. The cytoplasm of the spine has special components that distinguish it from dendritic stems. It is possible to note a peculiar triad in the cytoplasm of the spine: subsynaptic specialization of active zones - spiny apparatus - mitochondria. Given the variety of complex and important functions performed by mitochondria, one can also expect complex functional manifestations in "triads" during synaptic transmission. It can be said that the cytoplasm of the dendritic spine and the spiny apparatus may be directly related to the synaptic function.

Dendritic spines and ends of dendrites are also very sensitive to extreme factors. With any type of poisoning (for example, alcoholic, hypoxic, heavy metals - lead, mercury, etc.), the number of spines found on the dendrites of the cells of the cerebral cortex changes. In all likelihood, the spines do not disappear, but their cytoplasmic components are disturbed, and they are worse impregnated with salts of heavy metals. Since spines are one of the structural components of interneuronal contacts, malfunctions in them lead to serious impairment of brain function.

In some cases, with a short-term action of an extreme factor, at first glance, a paradorsal situation may occur, when the number of spines found on the dendrites of brain cells does not decrease, but increases. So, this is observed during experimental cerebral ischemia in its initial period. In parallel with an increase in the number of identified spinules, the functional state of the brain may improve. In this case, hypoxia is a factor that contributes to increased metabolism in the nervous tissue, better implementation of reserves that are not used in a normal situation, and the rapid combustion of toxins accumulated in the body. Ultrastructurally, this is manifested in a more intensive study of the cytoplasm of the spines, growth and enlargement of the cisterns of the spine apparatus. Probably, this phenomenon of the positive effect of hypoxia is observed when a person, experiencing great physical exertion under hypoxic conditions, conquers mountain peaks. These difficulties are then compensated by more intensive productive work, both of the brain and other organs.

Formation of dendrites

Dendrites and their interneuronal connections are formed during the ontogenetic development of the brain. Moreover, the dendrites, in particular the apical ones, in young individuals remain free for some time to form new contacts. The parts of the dendrite located closer to the cell body are probably associated with stronger and simpler natural conditioned reflexes, and the ends are left for the formation of new connections and associations.

In adulthood, there are no longer areas free from interneuronal contacts on the dendrites, but with aging, it is the ends of the dendrites that first suffer and in terms of saturation with contacts

in old individuals, they resemble the dendrites of childhood. This occurs both due to the weakening of transport protein-synthesizing processes in the cell, and due to impaired blood supply to the brain. Perhaps it is here that lies the morphological basis for such a fact, widely known in neurology and in everyday life, when old people find it difficult to master something new, often forget current events and remember the past very well. The same is observed in case of poisoning.

As already noted, the increase and complication of the dendritic tree in phylogenesis is necessary not only for the perception of a large number of incoming impulses, but also for preliminary processing.

The dendrites of the neurons of the central nervous system have a synaptic function throughout, and the terminal sections are in no way inferior to the middle ones in this. If we are talking about the distal (terminal) areas of the apical dendrites of the pyramidal neurons of the cerebral cortex, then their share in the implementation of interneuronal interactions is even more significant than proximal ones. There, in addition to a larger number of terminal synaptic plaques on the trunk itself and on the branches of the apical dendrite, there are also contacts on dendritic spines.

Studying this problem using electron microscopy, the researchers also became convinced that the end sections of the dendrites are densely covered with synaptic plaques and, thus, are directly involved in interneuronal interactions. Electron microscopy has also shown that dendrites can form contacts with each other. These contacts can be either parallel, to which most authors attribute electrotonic properties, or typical asymmetric synapses with well-defined organelles that provide chemical transmission. Such dendro-dendritic contacts are only just beginning to attract the attention of researchers. So, the dendrite throughout its entire length performs a synaptic function. How is the surface of the dendrite adapted to provide contacts with axon endings?

The surface membrane of the dendrite is designed to be used to the maximum for interneuronal contacts. The whole dendrite is pitted with depressions, folds, pockets, has various irregularities of the kind of microoutgrowths, spikes, mushroom-like appendages, etc. All these reliefs of dendritic trunks correspond to the shape and size of the incoming synaptic endings. Moreover, in different parts of the nervous system and in different animals, the relief of the dendritic surface has specific features. Of course, the most remarkable outgrowth of the dendritic membrane is the dendritic spine.

Dendrites are very sensitive to the action of various extreme factors. Violations in them lead to many diseases, such as mental disorders.

reflexes- this is the body's response to irritation of sensitive nerve formations - receptors, realized with the participation of the nervous system.

Types of reflexes conditional and unconditional

reflex arc

With the help of a reflex, excitation spreads along reflex arcs and the process of inhibition is carried out.

reflex arc- this is the path along which nerve impulses are conducted during the implementation of the reflex.

Reflex arc diagram

5 links of the reflex arc:

1. Receptor - perceives irritation and converts it into a nerve impulse.

2. Sensitive (centripetal) neuron - transmits excitation to the center.

3. Nerve center - excitation switches from sensory to motor neurons (there is an intercalary neuron in the three-neuron arc).

4. Motor (centrifugal) neuron - carries excitation from the central nervous system to the working organ.

5. Working body - reacts to the received irritation.

Information from the receptors of the working organ enters the nerve center to confirm the effectiveness of the reaction and, if necessary, coordinate it.

Scheme of the reflex arc of the knee jerk (a simple arc of two neurons)

Scheme of the reflex arc of the flexion reflex (a complex arc of several neurons)

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The source of information:

Biology in tables and diagrams. / Edition 2e, - St. Petersburg: 2004.

Rezanova E.A. Human biology. In tables and diagrams./ M.: 2008.