Receptors are presented. Educational portal. Receptors and their role in the human body

• By position
  • Exteroceptors: located on or near the surface of the body and perceive external stimuli
  • Interoreceptors are located in the internal organs and perceive internal stimuli
    • Proprioceptors receptors of the musculoskeletal system, allowing you to determine, for example, tension and the degree of stretching of muscles and tendons. They are a type of interoreceptors.

• Ability to perceive different stimuli
  • Monomodal responding to only one type of stimulus
  • Polymodal responding to several types of stimuli.

• According to an adequate stimulus
  • Chemoreceptors sense the effects of dissolved or volatile chemicals.
  • Osmoreceptors perceive changes in the osmotic concentration of the liquid.
  • Mechanoreceptors perceive mechanical stimuli
  • Photoreceptors perceive visible and ultraviolet light
  • Thermoreceptors perceive a decrease or increase in temperature
  • Pain receptors, the stimulation of which leads to pain. There is no such physical stimulus as pain, so separating them into a separate group based on the nature of the stimulus is to some extent arbitrary. In fact, they are high-threshold sensors of various damaging factors. However, a unique feature of nociceptors, which does not allow them to be classified, for example, as “high-threshold thermoreceptors,” is that many of them are polymodal: the same nerve ending can be excited in response to several different damaging stimuli.
  • Electroreceptors perceive changes in the electric field
  • Magnetic receptors perceive changes in the magnetic field

Humans have the first six types of receptors. Taste and smell are based on chemoreception, touch, hearing and balance are based on mechanoreception, as well as sensations of body position in space, and vision is based on photoreception. Thermoreceptors are found in the skin and some internal organs. Most interoreceptors trigger involuntary, and in most cases unconscious, autonomic reflexes. Thus, osmoreceptors are included in the regulation of kidney activity, chemoreceptors that perceive pH, concentrations of carbon dioxide and oxygen in the blood are included in the regulation of respiration, etc.

Sometimes it is proposed to distinguish a group of electromagnetic receptors, which includes photo-, electro- and magnetoreceptors. Magnetoreceptors have not been precisely identified in any group of animals, although they are believed to be some cells in the retina of birds, and possibly a number of other cells.

The table shows data on some types of receptors

Nature of the stimulus	Receptor type	Location and comments
electric field	Ampulla of Lorenzini and other types	Available in fish, cyclostomes, amphibians, as well as the platypus and echidna
Chemical substance	chemoreceptor
humidity	hygroreceptor	They belong to osmoreceptors or mechanoreceptors. Located on the antennae and mouthparts of many insects
mechanical impact	mechanoreceptor	In humans, they are present in the skin and internal organs
pressure	baroreceptor	Refers to mechanoreceptors
body position	proprioceptor	They belong to mechanoreceptors. In humans, these are neuromuscular spindles, Golgi tendon organs, etc.
osmotic pressure	osmoreceptor	Mainly interoreceptors; in humans, they are present in the hypothalamus, and also, probably, in the kidneys, the walls of the gastrointestinal tract, and possibly in the liver. There is evidence of a wide distribution of osmoreceptors in all tissues of the body
light	photoreceptor
temperature	thermoreceptor	React to temperature changes. In humans, they are present in the skin and hypothalamus
tissue damage	nociceptor	In most tissues with different frequencies. Pain receptors are free nerve endings of unmyelinated type C fibers or weakly myelinated type Aδ fibers.
a magnetic field	magnetic receptors	The exact location and structure are unknown, but their presence in many groups of animals has been proven by behavioral experiments.

Receptor called a specialized cell, evolutionarily adapted to the external or internal environment of a certain stimulus and to convert its energy from a physical or chemical form into a nervous form.

CLASSIFICATION OF RECEPTORS

The classification of receptors is based primarily on on the nature of sensations that arise in humans when they are irritated. Distinguish visual, auditory, olfactory, tactile receptors, thermoreceptors, proprioceptors and vestibuloreceptors (receptors for the position of the body and its parts in space). The question of the existence of special receptors .

Receptors by location divided into external , or exteroceptors, And internal , or interoreceptors. Exteroceptors include auditory, visual, olfactory, taste and tactile receptors. Interoreceptors include vestibuloreceptors and proprioceptors (receptors of the musculoskeletal system), as well as interoreceptors that signal the state of internal organs.

By the nature of contact with the external environment receptors are divided into distant receiving information at a distance from the source of stimulation (visual, auditory and olfactory), and contact – excited by direct contact with a stimulus (gustatory and tactile).

Depending on the nature of the type of perceived stimulus , to which they are optimally tuned, there are five types of receptors.

• Mechanoreceptors are excited by their mechanical deformation; located in the skin, blood vessels, internal organs, musculoskeletal system, auditory and vestibular systems.
• Chemoreceptors perceive chemical changes in the external and internal environment of the body. These include taste and olfactory receptors, as well as receptors that respond to changes in the composition of blood, lymph, intercellular and cerebrospinal fluid (changes in O 2 and CO 2 tension, osmolarity and pH, glucose levels and other substances). Such receptors are found in the mucous membrane of the tongue and nose, carotid and aortic bodies, and medulla oblongata.
• Thermoreceptors react to temperature changes. They are divided into heat and cold receptors and are found in the skin, mucous membranes, blood vessels, internal organs, hypothalamus, middle, oblongata and.
• Photoreceptors The retina of the eye perceives light (electromagnetic) energy.
• Nociceptors , the excitation of which is accompanied by painful sensations (pain receptors). The irritants of these receptors are mechanical, thermal and chemical (histamine, bradykinin, K + , H +, etc.) factors. Painful stimuli are perceived by free nerve endings, which are found in the skin, muscles, internal organs, dentin, and blood vessels. From a psychophysiological point of view, receptors are divided in accordance with the sensations formed into visual, auditory, gustatory, olfactory And tactile.

Depending on the structure of the receptors they are divided into primary , or primary sensory, which are specialized endings of the sensory, and secondary , or secondary sensory cells, which are cells of epithelial origin capable of forming a receptor potential in response to the action of adequate.

Primary sensory receptors can themselves generate action potentials in response to stimulation by an adequate stimulus if the magnitude of their receptor potential reaches a threshold value. These include olfactory receptors, most skin mechanoreceptors, thermoreceptors, pain receptors or nociceptors, proprioceptors and most interoreceptors of internal organs. The neuron body is located in the spinal cord or ganglion. In the primary receptor, the stimulus acts directly on the endings of the sensory neuron. Primary receptors are phylogenetically more ancient structures; they include olfactory, tactile, temperature, pain receptors and proprioceptors.

Secondary sensory receptors respond to the action of a stimulus only by the appearance of a receptor potential, the magnitude of which determines the amount of mediator released by these cells. With its help, secondary receptors act on the nerve endings of sensitive neurons, generating action potentials depending on the amount of mediator released from the secondary receptors. In secondary receptors there is a special cell synaptically connected to the end of the dendrite of the sensory neuron. This is a cell, such as a photoreceptor, of epithelial nature or neuroectodermal origin. Secondary receptors are represented by taste, auditory and vestibular receptors, as well as chemosensitive cells of the carotid glomerulus. Retinal photoreceptors, which have a common origin with nerve cells, are often classified as primary receptors, but their lack of ability to generate action potentials indicates their similarity to secondary receptors.

By speed of adaptation receptors are divided into three groups: quickly adaptable (phase), slow to adapt (tonic) and mixed (phasotonic), adapting at an average speed. An example of rapidly adapting receptors are the vibration (Pacini corpuscles) and touch (Meissner corpuscles) receptors on the skin. Slowly adapting receptors include proprioceptors, lung stretch receptors, and pain receptors. Retinal photoreceptors and skin thermoreceptors adapt at an average speed.

Most receptors are excited in response to stimuli of only one physical nature and therefore belong to monomodal . They can also be excited by some inappropriate stimuli, for example, photoreceptors - by strong pressure on the eyeball, and taste buds - by touching the tongue to the contacts of a galvanic battery, but in such cases it is impossible to obtain qualitatively distinct sensations.

Along with monomodal there are multimodal receptors, the adequate stimuli of which can be irritants of different nature. This type of receptor includes some pain receptors, or nociceptors (Latin nocens - harmful), which can be excited by mechanical, thermal and chemical stimuli. Thermoreceptors have polymodality, reacting to an increase in potassium concentration in the extracellular space in the same way as to an increase in temperature.

The article talks about what receptors are, why they serve humans, and, in particular, discusses the topic of receptor antagonists.

Biology

Life on our planet has existed for almost 4 billion years. During this period, incomprehensible to human perception, many things have changed on it and, probably, this process will continue forever. But if we consider any biological organism from a scientific point of view, then its structure, coherence and, in general, the very fact of existence are amazing, and this applies to even the simplest species. And there’s nothing to say about the human body! Any area of ​​its biology is unique and interesting in its own way.

In this article we will look at what receptors are, why they are needed and what they are. We will try to understand this in as much detail as possible.

Action

According to the encyclopedia, a receptor is a combination of the endings of nerve fibers in some neurons that are distinguished by sensitivity, and specific formations and special cells of living tissues. Together they are engaged in transforming the influence of factors of various kinds, which are often called stimuli, into a special one. Now we know what a receptor is.

Some types of human receptors perceive information and influence through special cells of epithelial origin. In addition, modified nerve cells also take part in processing information about stimuli, but their difference is that they cannot generate nerve impulses themselves, but only act on the innervating endings. For example, this is how taste buds work (they are located in the epithelium on the surface of the tongue). Their action is based on chemoreceptors, which are responsible for sensing and processing the effects of chemical or volatile substances.

Now we know what they are and how they work.

Purpose

Simply put, receptors are responsible for the functioning of almost all senses. And in addition to the most obvious ones, such as vision or hearing, they enable a person to sense other phenomena: pressure, temperature, humidity, etc. So we looked at the question of what receptors are. But let's look at them in more detail.

Stimuli that activate certain receptors can be very different effects and actions, for example, deformation of a mechanical property (wounds and cuts), aggression of chemicals, and even an electric or magnetic field! True, which receptors are responsible for the perception of the latter has not yet been precisely established. We only know that they definitely exist, but they are developed differently in everyone.

Kinds

They are divided into types according to their location in the body and the irritant, thanks to which we receive signals to the nerve endings. Let us consider in more detail the adequate stimulus:

• Chemoreceptors are responsible for taste and smell; their work is based on the effects of volatile and other chemicals.
• Osmoreceptors - are involved in determining changes in osmotic fluid, i.e., increase or decrease (this is something like the balance between extracellular and intracellular fluids).
• Mechanoreceptors - receive signals based on physical influence.
• Photoreceptors - thanks to them our eyes receive the visible spectrum of light.
• Thermoreceptors are responsible for sensing temperature.
• Pain receptors.

receptors?

To put it simply, these are substances that can bind to receptors, but do not change the course of their work. An agonist, on the contrary, not only binds, but also actively influences the receptor. For example, the latter include some narcotic substances used for anesthesia. They desensitize the receptor. If they are called partial, then their action is incomplete.

Receptors are divided into external, or exteroceptors, and internal, or interoreceptors. Exteroceptors are located on the outer surface of the animal or human body and perceive stimuli from the outside world (light, sound, thermal, etc.). Interoceptors are found in various tissues and internal organs (heart, lymphatic and blood vessels, lungs, etc.); perceive stimuli signaling the state of internal organs (visceroceptors), as well as the position of the body or its parts in space (vestibuloceptors). A type of interoceptors are proprioceptors located in muscles, tendons and ligaments and perceive the static state of muscles and their dynamics. Depending on the nature of the perceived adequate stimulus, there are mechanoreceptors, photoreceptors, chemoreceptors, thermoreceptors, etc. Receptors sensitive to ultrasound have been found in dolphins, bats and moths, and in some fish - to electric fields. Less studied is the existence of receptors sensitive to magnetic fields in some birds and fish. Monomodal receptors perceive stimulation of only one type (mechanical, light or chemical); among them are receptors that differ in the level of sensitivity and relation to the irritating stimulus. Thus, vertebrate photoreceptors are divided into more sensitive rod cells, which function as receptors for twilight vision, and less sensitive cone cells, which provide daytime light perception and color vision in humans and a number of animals; skin mechanoreceptors - more sensitive phase receptors that respond only to the dynamic phase of deformation, and static receptors that also respond to constant deformation, etc. As a result of this specialization, the receptors highlight the most significant properties of the stimulus and carry out a subtle analysis of the perceived irritations. Polymodal receptors respond to stimuli of different qualities, for example chemical and mechanical, mechanical and thermal. In this case, specific information encoded in molecules is transmitted to the central nervous system along the same nerve fibers in the form of nerve impulses, undergoing repeated energy amplification along the way. Historically, the division of receptors has been preserved into distant (visual, auditory, olfactory), which perceive signals from a source of irritation located at some distance from the body, and contact - in direct contact with the source of irritation. There are also primary (primary-sensing) and secondary (secondary-sensing) receptors. In primary receptors, the substrate that perceives external influences is embedded in the sensory neuron itself, which is directly (primarily) excited by the stimulus. In secondary receptors, between the active agent and the sensory neuron there are additional, specialized (receptive) cells in which the energy of external stimuli is converted (transformed) into nerve impulses.

All receptors are characterized by a number of common properties. They are specialized for the reception of certain irritations characteristic of them, called adequate. When stimulation occurs in the receptors, a change in the difference in bioelectric potentials on the cell membrane occurs, the so-called receptor potential, which either directly generates rhythmic impulses in the receptor cell or leads to their occurrence in another neuron connected to the receptor through a synapse. The frequency of impulses increases with increasing intensity of stimulation. With prolonged exposure to the stimulus, the frequency of impulses in the fiber extending from the receptor decreases; This phenomenon of decreasing receptor activity is called physiological adaptation. For different receptors, the time of such adaptation is not the same. Receptors are distinguished by high sensitivity to adequate stimuli, which is measured by the absolute threshold, or the minimum intensity of stimulation that can bring the receptors into a state of excitation. So, for example, 5-7 quanta of light falling on the eye receptor cause a light sensation, and 1 quanta is enough to excite an individual photoreceptor. The receptor can also be excited by an inadequate stimulus. By applying an electric current, for example, to the eye or ear, one can induce the sensation of light or sound. Sensations are associated with the specific sensitivity of the receptor, which arose during the evolution of organic nature. The figurative perception of the world is associated primarily with information coming from exteroceptors. Information from interoceptors does not lead to clear sensations. The functions of various receptors are interrelated. The interaction of vestibular receptors, as well as skin receptors and proprioceptors with the visual ones, is carried out by the central nervous system and underlies the perception of the size and shape of objects, their position in space. Receptors can interact with each other without the participation of the central nervous system, that is, due to direct communication with each other. Such interaction, established on visual, tactile and other receptors, is important for the mechanism of spatiotemporal contrast. The activity of the receptors is regulated by the central nervous system, which adjusts them depending on the needs of the body. These influences, the mechanism of which has not been sufficiently studied, are carried out through special efferent fibers that approach certain receptor structures.

The functions of the receptors are studied by recording bioelectric potentials directly from the receptors or associated nerve fibers, as well as by recording reflex reactions that occur when the receptors are irritated.

Pharmacological receptors (RF), cellular receptors, tissue receptors, located on the membrane of the effector cell; perceive regulatory and trigger signals of the nervous and endocrine systems, the action of many pharmacological drugs that selectively affect this cell, and transform these effects into its specific biochemical or physiological reaction. The most studied are the RFs through which the action of the nervous system is carried out. The influence of the parasympathetic and motor parts of the nervous system (the mediator acetylcholine) is transmitted by two types of RF: N-cholinoceptors transmit nerve impulses to skeletal muscles and in the nerve ganglia from neuron to neuron; M-cholinergic receptors are involved in the regulation of heart function and smooth muscle tone. The influence of the sympathetic nervous system (transmitter norepinephrine) and the hormone of the adrenal medulla (adrenaline) is transmitted by alpha and beta adrenoceptors. Excitation of alpha adrenoceptors causes vasoconstriction, a rise in blood pressure, pupil dilation, contraction of a number of smooth muscles, etc.; stimulation of beta-adrenoceptors - increased blood sugar, activation of enzymes, vasodilation, relaxation of smooth muscles, increased frequency and strength of heart contractions, etc. Thus, the functional effect is carried out through both types of adrenoceptors, and the metabolic effect is carried out mainly through beta-adrenoceptors. RFs have also been discovered that are sensitive to dopamine, serotonin, histamine, polypeptides and other endogenous biologically active substances and to pharmacological antagonists of some of these substances. The therapeutic effect of a number of pharmacological drugs is due to their specific action on specific receptors.

15. Conversion of stimulus energy in receptors. Receptor and generator potentials. Weber-Fechner law. Absolute and differential sensitivity thresholds.

As a result of the action adequate stimulus For most receptors, the permeability of the cell membrane for cations increases, which leads to its depolarization. An exception to the general rule are photoreceptors, where, after absorbing the energy of light quanta, hyperpolarization of the membrane occurs due to the control features of ion channels. The change in the membrane potential of receptors in response to a stimulus is receptor potential- input signal of primary sensory neurons. If the magnitude of the receptor potential reaches or exceeds a critical level of depolarization, action potentials are generated, with the help of which sensory neurons transmit information about current stimuli to the central nervous system.

Generation of action potentials occurs in the node of Ranvier of myelinated fibers closest to the receptors or the part of the membrane of the non-myelinated fiber closest to the receptors. Minimum strength of adequate stimulus, sufficient to generate action potentials in the primary sensory neuron, is defined as its absolute threshold. Minimum gain stimulus strength, accompanied by a significant change in the response of the sensory neuron, represents the differential threshold of its sensitivity.

Weber (1831) and Fechner (1860) proved the relationship between the absolute threshold and the differential threshold of the stimulus.

J- initial stimulus

JD - increase in irritation

K-constant

Information about the strength of the stimulus acting on the receptors encoded in two ways: the frequency of action potentials arising in a sensory neuron (frequency coding), and the number of sensory neurons excited in response to a stimulus. With an increase in the strength of the stimulus acting on the receptors the amplitude of the receptor potential increases, which, as a rule, is accompanied by an increase in the frequency of action potentials in the first-order sensory neuron. The wider the available frequency range of action potentials in sensory neurons, the greater the number of intermediate values ​​of stimulus strength that the sensory system is able to distinguish. Primary sensory neurons of the same modality differ in their excitation threshold, therefore, when exposed to weak stimuli, only the most sensitive neurons are excited, but with an increase in the strength of the stimulus, less sensitive neurons with a higher stimulation threshold also respond to it. The more primary sensory neurons are excited simultaneously, the stronger their joint effect on the common second-order neuron will be, which will ultimately affect the subjective assessment of the intensity of the current stimulus.

Duration of sensation depends on the actual time between the onset and cessation of influence on the receptors, as well as on their ability to reduce or even stop the generation of nerve impulses with prolonged exposure to an adequate stimulus. With prolonged stimulus receptor sensitivity threshold to it can increase, which is defined as receptor adaptation. Adaptation Mechanisms are not the same in receptors of different modalities, they are quickly distinguished adaptable(for example, tactile receptors in the skin) and slow adapting receptors(e.g. proprioceptors of muscles and tendons). Fast adapting receptors are more excited in response to a rapid increase in stimulus intensity ( phasic response), and their rapid adaptation helps to free perception from biologically insignificant information (for example, contact between skin and clothing). Excitation of slowly adapting receptors depends little on the rate of change of the stimulus and persists during its long-term action ( tonic response), so for example slow adaptation of proprioceptors allows a person to receive the information he needs to maintain a pose for as long as necessary.

Exist sensory neurons, generating action potentials spontaneously, i.e. in the absence irritation(for example, sensory neurons of the vestibular system), such activity is called background. The frequency of nerve impulses in these neurons can increase or decrease depending on the intensity of the effect on secondary stimulus receptors In addition, it can be determined by the direction in which the sensitive hairs of the mechanoreceptors deviate. For example, deviation of the hairs of secondary mechanoreceptors in one direction is accompanied by an increase in the background activity of the sensory neuron to which they belong, and in the opposite direction - a decrease in its background activity. This method of reception allows one to obtain information about both the intensity of the stimulus and the direction in which it acts.

16. Coding of information in sensory systems.

Coding- the process of converting information into a conditional form (code) convenient for transmission over a communication channel. The universal code of the nervous system is nerve impulses that travel along nerve fibers. In this case, the content of information is determined not by the amplitude of the pulses (they obey the “All or nothing” law), but by the frequency of the pulses (time intervals between individual pulses), their combination into bursts, the number of pulses in a burst, and the intervals between bursts. The transmission of a signal from one cell to another in all sections of the analyzer is carried out using a chemical code, i.e. various mediators. To store information in the central nervous system, encoding is carried out using structural changes in neurons (memory mechanisms). Coded characteristics of the stimulus. The analyzers encode the qualitative characteristics of the stimulus (for example, light, sound), the strength of the stimulus, the time of its action, as well as space, i.e. the place of action of the stimulus and its localization in the environment. All sections of the analyzer take part in encoding all the characteristics of the stimulus.

In the peripheral region analyzer, coding of the quality of the stimulus (type) is carried out due to the specificity of the receptors, i.e. the ability to perceive a stimulus of a certain type to which it is adapted in the process of evolution, i.e. to an adequate stimulus. Thus, a light beam excites only the receptors of the retina; other receptors (smell, taste, tactile, etc.) usually do not respond to it.

Stimulus strength can be encoded by a change in the frequency of impulses generated by receptors when the strength of the stimulus changes, which is determined by the total number of impulses per unit time. This is the so called frequency coding.

Space is encoded by the size of the area on which the receptors are excited; this is spatial encoding. The time of action of the stimulus on the receptor is encoded by the fact that it begins to be excited with the onset of the stimulus and stops being excited immediately after the stimulus is turned off (temporal coding).

In the wiring department analyzer, coding is carried out only at “switching stations,” i.e., when transmitting a signal from one neuron to another, where the code changes. Information is not encoded in nerve fibers; they act as wires through which information encoded in receptors and processed in the centers of the nervous system is transmitted. There can be different intervals between impulses in a separate nerve fiber, impulses are formed into packets with different numbers, and there can also be different intervals between individual packets. All this reflects the nature of the information encoded in the receptors. In this case, the number of excited nerve fibers in the nerve trunk can also change, which is determined by a change in the number of excited receptors or neurons at the previous signal transition from one neuron to another. At switching stations, for example in the thalamus, information is encoded, firstly, by changing the volume of impulses at the input and output, and secondly, by spatial coding, i.e. due to the connection of certain neurons with certain receptors. In both cases

the stronger the stimulus, the more neurons are excited.

At the cortical end of the analyzer frequency-spatial coding occurs, the neurophysiological basis of which is the spatial distribution of ensembles of specialized neurons and their connections with certain types of receptors. Impulses arrive from receptors in certain areas of the cortex at different time intervals. Information arriving in the form of nerve impulses is recoded into structural and biochemical changes in neurons (memory mechanisms). The cerebral cortex carries out the highest analysis and synthesis of incoming information. Analysis consists in the fact that, with the help of the sensations that arise, we distinguish between the current stimuli (qualitatively - light, sound, etc.) and determine the strength, time and place, i.e. the space on which the stimulus acts, as well as its localization (source of sound, light, smell). Synthesis is realized in the recognition of a known object, phenomenon or in the formation of an image of an object or phenomenon encountered for the first time.

So, the process of transmitting a sensory message is accompanied by repeated recoding and ends with higher analysis and synthesis, which occurs in the cortical section of the analyzers. After this, the choice or development of a program for the body’s response takes place.

17. Structural and functional characteristics of the cerebral cortex. Localization of functions in the cerebral cortex.

18. Receptive field. Topical organization of sensory systems.

The receptive field of a sensory neuron is an area with receptors that, when exposed to a certain stimulus, lead to a change in the excitation of this neuron.

The concept of receptive fields can be applied to the entire nervous system. If many sensory receptors synapse on a single neuron, they together form the receptive field of that neuron. For example, the receptive field of the ganglion (ganglionic) cell of the retina is represented by photoreceptor cells (English) Russian. (rods or cones), and a group of ganglion cells in turn creates a receptive field for one of the neurons of the brain. As a result, impulses from many photoreceptors converge to one neuron of a higher synaptic level; and this process is called convergence. The receptive field is the area occupied by the totality of all receptors, the stimulation of which leads to excitation of the sensory neuron (Fig. 17.1). The maximum value of the receptive field of a primary sensory neuron is determined by the space occupied by all the branches of its peripheral process, and the number of receptors present in this space indicates the density of innervation. A high density of innervation is combined, as a rule, with small sizes of receptive fields and, accordingly, high spatial resolution, which makes it possible to distinguish between stimuli acting on neighboring receptive fields. Small receptive fields are typical, for example, for the central fovea of ​​the retina and for the fingers, where the density of receptors is much higher than in the periphery of the retina or in the skin of the back, which are characterized by large receptive fields and lower spatial resolution. The receptive fields of neighboring sensory neurons can partially overlap each other, so information about the stimuli acting on them is transmitted not through one, but through several parallel axons, which increases the reliability of its transmission.

Rice. 17.1. Receptive fields of primary sensory neurons and second-order sensory neurons.

A. The receptive fields of primary sensory neurons are limited to the region of their sensory endings. The receptive field of a switching neuron is formed from the sum of the receptive fields of primary sensory neurons converging on it.

B. Stimulation of the central or peripheral region of the receptive field of a sensory neuron of the second and subsequent orders is accompanied by the opposite effect. As can be seen in the diagram, irritation of the center of the receptive field will cause excitation of the projection neuron, and irritation of the peripheral region will cause inhibition with the help of interneurons of the switching nucleus (lateral inhibition). As a result of the contrast created between the center and periphery of the receptive field, information is highlighted for transmission to the next hierarchical level.

The magnitude of the receptive fields of sensory neurons of the second and subsequent orders is greater than that of primary sensory neurons, since central neurons receive information from several neurons of the previous level converging on them. From the center of the receptive field, information is transmitted directly to the sensory neurons of the next order, and from the periphery - to the inhibitory interneurons of the switching nucleus, therefore the center and periphery of the receptive field are reciprocal with respect to each other. As a result, signals from the center of the receptive field easily reach the next hierarchical level of the sensory system, while signals coming from the periphery of the receptive field are inhibited (in another version of the organization of the receptive field, signals from the periphery are more easily transmitted, rather than from the center). This functional organization of receptive fields ensures the selection of the most significant signals, easily distinguishable against a contrasting background.

The sensory pathway consists of a number of modality-specific neurons that are connected by synapses. This principle of organization is called a labeled line or topical organization. The essence of this principle lies in the spatially ordered arrangement of neurons at various levels of sensory systems according to the characteristics of their receptive fields.

From a morphological point of view, the receptive field is a section of the receptor surface with which a given nerve structure (fiber, neuron) is anatomically (rigidly) connected. From a functional point of view, the receptive field is a dynamic concept, meaning that the same neuron at different periods of time, depending, for example, on the characteristics of the impact, may be associated with a different number of receptors.

The principle of the labeled line was opposed by the theory of “response structure,” according to which receptors encode the qualitative features of stimuli by the structure of the impulse response. This theory assumed the absence of rigid connections between receptors and central neurons. The basis for it was experimental data showing that information encoding is carried out not by single impulses, but by a group of uniformly successive action potentials. Additional parameters of receptor activity, for example, pulse frequency or duration of interpulse intervals, can be used as signal signs.

For uniformly following pulses, the signal signs can be the number of pulses in a burst or the duration of the bursts, as well as the intervals between them and the frequency of their repetition. Such coding opens up endless possibilities, since a wide variety of variations with pulse bursts are possible. The spatiotemporal distribution of electrical activity of nerve fibers is called patterns. The various qualities of stimuli, according to this theory, are reflected by characteristic “patterns” of patterns. Neurons are able to decipher these signals and, depending on their structure, form a sensation that corresponds to the stimulus encoded by certain patterns.

A neuron, responding differently to different patterns, can be involved in multiple functions. Each shade of the quality of sensation arises as a result of the activity of a complex of neurons that form dynamic ensembles, the formation of which depends on the nature of the patterns coming from the receptors.

Each modality has its own form of information coding in accordance with the physical properties of the distinguished stimuli. Some qualities are recognized by sensory systems operating according to the principle of topical organization, others are encoded by patterns. For example, recognition of many qualities of visual images is carried out by labeled lines, and taste stimuli are encoded by patterns.

19. Reflex arc.

The structural basis of reflex activity is made up of neural chains of receptor, intercalary and effector neurons. They form the path along which nerve impulses pass from the receptor to the executive organ during the implementation of any reflex. This path is called the reflex arc. It includes:

1. receptors that perceive irritation;

2. afferent nerve fibers - processes of receptor neurons that carry excitation to the central nervous system;

3. neurons and synapses transmitting impulses to effector neurons;

4. efferent nerve fibers that conduct impulses from the central nervous system to the periphery;

5. an executive organ whose activity changes as a result of a reflex.

The simplest reflex arc can be schematically imagined as formed by only two neurons: receptor and effector, between which there is one synapse. This reflex arc is called bineuronal and monosynaptic.

The reflex arcs of most reflexes include not two, but a larger number of neurons: a receptor, one or more intercalary and an effector. Such reflex arcs are called multineuronal and polysynaptic. Various variants of polysynaptic reflex arcs are possible. This simplest arc includes only three neurons and two synapses between them. There are polysynaptic reflex arcs in which a receptor neuron is connected to several interneurons, each of which forms synapses on different or on the same effector neuron.

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

· somatic nervous system, innervating the skeletal muscles

· autonomic nervous system, innervating internal organs: heart, stomach, intestines, kidneys, liver, etc.

The reflex arc consists of five sections:

1. receptors, perceiving irritation and responding to it with excitement. Receptors can be the endings of long processes of centripetal nerves or microscopic bodies of various shapes from epithelial cells on which the processes of neurons end. Receptors are located in the skin, in all internal organs; clusters of receptors form the sense organs (eye, ear, etc.).

2. sensory (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, a synaptic connection between the sensory and motor neurons occurs.

4. 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.

5. effector- a working organ that produces an effect, a reaction in response to stimulation of the receptor. 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 simplest reflex arc can be schematically represented as formed by only two neurons: receptor and effector, between which there is one synapse. This reflex arc is called bineuronal and monosynaptic. Monosynaptic reflex arcs are very rare. An example of them is the arc of the myotatic reflex.

In most cases, reflex arcs include not two, but a larger number of neurons: a receptor, one or more intercalary and an effector. Such reflex arcs are called multineuronal and polysynaptic. An example of a polysynaptic reflex arc is the reflex of withdrawing a limb in response to painful stimulation.

The reflex arc of the somatic nervous system on the way from the central nervous system to the skeletal muscle is not interrupted anywhere, unlike the reflex arc of the autonomic nervous system, which on the way from the central nervous system to the innervated organ is necessarily interrupted with the formation of a synapse - the autonomic ganglion.

Autonomic ganglia, depending on location, can be divided into three groups:

1. vertebral ganglia - belong to the sympathetic nervous system. They are located on both sides of the spine, forming two border trunks (they are also called sympathetic chains)

2. prevertebral (prevertebral) ganglia are located at a greater distance from the spine, at the same time they are located at some distance from the organs they innervate. The prevertebral ganglia include the ciliary ganglion, superior and middle cervical sympathetic nodes, solar plexus, superior and inferior mesenteric ganglia.

3. intraorgan ganglia are located in the internal organs: in the muscular walls of the heart, bronchi, middle and lower third of the esophagus, stomach, intestines, gall bladder, bladder, as well as in the glands of external and internal secretion. Parasympathetic fibers are interrupted on the cells of these ganglia.

This difference between the somatic and autonomic reflex arc is due to the anatomical structure of the nerve fibers that make up the neural chain and the speed of transmission of the nerve impulse through them.

For any reflex to occur, the integrity of all parts of the reflex arc is necessary. Violation of at least one of them leads to the disappearance of the reflex.

20. Unconditioned reflexes, their characteristics. Instincts.

Unconditioned reflexes- this is an innate species-specific reaction of the body, reflexively arising in response to the specific influence of a stimulus, to the influence of a biologically significant (pain, food, tactile irritation, etc.) stimulus adequate for a given type of activity.

Unconditioned reflexes:

· Congenital hereditary reactions, most of them begin to function immediately after birth.

· Are specific, i.e. characteristic of all representatives of this species.

· Permanent and persist throughout life.

· Carried out by the lower parts of the central nervous system (subcortical nuclei, brain stem, spinal cord).

· Arise in response to adequate stimulation acting on a specific receptive field.

According to the level of complexity, unconditioned reflexes are divided into:

simple unconditioned reflexes

reflex acts

behavior reactions

· instincts

Simple unconditioned reflexes are elementary innate reactions to stimuli. For example, withdrawing a limb from a hot object, blinking an eyelid when a speck gets into the eye, etc. Simple unconditioned reflexes to the corresponding stimulus always appear and cannot be changed or corrected.

Reflex acts are actions determined by several simple unconditioned reflexes, always performed in the same way and regardless of the dog’s consciousness. Basically, reflex acts ensure the vital functions of the body, therefore they always manifest themselves reliably and cannot be corrected.

Some examples of reflex acts:

Breath;

Swallowing;

Belching

When training and raising a dog, you should remember that the only way to prevent the manifestation of one or another reflex act is to change or remove the stimulus that causes it. So, if you want your pet not to defecate while practicing obedience skills (and he will still do this if necessary, despite your prohibition, because this is a manifestation of a reflex act), then walk the dog before training. In this way, you will eliminate the corresponding stimuli that cause a reflex act that is undesirable for you.

Behavioral reactions are the dog’s desire to carry out certain actions, based on a complex of reflex acts and simple unconditioned reflexes.

Thus, behavioral reactions are the cause of many of the dog's actions, but in a real situation their manifestation can be controlled. We gave a negative example showing unwanted behavior in a dog. But attempts to develop the desired behavior in the absence of the necessary reactions will end in failure. For example, it is useless to train a search dog from a candidate who lacks an olfactory-search reaction. A dog with a passive-defensive reaction (a cowardly dog) will not make a guard.

Instinct- this is an innate, strictly constant, specific form of adaptive behavior for each species, stimulated by the basic biological needs of the body and specific environmental stimuli.

Natural selection influences behavior in the same way as it influences the structure of the body, its color and all other morphological and physiological characteristics and properties of organisms.

Given the relatively low complexity of the brain, natural selection leads to the improvement of hard-wired forms of behavior that ensure survival.

In general, natural selection has led to the emergence of organisms with increasingly complex and flexible behavior to ensure survival in changing environments. As a consequence of this trend, man appeared on earth.

Criteria and signs of instincts:

1) Inspiration (motivation) and the ability to act are among the hereditary properties of the species;

2) such actions do not require preliminary training (although training can develop and improve its implementation!);

3) are performed essentially identically in all normal representatives of the species;

4) are associated with the normal functioning of its organs (for example, the instinct to dig holes is combined with the corresponding structure of the paws adapted for digging);

5) adapted to the ecological conditions of the species’ habitat (i.e., ensure survival in specific environmental conditions).

Levels of reflex behavioral reactions (according to A.B. Kogan)

· First level: elementary unconditioned reflexes. These are simple unconditional reflex reactions, carried out at the level of individual segments of the spinal cord. Implemented in accordance with genetically determined programs. Stereotypical. They are carried out unconsciously.

· Second level: coordination unconditioned reflexes. These are complex acts of contraction and relaxation of various muscles or stimulation and inhibition of the functions of internal organs, and these reciprocal relationships are well coordinated.

Feedback is of great importance in the coordination of unconditioned reflexes.

They are formed on the basis of elementary unconditioned reflexes (the first level of reflex reactions).

These are locomotor acts and vegetative processes aimed at maintaining homeostasis.

· The third level of organization of reflex reactions is integrative unconditioned reflexes.

They arise under the influence of biologically important stimuli (food and pain).

Integrative unconditioned reflexes are complex behavioral acts that are systemic in nature with pronounced somatic and vegetative components. For example, locomotor acts are accompanied by increased blood circulation, respiration, etc.

· The fourth level is the most complex unconditioned reflexes (instincts).

Herbert Spencer was the first to suggest that instincts are also reflexes.

The most complex unconditioned reflexes are carried out according to genetically specified programs, the trigger stimulus triggers them entirely.

· Fifth level – elementary conditioned reflexes.

They are developed in the process of individual life.

At an early age, simple conditioned reflex reactions are formed. Over the course of life they become more complex. The cerebral cortex is involved in the formation of conditioned reflexes.

The conditioned reflex mechanism of behavior is distinguished by a high degree of reliability, which is ensured by the multichannel nature and interchangeability of nerve connections in the plastic structures of the central nervous system.

· The sixth level of behavioral acts is complex forms of mental activity.

It is based on the integration of elementary conditioned reflexes and analytical-synthetic mechanisms of abstraction.

21. Conditioned reflexes, their characteristics.

Conditioned reflex is a complex multicomponent reaction that is developed on the basis of unconditioned reflexes using a previous indifferent stimulus. It has a signaling character and the body meets the impact of an unconditioned stimulus prepared.

Conditioned reflexes:

· Reactions acquired during individual life.

· Individual.

· Impermanent - can appear and disappear.

· They are primarily a function of the cerebral cortex.

· Occurs in response to any stimuli acting on different receptive fields.

Classification of conditioned reflexes

· According to the degree of proximity of the signal stimulus to the biology of the animal:

Natural conditioned reflexes

· Artificial conditioned reflexes

Based on the localization and properties of the afferent link of the conditioned reflex arc:

Exteroceptive

Interoceptive

Proprioceptive

According to the modality of the adequate stimulus:

Mechano-, photo-, chemo-, thermo-, osmoreceptor conditioned reflexes.

· Properties of the efferent link of conditioned reflexes:

· Vegetative

· Somatic

According to the degree (depth) of abstraction:

· Conditioned reflexes of I, II and higher orders.

· The tertiary conditioned reflex was developed in the 20s of the 20th century by I.P. Pavlov’s collaborator, D.S. Fursikov.

· The IV order reflex cannot be developed in dogs, but it can be developed in dolphins.

· In horses, the depth of abstraction is reflexes of the V – VI orders.

By structure:

simple and complex

According to the time relationship between signal and reinforcement:

· Cash (a reinforcing stimulus is given during the action of the signal stimulus).

· Trace (take a pause between the end of the conditioned stimulus and the beginning of reinforcement; as the experiment becomes more complex, the pause is from 15-20 s to 4-5 minutes).

Conditioned reflexes are reflexes to future events. Biological significance of conditionals

reflexes consist in their preventive role; they have an adaptive effect for the body

importance, preparing the body for future useful behavioral activities and helping it avoid harmful effects, subtly and effectively adapt to the surrounding natural and social environment. Conditioned reflexes are formed due to the plasticity of the nervous system.

22. Rules for the development of conditioned reflexes.

To develop a conditioned reflex it is necessary:

1) the presence of two stimuli, one of which is unconditioned (food, painful stimulus, etc.), causing an unconditioned reflex reaction, and the other is conditioned (signal), signaling the upcoming unconditional stimulus (light, sound, type of food, etc. .);

2) multiple combinations of conditioned and unconditioned stimuli (although the formation of a conditioned reflex is possible with their single combination);

3) the conditioned stimulus must precede the action of the unconditional;

4) any stimulus from the external or internal environment can be used as a conditioned stimulus, which should be as indifferent as possible, not cause a defensive reaction, not have excessive force and be able to attract attention;

5) the unconditioned stimulus must be strong enough, otherwise a temporary connection will not be formed;

6) arousal from an unconditioned stimulus should be stronger than from a conditioned one;

7) it is necessary to eliminate extraneous stimuli, as they can cause inhibition of the conditioned reflex;

8) the animal in which the conditioned reflex is developed must be healthy;

9) when developing a conditioned reflex, motivation must be expressed, for example, when developing a food salivary reflex, the animal must be hungry, but in a well-fed animal, this reflex is not developed.

Conditioned reflexes are easier to develop in response to environmentally similar influences for a given animal. In this regard, conditioned reflexes are divided into natural and artificial. Natural conditioned reflexes are developed to agents that, under natural conditions, act together with a stimulus that causes an unconditioned reflex (for example, the type of food, its smell, etc.). All other conditioned reflexes are artificial, i.e. are produced in response to agents that are not normally associated with the action of an unconditioned stimulus, for example, the food salivary reflex to a bell.

The physiological basis for the emergence of conditioned reflexes is the formation of functional temporary connections in the higher parts of the central nervous system. A temporary connection is a set of neurophysiological, biochemical and ultrastructural changes in the brain that arise during the combined action of conditioned and unconditioned stimuli. I.P. Pavlov suggested that during the development of a conditioned reflex, a temporary nervous connection is formed between two groups of cortical cells - the cortical representations of the conditioned and unconditioned reflexes. Excitation from the center of the conditioned reflex can be transmitted to the center of the unconditioned reflex from neuron to neuron.

Consequently, the first way of forming a temporary connection between the cortical representations of the conditioned and unconditioned reflexes is intracortical. However, when the cortical representation of the conditioned reflex is destroyed, the developed conditioned reflex is preserved. Apparently, the formation of a temporary connection occurs between the subcortical center of the conditioned reflex and the cortical center of the unconditioned reflex. When the cortical representation of the unconditioned reflex is destroyed, the conditioned reflex is also preserved. Consequently, the development of a temporary connection can occur between the cortical center of the conditioned reflex and the subcortical center of the unconditioned reflex.

Separation of the cortical centers of the conditioned and unconditioned reflexes by crossing the cerebral cortex does not prevent the formation of the conditioned reflex. This indicates that a temporary connection can be formed between the cortical center of the conditioned reflex, the subcortical center of the unconditioned reflex and the cortical center of the unconditioned reflex.

There are different opinions on the issue of the mechanisms for the formation of temporary connections. Perhaps the formation of a temporary connection occurs according to the dominant principle. The source of excitation from an unconditioned stimulus is always stronger than from a conditioned one, since the unconditioned stimulus is always biologically more significant for the animal. This focus of excitation is dominant, therefore attracts excitation from the focus of conditioned stimulation. If the excitation has passed along some nerve circuits, then next time it will pass along these paths much easier (the phenomenon of “blazing a path”). This is based on: the summation of excitations, a long-term increase in the excitability of synaptic formations, an increase in the amount of mediator in synapses, and an increase in the formation of new synapses. All this creates structural prerequisites for facilitating the movement of excitation along certain neural circuits.

Another idea about the mechanism of formation of a temporary connection is the convergent theory. It is based on the ability of neurons to respond to stimulation of different modalities. According to P.K. Anokhin, conditioned and unconditioned stimuli cause widespread activation of cortical neurons due to the inclusion of the reticular formation. As a result, the ascending signals (conditioned and unconditioned stimuli) overlap, i.e. these excitations meet on the same cortical neurons. As a result of the convergence of excitations, temporary connections arise and stabilize between the cortical representations of the conditioned and unconditioned stimuli.

23. Conditioned reflexes of the second and higher order. Dynamic stereotype.

Receptors

Coordination of the body's vital activity is impossible without information continuously coming from the external environment. Special organs or cells that perceive signals are called receptors; the signal itself is called a stimulus. Various receptors can perceive information from both the external and internal environment.

According to their internal structure, receptors can be either simple, consisting of a single cell, or highly organized, consisting of a large number of cells that are part of a specialized sensory organ. Animals can perceive the following types of information:

Light (photoreceptors);

Chemicals – taste, smell, moisture (chemoreceptors);

Mechanical deformations - sound, touch, pressure, gravity ( mechanoreceptors);

Temperature (thermoreceptors);

Electricity ( electroreceptors).

Receptors convert the energy of the stimulus into an electrical signal that excites neurons. The mechanism of receptor excitation is associated with a change in the permeability of the cell membrane to potassium and sodium ions. When stimulation reaches a threshold value, a sensory neuron is excited, sending an impulse to the central nervous system. We can say that receptors encode incoming information in the form of electrical signals.

As already noted, the sensory cell sends information according to the “all or nothing” principle (there is a signal / there is no signal). In order to determine the intensity of a stimulus, the receptor organ uses several cells in parallel, each of which has its own sensitivity threshold. There is also relative sensitivity - by how many percent the signal intensity must be changed for the sensory organ to detect the change. Thus, in humans, the relative sensitivity of light brightness is approximately 1%, sound intensity is 10%, and gravity is 3%. These patterns were discovered by Bouguer and Weber; they are valid only for the average zone of stimulus intensity. Sensors are also characterized by adaptation - they react primarily to sudden changes in the environment, without “clogging” the nervous system with static background information.

The sensitivity of a sensory organ can be significantly increased through summation, when several adjacent sensory cells are connected to a single neuron. A weak signal entering the receptor would not cause the neurons to fire if they were connected to each of the sensory cells separately, but it causes the neuron to fire, in which information from several cells is summed up at once. On the other hand, this effect reduces the resolution of the organ. Thus, the rods in the retina, unlike the cones, have increased sensitivity, since one neuron is connected to several rods at once, but they have a lower resolution. The sensitivity to very small changes in some receptors is very high due to their spontaneous activity, when nerve impulses occur even in the absence of a signal. Otherwise, weak impulses would not be able to overcome the sensitivity threshold of the neuron. The sensitivity threshold can change due to impulses coming from the central nervous system (usually via feedback), which changes the sensitivity range of the receptor. Finally, lateral inhibition plays an important role in increasing sensitivity. Neighboring sensory cells, when excited, have an inhibitory effect on each other. This enhances the contrast between neighboring areas.

The organ of balance in mammals is vestibular apparatus, located in the inner ear. Its receptor cells are equipped with hairs. Head movement causes the hairs to deflect and the potential to change. If, when the position of the head changes, this deviation is enhanced by otoconia - calcium carbonate crystals located on top of the hairs of the oval and round sacs, then sensitivity to the speed of rotation is ensured by the inertia of the gelatinous mass - cupula - located in the semicircular canals.

The lateral organs react to the speed and direction of the water flow, providing animals with information about changes in the position of their own body, as well as about nearby objects. They consist of sensory cells with bristles at the ends, which usually lie in subcutaneous canals. Short tubes passing through the scales extend outward, forming the lateral line. Lateral organs are found in cyclostomes, fish and aquatic amphibians.

The organ of hearing that perceives sound waves in air or water is called ear. All vertebrates have ears, but if in fish they are small protrusions, then in mammals they progress into a system of outer, middle and inner ears with a complex cochlea. The outer ear is present in reptiles, birds and animals; in the latter it is represented by mobile cartilaginous auricle. In mammals that have switched to an aquatic lifestyle, the external ear is reduced. In mammals, the main element of the ear is eardrum– separates the outer ear from the middle ear. Its vibrations, excited by sound waves, are amplified by three auditory ossicles - the malleus, the incus and the stapes. Next, the vibrations are transmitted through the oval window to a complex system of canals and cavities of the inner ear, filled with fluid; mutual movement of the basilar and tectorial membranes converts a mechanical signal into an electrical one, which is then sent to the central nervous system. Eustachian tube, connecting the middle ear to the pharynx, equalizes pressure and prevents damage to the auditory organs when it changes.

As it moves away from the base of the cochlea, the basilar membrane expands; its sensitivity changes in such a way that high-frequency sounds stimulate nerve endings only at the base of the cochlea, and low-frequency sounds only at its apex. Sounds consisting of several frequencies stimulate different areas of the membrane; Nerve impulses are summed up in the auditory zone of the cerebral cortex, resulting in the sensation of one mixed sound. The difference in sound volume is due to the fact that each section of the basilar membrane contains a set of cells with different sensitivity thresholds.

In insects, the eardrum is located on the front legs, chest, abdomen or wings. Many insects are susceptible to ultrasound (for example, butterflies can detect sound waves with a frequency of up to 240 kHz).

They can react to temperature as specialized organs - Ruffini corpuscles (warmth) and Krause cones(cold) and free nerve endings located in the skin.

Some groups of fish have developed paired electrical organs, designed for protection, attack, signaling and orientation in space. They are located on the sides of the body or near the eyes and consist of electrical plates collected in columns - modified cells that generate electric current. The plates in each column are connected in series, and the columns themselves are connected in parallel. The total number of records is hundreds of thousands and even millions. The voltage at the ends of electrical organs can reach 1200 V. The frequency of discharges depends on their purpose and can be tens and hundreds of hertz; in this case, the voltage in the discharge ranges from 20 to 600 V, and the current strength - from 0.1 to 50 A. Electric discharges of stingrays and eels are dangerous to humans.

The sensations of taste and smell are associated with the action of chemicals. In mammals, taste stimuli interact with specific molecules in sensory cells that form taste buds. There are four types of taste sensations: sweet, salty, sour and bitter. It is still unknown how taste depends on the internal structure of the chemical.

Odorous substances in the air penetrate the mucus and stimulate the olfactory cells. Perhaps there are several basic odors, each of which affects a specific group of receptors.

Insects have extremely sensitive organs of taste and smell, hundreds and thousands of times more effective than human ones. The taste organs of insects are located on the antennae, labial palps and paws. The olfactory organs are usually located on the antennae.

The most primitive photoreceptor systems (eye spots) are found in protozoa. The simplest light-sensitive eyes, consisting of visual and pigment cells, are found in some coelenterates and lower worms. They are able to distinguish between light and dark, but are not able to create an image. More complex organs of vision in some annelids, mollusks and arthropods are equipped with a light-refracting apparatus.

The most perfect eyes - the so-called camera vision– possessed by cephalopods and vertebrates (especially birds). The eyes of vertebrates consist of eyeballs connected to the brain, and peripheral parts: eyelids, which protect the eyes from damage and bright light, lacrimal glands, which moisturize the surface of the eye, and oculomotor muscles. The eyeball has a spherical shape with a diameter of about 24 mm (hereinafter, all figures are given for the human eye) and weighs 6–8 g. Outside, the eyeball is protected by the sclera (in humans - 1 mm thick), which passes in front into a thin and transparent cornea (0 .6 mm), refracting light. Under this layer is the choroid, which supplies blood to the retina. The part of the eyeball facing the light contains a protein biconvex lens (lens) that serves for accommodation Iris. The color of the eyes depends on its pigmentation. In the middle of the iris there is a hole with a diameter of about 3.5 mm -