Nervous system. Neurons and nervous tissue

The human nervous system is divided in different ways. Anatomically, it consists of the central nervous system (CNS) and the peripheral nervous system (PNS). The central nervous system includes the brain and spinal cord, and the PNS, which provides communication between the central nervous system and various parts of the body, includes the cranial and spinal nerves, as well as nerve ganglia and nerve plexuses lying outside the spinal cord and brain.

A neuron, like other cells, has a nucleus and a number of tiny structures - organelles

Concentrations of ions (electrically charged atoms) - mainly sodium and potassium, as well as organic matter- outside the neuron and inside it are not the same, therefore the nerve cell at rest is charged negatively from the inside, and positively charged from the outside; as a result, a potential difference appears on the cell membrane (the so-called “resting potential” is approximately -70 millivolts). Any change that reduces the negative charge within the cell and thereby the potential difference across the membrane is called depolarization.

The plasma membrane surrounding the neuron is a complex formation consisting of lipids (fats), proteins and carbohydrates. It is practically impenetrable to ions. But some of the protein molecules in the membrane form channels through which certain ions can pass. However, these channels, called ion channels, are not constantly open, but, like gates, can open and close.

When a neuron is stimulated, some of the sodium (Na

In many neurons, depolarization also causes the opening of potassium (

Electrochemical changes at the point of stimulation cause depolarization at an adjacent point on the membrane, triggering the same cycle of changes in it. This process is constantly repeated, and in each new point, where depolarization occurs, an impulse of the same magnitude is born as at the previous point. Thus, along with the renewed electrochemical cycle, the nerve impulse spreads along the neuron from point to point.

Axonal fibers in the PNS are surrounded by neurilemma, a sheath of Schwann cells that are located along the axon, like beads on a string. A significant number of these axons are covered with an additional sheath of myelin (a protein-lipid complex); they are called myelinated (pulpy). Fibers surrounded by neurilemma cells, but not covered with a myelin sheath, are called unmyelinated (unmyelinated). Myelinated fibers are found only in vertebrates. The myelin sheath is formed from the plasma membrane of Schwann cells, which is wound around the axon like a roll of ribbon, forming layer upon layer. The section of the axon where two adjacent Schwann cells touch each other is called the node of Ranvier. In the central nervous system, the myelin sheath of nerve fibers is formed by a special type of glial cells - oligodendroglia. Each of these cells forms the myelin sheath of several axons at once. Unmyelinated fibers in the CNS lack a sheath of any special cells.

The myelin sheath speeds up the conduction of nerve impulses that “jump” from one node of Ranvier to another, using this sheath as a connecting electrical cable. The speed of impulse conduction increases with the thickening of the myelin sheath and ranges from 2 m/s (along unmyelinated fibers) to 120 m/s (along fibers especially rich in myelin). For comparison: the speed of propagation of electric current through metal wires is from 300 to 3000 km/s.

In a number of animals (for example, the lobster), a particularly close connection is established between the neurons of certain nerves with the formation of either an unusually narrow synapse, the so-called. gap junction, or, if the neurons are in direct contact with each other, tight junction. Nerve impulses pass through these connections not with the participation of a neurotransmitter, but directly, through electrical transmission. Mammals, including humans, also have a few tight junctions of neurons.

Cut or damaged PNS neuron fibers surrounded by the neurilemma can regenerate if the cell body remains intact. Below the site of transection, the neurilemma is preserved as a tubular structure, and that part of the axon that remains connected to the cell body grows along this tube until it reaches the nerve ending. In this way, the function of the damaged neuron is restored. Axons in the central nervous system that are not surrounded by a neurilemma are apparently unable to re-grow to the site of their previous termination. However, many neurons of the central nervous system can produce new short processes - branches of axons and dendrites that form new synapses.

The central nervous system is made up of gray and white matter. Gray matter is composed of cell bodies, dendrites, and unmyelinated axons, organized into complexes that include countless synapses and serve as information processing centers for many functions of the nervous system. White matter consists of myelinated and unmyelinated axons that act as conductors transmitting impulses from one center to another. The gray and white matter also contains glial cells.

CNS neurons form many circuits that perform two main functions: they provide reflex activity, as well as complex information processing in higher brain centers. These higher centers, such as the visual cortex (visual cortex), receive incoming information, process it, and transmit a response signal along the axons.

The result of the activity of the nervous system is one or another activity, which is based on the contraction or relaxation of muscles or the secretion or cessation of secretion of glands. It is with the work of muscles and glands that any way of our self-expression is connected.

Incoming sensory information is processed, passing through a sequence of centers connected by long axons that form specific pathways, for example pain, visual, auditory. Sensory (ascending) pathways go in an ascending direction to the centers of the brain. Motor (descending) tracts connect the brain with motor neurons of the cranial and spinal nerves.

The pathways are usually organized in such a way that information (for example, pain or tactile) from the right side of the body enters the left side of the brain and vice versa. This rule also applies to the descending motor pathways: the right half of the brain controls the movements of the left half of the body, and the left half controls the movements of the right. There are, however, a few exceptions to this general rule.

The cerebral hemispheres - the largest part of the brain - contain higher nerve centers that form the basis of consciousness, intelligence, personality, speech, and understanding. In each of the cerebral hemispheres, the following formations are distinguished: underlying isolated accumulations (nuclei) of gray matter, which contain many important centers; a large mass of white matter located above them; covering the outside of the hemispheres is a thick layer of gray matter with numerous convolutions that makes up the cerebral cortex.

The cerebellum also consists of an underlying gray matter, an intermediate mass of white matter, and an outer thick layer of gray matter that forms many convolutions. The cerebellum primarily provides coordination of movements.

The brainstem is formed by a mass of gray and white matter that is not divided into layers. The trunk is closely connected with the cerebral hemispheres, the cerebellum and the spinal cord and contains numerous centers of sensory and motor pathways. The first two pairs of cranial nerves arise from the cerebral hemispheres, while the remaining ten pairs arise from the trunk. The trunk regulates vital functions such as breathing and blood circulation.

Neuron (nerve cell)- the main structural and functional element of the nervous system; Humans have more than one hundred billion neurons. A neuron consists of a body and processes, usually one long process - an axon and several short branched processes - dendrites. Along dendrites, impulses follow to the cell body, along an axon - from the cell body to other neurons, muscles or glands. Thanks to the processes, neurons contact each other and form neural networks and circles through which nerve impulses circulate. A neuron, or nerve cell, is a functional unit of the nervous system. Neurons are susceptible to stimulation, that is, they are capable of being excited and transmitting electrical impulses from receptors to effectors. Based on the direction of impulse transmission, afferent neurons (sensory neurons), efferent neurons (motor neurons) and interneurons are distinguished. Each neuron consists of a soma (a cell with a diameter of 3 to 100 microns, containing a nucleus and other cellular organelles immersed in the cytoplasm) and processes - axons and dendrites. Based on the number and location of processes, neurons are divided into unipolar neurons, pseudounipolar neurons, bipolar neurons and multipolar neurons. .

The main functions of a nerve cell are the perception of external stimuli (receptor function), their processing (integrative function) and the transmission of nerve influences to other neurons or various working organs (effector function)

The peculiarities of the implementation of these functions make it possible to divide all neurons of the central nervous system into two large groups:

1) Cells that transmit information over long distances (from one part of the central nervous system to another, from the periphery to the center, from the center to the executive organ). These are large afferent and efferent neurons that have a large number of synapses on their body and processes, both inhibitory and excitatory, and are capable of complex processes of processing influences coming through them.

2) Cells that provide interneural connections within organic nervous structures (intermediate neurons of the spinal cord, cerebral cortex, etc.). These are small cells that perceive nerve influences only through excitatory synapses. These cells are not capable of complex processes of integration of local synoptic influences of potentials; they serve as transmitters of excitatory or inhibitory influences on other nerve cells.

Perceiving function of a neuron. All irritations entering the nervous system are transmitted to the neuron through certain sections of its membrane located in the area of ​​synaptic contacts. 6.2 Integrative function of a neuron. The overall change in the membrane potential of a neuron is the result of a complex interaction (integration) of local EPSPs and IPSPs of all the numerous activated synapses on the cell body and dendrites.

Effector function of a neuron. With the advent of AP, which, unlike local changes in membrane potential (EPSP and IPSP), is a spreading process, the nerve impulse begins to be conducted from the body of the nerve cell along the axon to another nerve cell or working organ, i.e. the effector function of the neuron is carried out.

Synapses in the central nervous system.

Synapse is a morphofunctional formation of the central nervous system, which ensures signal transmission from a neuron to another neuron or from a neuron to an effector cell. All CNS synapses can be classified as follows.

1. By localization: central and peripheral (neuromuscular, neurosecretory synapse of the autonomic nervous system).

2. According to development in ontogenesis: stable and dynamic, emerging in the process of individual development.

3. By final effect: inhibitory and excitatory.

4. According to the signal transmission mechanism: electrical, chemical, mixed.

5. Chemical synapses can be classified:

A) by contact form- terminal (flask-shaped connection) and transient (varicose dilatation of the axon);

b) by the nature of the mediator– cholinergic, adrenergic, dopaminergic

Electrical synapses. It is now recognized that there are electrical synapses in the central nervous system. From a morphological point of view, an electrical synapse is a gap-like formation (slit dimensions up to 2 nm) with ion bridges-channels between two contacting cells. Current loops, in particular in the presence of an action potential (AP), almost unhinderedly jump through such a gap-like contact and excite, i.e. induce the generation of APs in the second cell. In general, such synapses (they are called ephapses) provide very rapid transmission of excitation. But at the same time, with the help of these synapses it is impossible to ensure unilateral conduction, since most of these synapses have bilateral conductivity. In addition, they cannot be used to force an effector cell (a cell that is controlled through a given synapse) to inhibit its activity. An analogue of the electrical synapse in smooth muscles and in cardiac muscle are gap junctions of the nexus type.

Chemical synapses. In structure, chemical synapses are the ends of an axon (terminal synapses) or its varicose part (passing synapses), which is filled with a chemical substance - a mediator. In the synapse, there is a presynaptic element, which is limited by the presynaptic membrane, a postsynaptic element, which is limited by the postsynaptic membrane, as well as an extrasynaptic region and a synaptic cleft, the size of which is on average 50 nm.

Reflex arc. Classification of reflexes.

Reflex- the body’s reaction to changes in the external or internal environment, carried out through the central nervous system in response to irritation of receptors.

All reflex acts of the whole organism are divided into unconditioned and conditioned reflexes. Unconditioned reflexes are inherited, they are inherent in everyone biological species; their arches are formed at the time of birth and normally remain throughout life. However, they can change under the influence of illness. Conditioned reflexes arise with individual development and accumulation of new skills. The development of new temporary connections depends on changing environmental conditions. Conditioned reflexes are formed on the basis of unconditioned ones and with the participation of higher parts of the brain. They can be classified into different groups according to a number of characteristics.

1. According to biological significance

A.) food

B.) defensive

B.) sexual

G.) approximate

D.) postural-tonic (reflexes of body position in space)

E.) locomotor (reflexes of body movement in space)

2. By location of receptors, the irritation of which is caused by this reflex act

A.) exteroceptive reflex - irritation of receptors on the external surface of the body

B.) viscero- or interoreceptive reflex - arising from irritation of receptors of internal organs and blood vessels

B.) proprioceptive (myotatic) reflex - irritation of receptors of skeletal muscles, joints, tendons

3. According to the location of the neurons involved in the reflex

A.) spinal reflexes - neurons located in the spinal cord

B.) bulbar reflexes - carried out with the obligatory participation of neurons of the medulla oblongata

B.) mesencephalic reflexes - carried out with the participation of midbrain neurons

D.) diencephalic reflexes - neurons of the diencephalon are involved

D.) cortical reflexes - carried out with the participation of neurons in the cerebral cortex

Reflex arc- this is the path along which irritation (signal) from the receptor passes to the executive organ. The structural basis of the reflex arc is formed by neural circuits consisting of receptor, intercalary and effector neurons. It is these neurons and their processes that form the path along which nerve impulses from the receptor are transmitted to the executive organ during the implementation of any reflex.

In the peripheral nervous system, reflex arcs (neural circuits) are distinguished

Somatic nervous system, innervating the skeletal muscles

The autonomic nervous system innervates internal organs: heart, stomach, intestines, kidneys, liver, etc.

The reflex arc consists of five sections:

1. Receptors that perceive irritation and respond to it with excitement. Receptors are located in the skin, in all internal organs; clusters of receptors form the sense organs (eye, ear, etc.).

2. Sensitive (centripetal, afferent) nerve fiber, transmitting excitation to the center; a neuron that has this fiber is also called sensitive. The cell bodies of sensory neurons are located outside the central nervous system - in ganglia along the spinal cord and near the brain.

3. Nerve center, where excitation switches from sensory neurons to motor neurons; The centers of most motor reflexes are located in the spinal cord. The brain contains centers for complex reflexes, such as protective, food, orientation, etc. In the nerve center

There is a synaptic connection between the sensory and motor neurons.

1. Motor (centrifugal, efferent) nerve fiber, carrying excitation from the central nervous system to the working organ; Centrifugal fiber is a long extension of a motor neuron. A motor neuron is a neuron whose process approaches the working organ and transmits a signal to it from the center.

2. Effector - a working organ that produces an effect, a reaction in response to receptor irritation. Effectors can be muscles that contract when they receive stimulation from the center, gland cells that secrete juice under the influence of nervous stimulation, or other organs.

The concept of the nerve center.

Nerve center- a set of nerve cells, more or less strictly localized in the nervous system and certainly involved in the implementation of a reflex, in the regulation of one or another function of the body or one of the aspects of this function. In the simplest cases, the nerve center consists of several neurons forming a separate node (ganglion).

In every N. c. Through the input channels - the corresponding nerve fibers - information from the sense organs or from other nervous systems arrives in the form of nerve impulses. This information is processed by the neurons of the central nervous system, whose processes (axons) do not extend beyond its boundaries. The final link is the neurons, the processes of which leave the N. c. and deliver its command impulses to peripheral organs or other N. c. (output channels). The neurons that make up the neural network are connected to each other through excitatory and inhibitory synapses and form complex complexes, so-called neural networks. Along with neurons that are excited only in response to incoming nerve signals or the action of various chemical stimuli contained in the blood, the composition of N. c. may include pacemaker neurons that have their own automaticity; They have the ability to periodically generate nerve impulses.

Localization of N. c. determined on the basis of experiments with irritation, limited destruction, removal or transection of certain parts of the brain or spinal cord. If, when a given area of ​​the central nervous system is irritated, one or another physiological reaction occurs, and when it is removed or destroyed, it disappears, then it is generally accepted that the nervous system is located here, influencing this function or participating in a certain reflex.

Properties of nerve centers.

The nerve center (NC) is a collection of neurons in various parts of the central nervous system that provide regulation of any function of the body.

The following features are characteristic for the conduction of excitation through nerve centers:

1. Single-line conduction, it goes from the afferent, through the intercalary to the efferent neuron. This is due to the presence of interneuron synapses.

2. The central delay in the conduction of excitation, i.e., along the NC excitation is much slower than along the nerve fiber. This is explained by synaptic delay because most of the synapses are in the central link of the reflex arc, where the conduction speed is the lowest. Based on this, reflex time is the time from the onset of exposure to the stimulus to the appearance of the response. The longer the central delay, the longer the reflex time. However, it depends on the strength of the stimulus. The larger it is, the shorter the reflex time and vice versa. This is explained by the phenomenon of summation of excitations in synapses. In addition, it is determined by the functional state of the central nervous system. For example, when the NC is tired, the duration of the reflex reaction increases.

3. Spatial and temporal summation. Temporal summation occurs, as in synapses, due to the fact that the more nerve impulses arrive, the more neurotransmitter is released in them, the higher the amplitude of the EPSP. Therefore, a reflex reaction can occur to several successive subthreshold stimuli. Spatial summation is observed when impulses from several neuron receptors go to the nerve center. When subthreshold stimuli act on them, the resulting postsynaptic potentials are summed up 11 and a propagating AP is generated in the neuron membrane.

4. Transformation of the rhythm of excitation - a change in the frequency of nerve impulses when passing through the nerve center. The frequency may decrease or increase. For example, increasing transformation (increase in frequency) is due to the dispersion and multiplication of excitation in neurons. The first phenomenon occurs as a result of the division of nerve impulses into several neurons, the axons of which then form synapses on one neuron. Second, the generation of several nerve impulses during the development of an excitatory postsynaptic potential on the membrane of one neuron. The downward transformation is explained by the summation of several EPSPs and the appearance of one AP in the neuron.

5. Post-tetanic potentiation, this is an increase in the reflex response as a result of prolonged excitation

neurons center. Under the influence of many series of nerve impulses passing from high frequency through synapses, a large amount of neurotransmitter is released at interneuron synapses. This leads to a progressive increase in the amplitude of the excitatory postsynaptic potential and long-term (several hours) excitation of neurons.

6. Aftereffect is a delay in the end of the reflex response after the cessation of the stimulus. Associated with the circulation of nerve impulses along closed circuits of neurons.

7. The tone of the nerve centers is a state of constant increased activity. It is caused by the constant supply of nerve impulses to the NC from peripheral receptors, the stimulating influence of metabolic products and other humoral factors on neurons. For example, the manifestation of the tone of the corresponding centers is the tone of a certain muscle group.

8. automaticity or spontaneous activity of nerve centers. Periodic or constant generation of nerve PULSES by neurons, which arise spontaneously in them, i.e. in the absence of signals from other neurons or receptors. It is caused by fluctuations in the metabolic processor in neurons and the effect of humoral factors on them.

9. Plasticity of nerve centers. This is their ability to change functional properties. In this case, the center acquires the ability to perform new functions or restore old ones after damage. The basis of plasticity N.Ts. lies the plasticity of synapses and membranes of neurons, which can change their molecular structure.

10. Low physiological lability and fatigue. N.Ts. can conduct pulses of only a limited frequency. Their fatigue is explained by fatigue of synapses and deterioration of neuronal metabolism.

Inhibition in the central nervous system.

Inhibition in the central nervous system prevents the development of excitation or weakens ongoing excitation. An example of inhibition could be the cessation of a reflex reaction against the background of the action of another stronger stimulus. Initially, a unitary-chemical theory of inhibition was proposed. It was based on Dale's principle: one neuron - one transmitter. According to it, inhibition is provided by the same neurons and synapses as excitation. Subsequently, the correctness of the binary chemical theory was proven. According to the latter, inhibition is provided by special inhibitory neurons, which are intercalary. These are Renshaw cells of the spinal cord and Purkinje neurons. Inhibition in the central nervous system is necessary for the integration of neurons into a single nerve center. The following inhibitory mechanisms are distinguished in the central nervous system:

1| Postsynaptic. It occurs in the postsynaptic membrane of the soma and dendrites of neurons, i.e. after the transmitting synapse. In these areas, specialized inhibitory neurons form axo-dendritic or axosomatic synapses (Fig.). These synapses are glycinergic. As a result of the effect of NLI on the glycine chemoreceptors of the postsynaptic membrane, its potassium and chloride channels open. Potassium and chloride ions enter the neuron, and IPSP develops. The role of chlorine ions in the development of IPSP: small. As a result of the resulting hyperpolarization, the excitability of the neuron decreases. The conduction of nerve impulses through it stops. The alkaloid strychnine can bind to glycerol receptors on the postsynaptic membrane and turn off inhibitory synapses. This is used to demonstrate the role of inhibition. After the administration of strychnine, the animal develops cramps in all muscles.

2. Presynaptic inhibition. In this case, the inhibitory neuron forms a synapse on the axon of the neuron that approaches the transmitting synapse. Those. such a synapse is axo-axonal (Fig.). The mediator of these synapses is GABA. Under the influence of GABA, chloride channels of the postsynaptic membrane are activated. But in this case, chlorine ions begin to leave the axon. This leads to a small local but long-lasting depolarization of its membrane.

A significant part of the sodium channels of the membrane is inactivated, which blocks the conduction of nerve impulses along the axon, and consequently the release of the neurotransmitter at the transmitting synapse. The closer the inhibitory synapse is located to the axon hillock, the stronger its inhibitory effect. Presynaptic inhibition is most effective when information processing, since the conduction of excitation is not blocked in the entire neuron, but only at its one input. Other synapses located on the neuron continue to function.

3. Pessimal inhibition. Discovered by N.E. Vvedensky. Occurs at a very high frequency of nerve impulses. A persistent, long-term depolarization of the entire neuron membrane and inactivation of its sodium channels develops. The neuron becomes unexcitable.

Both inhibitory and excitatory postsynaptic potentials can simultaneously arise in a neuron. Due to this, the necessary signals are isolated.

Principles of coordination of reflex processes.

The reflex reaction in most cases is carried out not alone, but whole group reflex arcs and nerve centers. Coordination of reflex activity is the interaction of nerve centers and nerve impulses passing through them, which ensures the coordinated activity of organs and systems of the body. It is carried out through the following processes:

1. Temporary and spatial relief. This is an increase in the reflex response when exposed to a number of sequential stimuli or their simultaneous impact on several receptive fields. Explained by the phenomenon of summation in nerve centers.

2. Occlusion is the opposite phenomenon of relief. When the reflex response to two or more suprathreshold stimuli is less than the responses to their separate effects. It is associated with the convergence of several excitatory impulses on one neuron.

3. The principle of a common final path. Designed by C. Sherrington. It is based on the phenomenon of convergence. According to this principle, synapses of several afferents that are part of several reflex arcs can form on one efferent motor neuron. This neuron is called the common terminal pathway and is involved in several reflex responses. If the interaction of these reflexes leads to an increase in the general reflex reaction, such reflexes are called allied. If there is a struggle between afferent signals for the motor neuron - the final path, then it is antagonistic. As a result of this struggle, secondary reflexes are weakened, and the common final path is freed up for vital ones.

4. Reciprocal inhibition. Discovered by C. Sherrington. This is the phenomenon of inhibition of one Center as a result of excitation of another. Those. in this case, the antagonistic center is inhibited. For example, when the centers of flexion of the left leg are excited, the centers of the extensor muscles of the same leg and the centers of the flexors of the right leg are inhibited by a reciprocal mechanism. The inhalation and exhalation centers of the medulla oblongata are in a reciprocal relationship. sleep and wakefulness centers, etc.

5. The principle of dominance. Opened by A.A. Ukhtomsky. The dominant is the predominant focus of excitation in the central nervous system, subjugating other NCs. The dominant center provides a complex of reflexes that are necessary at the moment to achieve specific purpose. Under certain conditions, drinking, food, defensive, sexual and other dominants arise. The properties of the dominant focus are increased excitability, persistence of excitation, high ability for summation, and inertia. These properties are due to the phenomena of facilitation, irradiation, with a simultaneous increase in the activity of intercalary inhibitory neurons, which inhibit neurons of other centers.

6. The principle of reverse afferentation. The results of the reflex act are perceived by reverse afferentation neurons and information from them comes back to the nerve center. There they are compared with the excitation parameters and the reflex reaction is corrected.

Methods for studying the functions of the central nervous system.

1. Method of cutting the brain stem at various levels. For example, between the medulla oblongata and the spinal cord.

2. Method of extirpation (removal) or destruction of parts of the brain.

3.Method of irritation of various parts and centers of the brain.

4. Anatomical and clinical method. Clinical observations of changes in the functions of the central nervous system when any of its parts are damaged, followed by a pathological examination.

5. Electrophysiological methods:

A. Electroencephalography is the recording of brain biopotentials from the surface of the scalp. The technique was developed and introduced into the clinic by G. Berger.

b. Registration of biopotentials of various nerve centers is used in conjunction with stereotactic technique, in which electrodes are inserted into a strictly defined nucleus using micromanipulators using the evoked potential method, recording the electrical activity of brain areas during electrical stimulation of peripheral receptors or other areas;

6. Method of intracerebral administration of substances using microinophoresis.

7. Chronoreflexometry - determination of reflex time.

Spinal cord reflexes.

Reflex function. The nerve centers of the spinal cord are segmental, or working, centers. Their neurons are directly connected to receptors and working organs. In addition to the spinal cord, such centers are present in the medulla oblongata and midbrain. Suprasegmental centers, for example, the diencephalon and cerebral cortex, do not have a direct connection with the periphery. They control it through segmental centers. Motor neurons of the spinal cord innervate all the muscles of the trunk, limbs, neck, as well as the respiratory muscles - the diaphragm and intercostal muscles.

Neuron is a structural and functional unit of nervous tissue. This is a specialized cell, which, along with general physiological properties (excitability, conductivity), also has a number of specific properties:

- Perceive information- translate stimulus information into the biological language of the cell.

- Process information- i.e. carry out information analysis, synthesis - combining various parts of information after analysis to obtain a new quality.

- Encode information- transform information into a form convenient for storage in the brain.

- Form a team a control signal that spreads to other cells, neurons, muscle cells.

- Transfer of information neuron to other structures.

Neurons are able to communicate with other cells and exert an informational influence on them (the place of contact is the synapse).

The neuron carries out all its activities in counting 3 physiological properties(in addition to excitability and conductivity):

Reception;

Electrogenesis;

Neurosecretion.

In general terms, all neurons have a body - soma and processes - dendrites and axons.

They are conventionally divided according to structure and functions into the following groups:

Body shape: polygonal, pyramidal, round, oval.

According to the number and nature of processes:

Unipolar– having one shoot

Pseudounipolar– one process extends from the body, which then divides into 2 branches.

Bipolar– 2 processes, one dendrite-like, the other an axon.

Multipolar– have 1 axon and many dendrites.

According to the mediator released by the neuron at the synapse: cholinergic, adrenergic, serotonergic, peptidergic, etc.

By function:

Afferent, or sensitive - serve to perceive signals from the external and internal environment and transmit them to the central nervous system.

Insert, or interneurons, intermediate - provide processing, storage and transmission of information to efferent neurons. They are the majority in the central nervous system.

Efferent or motor - they form control signals and transmit them to peripheral neurons and executive organs.

According to physiological role: excitatory and inhibitory.

General neuron functions The central nervous system is responsible for receiving, encoding, storing information and producing neurotransmitters. Neurons, through numerous synapses, receive signals in the form of postsynaptic potentials. Then they process this information and form a certain response. Consequently, they also perform integrative, i.e. unifying function.

Communication between neurons, as can be seen, is carried out through the gap between the ends of the axon of one neuron and the dendrites of another. If they lie in sufficient proximity, that is, the gap is small, then a synaptic node, or synapse, connecting these two neurons can form in this place.

Synapse similar to resistance in an electrical circuit. If this resistance is high, then the connection between neurons is weak and the excitation of one neuron does not cause excitation of another. If the “resistance” of the synapse is small, then there is a strong connection and the neuron is easily excited by the axon of another neuron connected to it.
The excitation of a neuron occurs according to the “all or nothing” principle. This means that the neuron can either be excited, and a nerve impulse travels from the cell along the axon to the synaptic nodes and further to other neurons, or not excited.

2. Humoral regulation. Functions, mechanisms of interaction of humoral substances with target cells. The place and role of the endocrine glands in the regulation of function.

Humoral regulation- one of the evolutionarily early mechanisms for regulating vital processes in the body, carried out through body fluids (blood, lymph, tissue fluid, oral cavity) with the help of hormones secreted by cells, organs, and tissues. In highly developed animals, including humans, humoral regulation is subordinated to nervous regulation and, together with it, constitutes a single system of neurohumoral regulation. Metabolic products act not only directly on effector organs, but also on the endings of sensory nerves (chemoreceptors) and nerve centers, causing certain reactions by humoral or reflex means. Humoral transmission of nerve impulses by chemicals, mediators, occurs in the central and peripheral nervous system. Along with hormones, intermediate metabolic products play an important role in humoral regulation.

Biological activity of liquid media the body is determined by the ratio of the content of catecholamines (adrenaline and norepinephrine, their precursors and breakdown products), acetylcholine, histamine, serotonin and other biogenic amines, some polypeptides and amino acids, the state of enzyme systems, the presence of activators and inhibitors, the content of ions, trace elements, etc.

Depending on the structure of the hormone, there are two types of interaction. If a hormone molecule lipophilic,(for example, steroid hormones), it can penetrate the lipid layer of the outer membrane of target cells. If the molecule is large or polar, then its penetration into the cell is impossible. Therefore, for lipophilic hormones, receptors are located inside target cells, and for hydrophilic- receptors are located in the outer membrane.

To obtain a cellular response In the case of hydrophilic molecules, the hormonal signal is affected by the intracellular signal transduction mechanism. This occurs with the participation of substances called second messengers. Hormone molecules are very diverse in shape, but “second messengers” are not. The reliability of signal transmission is ensured by the very high affinity of the hormone for its receptor protein.

Intermediaries- these are cyclic nucleotides (cAMP and cGMP), inositol triphosphate, calcium-binding protein - calmodulin, calcium ions, enzymes involved in the synthesis of cyclic nucleotides, as well as protein kinases - protein phosphorylation enzymes. All these substances are involved in the regulation of the activity of individual enzyme systems in target cells.

Exists two main ways of signal transmission into cells - targets from signaling molecules with a membrane mechanism of action: adenylate cyclase (or guanylate cyclase) systems; and the phosphoinositide mechanism.

Cyclase system - This is a system consisting of adenosine cyclophosphate, adenylate cyclase and phosphodiesterase contained in the cell, which regulates the permeability of cell membranes, is involved in the regulation of many metabolic processes of a living cell, and mediates the action of some hormones. That is, the role of the cyclase system is that they are second intermediaries in the mechanism of action of hormones.

The adenylate cyclase - cAMP system. The membrane enzyme adenylate cyclase can be found in two forms - activated and non-activated. Activation of adenylate cyclase occurs under the influence of a hormone-receptor complex, the formation of which leads to the binding of guanyl nucleotide (GTP) to a special regulatory stimulating protein (GS protein), after which the GS protein causes the addition of magnesium to adenylate cyclase and its activation. This is how the hormones activating adenylate cyclase act: glucagon, thyrotropin, parathyrin, vasopressin, gonadotropin, etc. Some hormones, on the contrary, suppress adenylate cyclase (somatostatin, angiotensin-P, etc.)

Under the influence of adenylate cyclase cAMP is synthesized from ATP, which causes activation of protein kinases in the cell cytoplasm, which ensure the phosphorylation of numerous intracellular proteins. This changes the permeability of membranes, i.e. causes metabolic and, accordingly, functional changes typical for the hormone. The intracellular effects of cAMP are also manifested in their influence on the processes of proliferation, differentiation, and the availability of membrane receptor proteins to hormone molecules.

"Guanylate cyclase - cGMP" system. Activation of membrane guanylate cyclase occurs not under the direct influence of the hormone-receptor complex, but indirectly through ionized calcium and oxidative membrane systems. This is how atrial natriuretic hormone, atriopeptide, a tissue hormone of the vascular wall, realizes its effects. In most tissues, the biochemical and physiological effects of cAMP and cGMP are opposite. Examples include stimulation of cardiac contractions under the influence of cAMP and inhibition of them by cGMP, stimulation of contractions of intestinal smooth muscles by cGMP and inhibition of cAMP.

In addition to the adenylate cyclase or guanylate cyclase systems, there is also a mechanism for transmitting information within the target cell with the participation of calcium ions and inositol triphosphate.

Inositol triphosphate - is a substance that is a derivative complex lipid- inositol phosphatide. It is formed as a result of the action of a special enzyme - phospholipase "C", which is activated as a result of conformational changes in the intracellular domain of the membrane receptor protein. This enzyme hydrolyzes the phosphoester bond in the phosphatidyl-inositol 4,5-bisphosphate molecule to form diacylglycerol and inositol triphosphate.

It is known that education diacylglycerol and inositol triphosphate leads to an increase in the concentration of ionized calcium inside the cell. This leads to the activation of many calcium-dependent proteins inside the cell, including the activation of various protein kinases. And here, as with the activation of the adenylate cyclase system, one of the stages of signal transmission inside the cell is protein phosphorylation, which leads to a physiological response of the cell to the action of the hormone.

In progress phosphoinositide mechanism signal transmission in the target cell involves a special calcium-binding protein - calmodulin. This is a low molecular weight protein (17 kDa), 30% consisting of negatively charged amino acids (Glu, Asp) and therefore capable of actively binding Ca+2. One calmodulin molecule has 4 calcium-binding sites. After interaction with Ca+2, conformational changes occur in the calmodulin molecule and the “Ca+2-calmodulin” complex becomes capable of regulating the activity (allosterically inhibiting or activating) many enzymes - adenylate cyclase, phosphodiesterase, Ca+2,Mg+2-ATPase and various protein kinases.

In different cells under the influence of the complex "Ca+2-calmodulin" on isoenzymes of the same enzyme (for example, on different types of adenylate cyclase), in some cases activation is observed, and in others, inhibition of the cAMP formation reaction is observed. Such various effects occur because the allosteric centers of isoenzymes may include different amino acid radicals and their reaction to the action of the Ca+2-calmodulin complex will be different.

Thus, in the role "second intermediaries" to transmit signals from hormones in target cells there may be: cyclic nucleotides (c-AMP and c-GMP); Ca ions; complex "Ca-calmodulin"; diacylglycerol; inositol triphosphate.

The mechanisms for transmitting information from hormones inside target cells using the listed intermediaries have common features : one of the stages of signal transmission is protein phosphorylation; cessation of activation occurs as a result of special mechanisms initiated by the process participants themselves - there are negative feedback mechanisms.

Hormones are the main humoral regulators of the physiological functions of the body, and their properties, biosynthesis processes and mechanisms of action are now well known. Hormones are highly specific substances in relation to target cells and have very high biological activity.

Endocrine glands – specialized organs that do not have excretory ducts and secrete secretions into the blood, cerebral fluid, and lymph through intercellular gaps.

Physiological role of endocrine glands is associated with their influence on the mechanisms of regulation and integration, adaptation, and maintaining the constancy of the internal environment of the body.

Classification of neurons

There is a wide variety of CNS neurons. Therefore, it is proposed various options their classifications. Most often, this classification is carried out according to three characteristics - morphological, functional and biochemical.

Morphological classification of neurons takes into account the number of processes in neurons and divides all neurons into three types - unipolar, bipolar and multipolar.

Unipolar neurons (from the Latin unus - one; synonyms - single-process, or unipolar, neurons) have one process. According to some researchers, neurons of this type are not found in the nervous system of humans and other mammals. However, some authors believe that unipolar neurons are observed in humans during early embryonic development, and in postnatal ontogenesis they are found in the mesencephalic nucleus of the trigeminal nerve (providing proprioceptive sensitivity of the masticatory muscles). A number of researchers classify amacrine neurons of the retina and interglomerular neurons of the olfactory bulb as unipolar cells.

Bipolar neurons (synonyms - bipolar, or bipolar, neurons) have two processes - an axon and a dendrite, usually extending from opposite poles of the cell. In the human nervous system, bipolar neurons themselves are found mainly in the peripheral parts of the visual, auditory and olfactory systems, for example, bipolar cells of the retina, spiral and vestibular ganglia. Bipolar neurons are connected by a dendrite to the receptor, and by an axon - to a neuron at the next level of organization of the corresponding sensory system.

However, much more often in the central nervous system of humans and other animals there is a type of bipolar neurons - the so-called pseudounipolar, or false unipolar, neurons. In them, both cell processes (axon and dendrite) extend from the cell body in the form of a single outgrowth, which is further divided in a T-shape into dendrite and axon: the first comes from the periphery of the receptors, the second goes to the central nervous system. These cells are found in the sensory spinal and cranial ganglia. They provide perception of pain, temperature, tactile, proprioceptive, baroreceptive and vibration signaling.

Multipolar neurons have one axon and many (2 or more) dendrites. They are most common in the human nervous system. Up to 60-80 variants of these cells have been described. However, they all represent varieties of spindle, stellate, basket, pyriform and pyramidal cells.

Based on the length of the axon, Golgi cells of type I (with a long axon) and Golgi cells of type II (with a short axon) are distinguished.

From the point of view of the localization of neurons, they can be divided into neurons of the central nervous system, i.e. located in the spinal cord (spinal neurons) and brain (bulbar, mesencephalic, cerebellar, hypothalamic, thalamic, cortical), as well as outside the central nervous system, i.e. included in the peripheral nervous system are the neurons of the autonomic ganglia, as well as the neurons that form the basis of the metasympathetic division of the autonomic nervous system.

Functional classification of neurons divides them according to the nature of the function they perform (in accordance with their place in the reflex arc) into three types: afferent (sensitive), efferent (motor) and associative.

1.Afferent neurons (synonyms - sensitive, receptor, centripetal), as a rule, are false unipolar nerve cells. The bodies of these neurons are not located in the central nervous system, but in the spinal ganglia or sensory ganglia of the cranial nerves. One of the processes extending from the body of the nerve cell follows to the periphery, to one or another organ and ends there with a sensory receptor that is capable of transforming the energy of an external stimulus (irritation) into a nerve impulse. The second process is directed to the central nervous system (spinal cord) as part of the dorsal roots of the spinal nerves or the corresponding sensory fibers of the cranial nerves. Typically, afferent neurons have small sizes and a dendrite well branched at the periphery. The functions of afferent neurons are closely related to the functions of sensory receptors. Thus, afferent neurons generate nerve impulses under the influence of changes in the external or internal environment

Some of the neurons involved in the processing of sensory information, which can be considered as afferent neurons of the higher parts of the brain, are usually divided depending on the sensitivity to the action of stimuli into monosensory, bisensory and polysensory.

Monosensory neurons are located more often in the primary projection zones of the cortex and respond only to signals of their sensory function. For example, a significant part of the neurons in the primary visual area of ​​the cerebral cortex reacts only to light stimulation of the retina.

Monosensory neurons are divided functionally according to their sensitivity to different qualities of a single stimulus. Thus, individual neurons of the auditory zone of the cerebral cortex can respond to presentations of a tone of 1000 Hz and not respond to tones of a different frequency. They are called monomodal. Neurons that respond to two different tones are called bimodal; neurons that respond to three or more are called polymodal.

Bisensory neurons are more often located in the secondary zones of the cortex of any analyzer and can respond to signals from both their own and other sensory systems. For example, neurons in the secondary visual area of ​​the cerebral cortex respond to visual and auditory stimuli.

Polysensory neurons are most often neurons of the associative areas of the brain; they are able to respond to irritation of the auditory, visual, skin and other receptive systems.

2. Efferent neurons (synonyms - motor, motor, secretory, centrifugal, cardiac, vasomotor, etc.) are designed to transmit information from the central nervous system to the periphery, to the working organs. For example, efferent neurons of the motor zone of the cerebral cortex - pyramidal cells - send impulses to the alpha motor neurons of the anterior horns of the spinal cord, i.e. they are efferent for this part of the cerebral cortex. In turn, alpha motor neurons of the spinal cord are efferent to its anterior horns and send signals to the muscles.

In terms of their structure, efferent neurons are multipolar neurons, the bodies of which are located in the gray matter of the central nervous system (or on the periphery in vegetative nodes of various orders). The axons of these neurons continue as somatic or autonomic nerve fibers (peripheral nerves) to the corresponding working organs, including skeletal and smooth muscles, as well as numerous glands. The main feature of efferent neurons is the presence of a long axon with a high speed of excitation.

Efferent neurons of different parts of the cerebral cortex connect these parts with each other via arcuate connections. Such connections provide intrahemispheric and interhemispheric relationships. All descending tracts of the spinal cord (pyramidal, rubrospinal, reticulospinal, etc.) are formed by axons of efferent neurons of the corresponding parts of the central nervous system. Neurons of the autonomic nervous system, for example, the nuclei of the vagus nerve, the lateral horns of the spinal cord also belong to efferent neurons.

3. Insert neurons (synonyms - interneurons, contact, associative, communicative, connecting, closing, conductor, conductor) transmit nerve impulses from an afferent (sensitive) neuron to an efferent (motor) neuron. The essence of this process is to transmit the signal received by the afferent neuron to the efferent neuron for execution in the form of a response from the body. I. P. Pavlov defined the essence of this as “the phenomenon of nervous closure.”

Interneurons are located within the gray matter of the central nervous system. By their structure, these are multipolar neurons. It is believed that functionally these are the most important neurons of the central nervous system, since they account for 97%, and according to some data, even 98-99% of total number neurons of the central nervous system. The area of ​​influence of interneurons is determined by their structure, including the length of the axon and the number of collaterals. For example, many interneurons have axons that end on the neurons of their own center, ensuring, first of all, their integration.

Some interneurons receive activation from neurons in other centers and then distribute this information to neurons in their center. This ensures an increase in the influence of the signal due to its repetition in parallel paths and lengthens the time of storing information in the center. As a result, the center where the signal arrived increases the reliability of the influence on the executive structure.

Other interneurons receive activation from collaterals of efferent neurons in their own center and then transmit this information back to their own center, forming feedbacks. This is how reverberating networks are organized, allowing information to be stored in the nerve center for a long time.

According to their function, interneurons can be exciting or brake. In this case, excitatory neurons can not only transmit information from one neuron to another, but also modify the transmission of excitation, in particular, enhance its effectiveness. For example, in the cerebral cortex there are “slow” pyramidal neurons that influence the activity of “fast” pyramidal neurons.

Obviously, among the interneurons one can also distinguish command neurons, pacemaker neurons, hormone-producing neurons (for example, neurons of the tuberoinfundibular region of the hypothalamus), need-motivational, gnostic and many other types of neurons.

Biochemical the classification of neurons is based on the chemical characteristics of the neurotransmitters used by neurons in the synaptic transmission of nerve impulses. There are many different groups of neurons, in particular, cholinergic (mediator - acetylcholine), adrenergic (mediator - norepinephrine), serotonergic (mediator - serotonin), dopaminergic (mediator - dopamine), GABAergic (mediator - gamma-aminobutyric acid - GABA) , purinergic (mediator - ATP and its derivatives), peptidergic (mediators - substance P, enkephalins, endorphins, vasoactive intestinal peptide, cholecystokinin, neurotensin, bombesin and other neuropeptides). In some neurons, the terminals simultaneously contain two types of neurotransmitter, as well as neuromodulators.

The distribution of neurons using different transmitters in the nervous system is uneven. Impaired production of certain mediators in certain brain structures is associated with the pathogenesis of a number of neuropsychiatric diseases. Thus, the content of dopamine is reduced in parkinsonism and increased in schizophrenia, a decrease in the level of norepinephrine and serotonin is typical for depressive states, and their increase is typical for manic states.

Hormone-producing neurons can also be divided into groups, depending on the nature of the hormone they produce (corticoliberin-, gonadoliberin-, thyroliberin-producing, prolactostatin-producing and others).

Other types of neuron classifications. Nerve cells of different parts of the nervous system can be active without influence, i.e. have the property of automaticity. They are called background active neurons. Other neurons exhibit impulse activity only in response to some kind of stimulation, i.e. they have no background activity.

Some neurons, due to their special significance in brain activity, received additional names after the researcher who first described the corresponding neurons. Among them are Betz pyramidal cells, localized in the neocortex; pyriform Purkinje cells, Golgi cells, Lugano cells (all in the cerebellar cortex); inhibitory Renshaw cells (spinal cord) and a number of other neurons.

The functions of a neuron as a whole formation are to provide information processes in the central nervous system, including with the help of transmitter substances (neurotransmitters). Neurons, as specialized cells, receive, encode, process, store and transmit information. Neurons form control (regulatory) commands for various internal organs and for skeletal muscles (due to which various locomotions are performed), and also ensure the implementation of all forms of mental activity - from elementary to the most complex, including thinking and speech. All this is ensured due to the unique ability of the neuron to generate electrical discharges and transmit information using specialized endings - synapses. However, the implementation of all neuron functions is possible only with working together neurons. Therefore, the decisive point in the activity of a neuron is its ability to generate action potentials, as well as its ability to perceive action potentials and transmitters from other neurons and transmit the necessary information to other neurons. All this is especially clearly manifested in the case when the neuron is a component of neural associations, in particular, an integral part of the reflex arc (see below). The implementation of the information function occurs with the participation of all parts of the neuron - dendrites, perikaryon and axon. At the same time, dendrites, together with the perikaryon, specialize in the perception of information, axons (together with the axon hillock of the perikaryon) specialize in transmitting information, and the perikaryon specializes in decision making (in in a broad sense this word). In addition, the body of the neuron (soma, or perikaryon), in addition to the informational one, performs a trophic function in relation to its processes and their synapses. Transection of an axon or dendrite leads to the death of processes lying distal to the transection, and, consequently, the synapses of these processes. The soma also ensures the growth of dendrites and axons.

Like all excitable cells, neurons have a membrane potential, the nature of which, as noted above, is mainly due to the nonequilibrium distribution of K + ions. For most neurons, the membrane potential reaches 50-70 mV. In background active neurons, i.e. possessing spontaneous activity, the value of the membrane potential periodically decreases (i.e., spontaneous depolarization is observed), as a result of which, when a critical level of depolarization is reached, an action potential is generated. However, most neurons generate action potentials only in response to a sensory stimulus. The threshold potential on average for the perikaryon is approximately 20-35 mV, for dendrites it is even higher, but in the area of ​​the axon hillock it is only 5-10 mV. Thus, the most excitable part of the perikaryon is the axon hillock. The action potentials of all neurons are characterized by a relatively small amplitude, which reaches 80-110 mV. The action potential in its shape (with intracellular abduction) is peak-shaped. It is characterized by the short duration of the spike (1-3 ms), the severity of trace hyperpolarization (this is especially typical for motor neurons of the spinal cord), as a result of which the excitability of the neuron often decreases. The duration of the absolute refractory phase for neurons is relatively short (within 2-3 ms), which provides relatively high level lability of neurons. At the same time, neurons are characterized by high fatigue, which indicates a relatively limited ability of neurons to recover. At the same time, it should be remembered that the long life span of a neuron, associated with the delayed onset of apoptosis, is to a certain extent ensured by the ability of neurons to timely, or rather, to stop their activity in advance, preventing the activation of apoptosis.

Action potential generation, in particular depolarization phase is explained by the entry of Na + ions from the extracellular environment into the neuron, and repolarization phase- the release of K + ions, as well as activation of the Na + -K + pump. Neurons also have calcium channels, which are more concentrated in the presynaptic membrane region of the axon terminals. It also contains a Ca 2+ pump, which ensures the removal of calcium ions from the presynaptic terminal into the extracellular environment. The concentration of Ca 2+ ions in the extracellular environment is the most important mechanism for regulating neuron excitability. An increase in the level of Ca 2+ in the blood (to certain values) reduces it, and a decrease leads to an excessive increase in excitability, which is often accompanied by the appearance of spontaneous generation of action potentials and the occurrence of a convulsive state. This dependence of excitability on Ca 2+ ions is associated with the presence of calcium channels in the perikaryon membrane, as well as Ca 2+ -dependent potassium channels. When the intracellular concentration of Ca 2+ ions increases in a neuron, this causes activation of Ca 2+ -dependent potassium channels, which increases permeability to K + ions. The consequence of this is the development of pronounced trace hyperpolarization, which is observed during the repolarization phase. It is important to note that trace hyperpolarization itself plays an important role in the activity of the neuron. This is due to the fact that in response to long-term depolarization, which can occur under the influence of a series of impulses arriving at neurons, the neuron usually generates not a single potential, but a series of action potentials. The pulse repetition rate in this series is determined by the magnitude of the trace hyperpolarization - the higher it is, the greater the interval between adjacent action potentials, i.e. the less often they are generated. This is why, for example, the maximum excitation rhythm in spinal cord motor neurons, in which the hyperpolarization phase lasts 100-150 ms, is only 40-50 Hz. At the same time, neurons in which the duration of the hyperpolarization phase is short (for example, some interneurons) can produce bursts of discharges with a frequency of up to 1000 Hz.

The mechanism for maintaining the concentration of K + ions in the intercellular environment is important for the physiology of the neuron. This is due to the fact that in the CNS, neurons and their processes are surrounded by narrow gap-like extracellular spaces (the width of the gap usually does not exceed 15 nm). Therefore, during the generation of an action potential, the concentration of K + ions in these spaces can increase significantly (instead of 4-5 mM, it can reach 10 mM), which will lead to disruption of neuron activity, up to the generation of convulsive discharges. In order to prevent this process, neuroglial cells, in particular astrocytes, take on the function of regulating the content of ions in the extracellular space. In particular, when there is an excess content of K + ions in the extracellular space, glial cells absorb them, and when their content is insufficient, they release these ions. Thus, astrocytes function as a buffer system in relation to K + ions, Ca 2+ and, probably, other ions.

Numerous dendrites and the plasma membrane of the perikaryon are rich in chemoreceptors, due to which signals transmitted through synapses are perceived. Each neuron has a large number of synapses, taking into account the total number of neurons in humans, which is approximately 10 11 (in this case, the total number of synaptic contacts between neurons, as mentioned above, approaches the astronomical figure of 10 15) provides the possibility of storing up to 10 19 in the central nervous system units of information. This amount of information is equivalent to almost all the knowledge accumulated by humanity to date.

It is also important to note that due to the interaction of the transmitter with the receptor on the postynaptic membrane of the neuron, two processes can occur - depolarization (excitatory postsynaptic potential) and hyperpolarization (inhibitory postsynaptic potential). These processes are integrated in space and time (respectively, spatial and temporal summation) on the membrane of the neuron and thereby either generate the generation of AP on the axon hillock, or, conversely, increase the MP (membrane potential) and thereby prevent the excitation of the neuron. This phenomenon, called synaptic interaction, plays an extremely important role in the activity of the neuron.

Regarding such a property of a neuron as conductivity, it should be emphasized that all its components - perikaryon, dendrites and axon - are capable of conducting an impulse. In this case, for the dendrite and, especially, for the axon, the conduction of excitation is the main function. As a rule, the neuron is dynamically polarized, i.e. capable of conducting a nerve impulse in only one direction - from the dendrite through the cell body to the axon. This phenomenon is called orthodromic spread of excitement. In some cases it is possible antidromic propagation of excitation, i.e. from the axon to the perikaryon and dendrites. In this aspect, it is important to note that thanks to collaterals and the presence of inhibitory interneurons, a number of CNS neurons can carry out the so-called return self-braking- during the generation of AP, excitation from neuron A spreads along the axon to another neuron or organ, but at the same time excitation along collaterals reaches the inhibitory neuron. Its activation leads to inhibition of neuron A.

From a functional point of view, a neuron can be in three main states - 1) in a state of rest, 2) in a state of activity, or excitation, and 3) in a state of inhibition.

1). At rest, the neuron has a stable level of membrane potential. At any moment, the neuron is ready to excite, i.e. generate an action potential, or go into a state of inhibition.

2). In a state of activity, i.e. when excited, a neuron generates an action potential or, more often, a group of action potentials (a series of action potentials, a burst of action potentials, a burst of excitation). The frequency of action potentials within a given series of action potentials, the duration of this series, as well as the duty cycle (intervals) between successive series - all these indicators vary widely and are a component of the neuron code. It was already noted above that Ca 2+ and K + ions play an important role in regulating the frequency of impulses.

Most often the state of activity is induced. This occurs due to the receipt of impulses to the neuron from other neurons. For some neurons, the active state arises spontaneously, i.e. automatically, and most often the automation of a neuron is manifested by the periodic generation of a series of impulses. An example of such neurons is pacemakers, i.e. The pacemakers are the neurons of the respiratory center of the medulla oblongata.

Often such neurons are called background active neurons. According to the nature of the reaction to incoming impulses, they are divided into inhibitory and excitatory. Inhibitory neurons reduce their background firing rate in response to external signal, and excited ones increase the frequency of background activity.

There are at least three types of background neuronal activity - continuous-arrhythmic, burst and group.

Continuously arrhythmic the type of activity is manifested in the fact that background active neurons generate impulses continuously with some slowdown or increase in the frequency of discharges. Such neurons usually provide tone to the nerve centers. Background active neurons have great importance in maintaining the level of excitation of the cortex and other brain structures. The number of background active neurons increases during wakefulness.

Pachechny The type of activity is that neurons produce a group of impulses with a short interpulse interval, after which there is a period of silence, and then a burst of impulses is generated again. Typically, the interpulse intervals in a burst are approximately 1-3 ms, and the interval between PD bursts is 15-120 ms. It is believed that this type of activity creates conditions for the transmission of signals while reducing the functionality of the conducting or perceptive structures of the brain.

Group the form of activity is characterized by the aperiodic appearance of a group of pulses (interpulse intervals range from 3 to 30 ms), followed by a period of silence.

3). The state of inhibition is manifested in the fact that a background-active neuron or a neuron receiving an exciting influence from the outside stops its impulse activity. A neuron can also enter a state of inhibition from a resting state. In all cases, inhibition is based on the phenomenon of hyperpolarization of the neuron (this is characteristic of postsynaptic inhibition) or the active cessation of incoming impulses from other neurons, which is observed under conditions of presynaptic inhibition.

An idea of ​​the role of incoming information for a neuron. The incoming information received by the dendrites is processed in the body of the neuron, launching a series of metabolic (metabolic) processes. Some of these processes are necessary to maintain the life of the neuron. Another part of the induced metabolic processes is converted into a response in the form of the generation of action potentials going to the target organ or to another neuron in the form of a series of impulses of a certain frequency. The third part of the processes is necessary to create a kind of buffer in the neuron to ensure the constancy of the output of action potentials from the neuron during quantitative fluctuations in the input. With a persistent increase in the number of received impulses, the accumulated reserve becomes excessive; accordingly, the axon increases the frequency of its impulses, but not gradually, but spasmodically, as if jumping to new level activity, the same relatively constant as the previous one. If the overload is not eliminated, then further abrupt increases in the pulse frequency are possible, and then an increase in the pulse power. If there is a lack of incoming stimuli, the accumulated reserve is exhausted first - the neuron tries to maintain the constancy of the response mode, i.e. output pulse. With a persistent and significant decrease in intake, the “reserves” are exhausted, and abrupt changes in the frequency of axonal impulses occur, only in the reverse order - towards a decrease. A decrease in the number of input stimuli below a certain critical level leads to the fact that the neuron not only cannot organize a response, but also does not have the resources to fully support its own vital functions. Complete blocking of input impulses leads to the death of the neuron. The stated hypothesis is to a certain extent consistent with the idea of ​​G. Sorokhtin (60s of the XX century) about negative impact on the activity of neurons with a deficit of incoming information (the hypothesis of a deficit of excitation).

The leading reason that distinguishes the human brain from the brains of other representatives of the animal world is the quantitative composition of brain neurons and the nature of their association.

In general, depending on the tasks and responsibilities assigned to neurons, they are divided into three categories:

- Sensory neurons receive and transmit impulses from receptors “to the center”, i.e. central nervous system. Moreover, the receptors themselves are specially trained cells of the sensory organs, muscles, skin and joints that can detect physical or chemical changes inside and outside our body, convert them into impulses and joyfully transmit them to sensory neurons. Thus, signals travel from the periphery to the center.

Next type:

- Motor (motor) neurons, which rumbling, fircha and beeping, carry signals coming from the brain or spinal cord to the executive organs, which are muscles, glands, etc. Yeah, that means the signals go from the center to the periphery.

well and intermediate (intercalary) neurons, simply put, they are “extension cords”, i.e. receive signals from sensory neurons and send these impulses further to other intermediate neurons, or directly to motor neurons.

In general, this is what happens: in sensory neurons, dendrites are connected to receptors, and axons are connected to other neurons (interneurons). In motor neurons, on the contrary, dendrites are connected to other neurons (interneurons), and axons are connected to some effector, i.e. a stimulator of muscle contraction or gland secretion. Well, accordingly, interneurons have both dendrites and axons connected to other neurons.

It turns out that the simplest path along which a nerve impulse can travel will consist of three neurons: one sensory, one intercalary and one motor.

Yeah, now let's remember the guy - a very “nervous pathologist”, with a malicious smile, knocking his “magic” hammer on his knee. Sound familiar? Now, this is the simplest reflex: when it hits the knee tendon, the muscle attached to it stretches and the signal from the sensory cells (receptors) located in it is transmitted along sensory neurons to the spinal cord. And already in it, sensory neurons contact either through intercalary or directly with motor neurons, which in response send impulses back to the same muscle, causing it to contract and the leg to straighten.

The spinal cord itself is conveniently nestled inside our spine. It is soft and vulnerable, which is why it hides in the vertebrae. The spinal cord is only 40-45 centimeters in length, as thick as a little finger (about 8 mm) and weighs some 30 grams! But, despite all its frailty, the spinal cord is the control center of a complex network of nerves spread throughout the body. Almost like a mission control center! :) Without it, neither the musculoskeletal system nor the main vital organs can function and work.

The spinal cord originates at the level of the edge of the occipital foramen of the skull and ends at the level of the first and second lumbar vertebrae. But below the spinal cord in the spinal canal there is such a dense bundle of nerve roots, funny called the cauda equina, apparently for its resemblance to it. So, the cauda equina is a continuation of the nerves coming out of the spinal cord. They are responsible for the innervation of the lower extremities and pelvic organs, i.e. transmit signals from the spinal cord to them.

The spinal cord is surrounded by three membranes: soft, arachnoid and hard. And the space between the soft and arachnoid membranes is also filled with a large amount of cerebrospinal fluid. Through the intervertebral foramina, spinal nerves depart from the spinal cord: 8 pairs of cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 or 2 coccygeal. Why steam? Yes, because the spinal nerve exits through two roots: posterior (sensitive) and anterior (motor), connected into one trunk. So, each such pair controls a certain part of the body. That is, for example, if you accidentally grabbed a hot pan (God forbid! Pah-pah-pah!), then a pain signal immediately arises in the endings of the sensory nerve, immediately entering the spinal cord, and from there - into paired motor nerve, which transmits the order: “Akhtung-akhtung! Remove your hand immediately!” Moreover, believe me, this happens very quickly - even before the brain registers the pain impulse. As a result, you manage to pull your hand away from the pan before you feel pain. Of course, this reaction saves us from severe burns or other damage.

In general, almost all of our automatic and reflex actions are controlled by the spinal cord, well, with the exception of those that are monitored by the brain itself. Well, for example: we perceive what we see with the help of the optic nerve going to the brain, and at the same time we turn our gaze to different sides with the help of the eye muscles, which are controlled by the spinal cord. Yes, and we cry the same on the orders of the spinal cord, which “manages” the lacrimal glands.

We can say that our conscious actions come from the brain, but as soon as we begin to perform these actions automatically and reflexively, they are transferred to the spinal cord. So, when we are just learning to do something, then, of course, we consciously think about and think through and comprehend every movement, which means we use the brain, but over time we can already do it automatically, and this means that the brain transfers the “reins of power” of this action to the spinal one, it’s just that he has already become bored and uninteresting... because our brain is very inquisitive, inquisitive and loves to learn!

Well, it’s time for us to get curious......