Definition and characteristics of receptors. Receptors and their classification

In some receptors (for example, taste And auditory human receptors), the stimulus is directly perceived by specialized cells of epithelial origin or modified nerve cells (sensitive elements of the retina), which do not generate nerve impulses, but act on the nerve endings that innervate them, changing the secretion of the mediator. In other cases, the only cellular element of the receptor complex is the nerve ending itself, often associated with special structures of the intercellular substance (for example, Pacinian corpuscle).

How receptors work

Stimuli for different receptors can be light, mechanical deformation, chemicals, changes temperature, as well as changes in the electric and magnetic fields. In receptor cells (whether directly nerve endings or specialized cells), the corresponding signal changes the conformation of sensitive cellular receptor molecules, which leads to a change in the activity of membrane ion receptors and a change in the membrane potential of the cell. If the receiving cell is the nerve ending itself (the so-called primary receptors), then depolarization of the membrane usually occurs, followed by the generation of a nerve impulse. Specialized receptor cells secondary receptors can both de- and hyperpolarize. In the latter case, a change in membrane potential leads to a decrease in the secretion of an inhibitory transmitter acting on the nerve ending and, ultimately, still to the generation of a nerve impulse. This mechanism is implemented, in particular, in the sensitive elements of the retina.

Cellular receptor molecules can be either mechano-, thermo- and chemosensitive ion channels, or specialized G-proteins (as in retinal cells). In the first case, the opening of channels directly changes the membrane potential (mechanosensitive channels in Pacinian corpuscles); in the second case, a cascade of intracellular signal transduction reactions is triggered, which ultimately leads to the opening of channels and a change in potential on the membrane.

Types of receptors

There are several classifications of receptors:

• By position in the body

  Exteroceptors (exteroceptors) - located on the surface or near the surface of the body and perceive external stimuli (signals from the environment)

  Interoreceptors (interoceptors) - located in internal organs and perceive internal stimuli (for example, information about the state of the internal environment of the body)

  • Proprioceptors (proprioceptors) are receptors of the musculoskeletal system, allowing one to determine, for example, the tension and 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 (for example, photoreceptors to light)

  Polymodal - responsive to several types of stimuli (for example, many pain receptors, as well as some invertebrate receptors that respond simultaneously to mechanical and chemical stimuli).

According to an adequate stimulus

• Hair follicle receptors - respond to hair deviation.

  Ruffini endings– stretch receptors. They are slow to adapt, have large receptive fields. They react to heat.

  Krause flask- a receptor that responds to cold.

Muscle and tendon receptors

• Muscle spindles– muscle stretch receptors are of two types:
  • with nuclear bag
  • with nuclear chain

Golgi tendon organ– muscle contraction receptors. When a muscle contracts, the tendon stretches and its fibers compress the receptor ending, activating it.

Ligament receptors

They are mostly free nerve endings (Types 1, 3 and 4), with a smaller group being encapsulated (Type 2). Type 1 is similar to Ruffini's endings, Type 2 is similar to Paccini's corpuscles.

Article on human anatomy and physiology

Receptors and their role in the human body

Vorobiev Anton Sergeevich

Receptor(from Latin recipere - to receive) - a sensitive nerve ending or a specialized cell that converts perceived irritation into nerve impulses.
The receptor is much more susceptible to external influences than other organs and nerve fibers. The sensitivity of this organ is especially high and is inversely proportional to the threshold. That is, if they say that the irritation threshold is low, this means that the sensitivity of the receptor is high. A receptor is a specialized apparatus.
Each receptor is designed to perceive one type of irritation.
All receptors are characterized by the presence of a specific membrane region containing a receptor protein that determines reception processes.
The main characteristic of the body's receptor apparatus is its adaptability to the perception of irritations, increased sensitivity to them and specialization to certain types of influence.
There are several classifications receptors:

• By position in the body
  • Exteroceptors (exteroceptors) - located on or near the surface of the body and perceive external stimuli (signals from the environment)
  • Interoreceptors (interoceptors) - located in internal organs and perceive internal stimuli (for example, information about the state of the internal environment of the body)
    • Proprioceptors (proprioceptors) are receptors of the musculoskeletal system, allowing one to determine, for example, the tension and degree of stretching of muscles and tendons. They are a type of interoreceptor
• Ability to perceive different stimuli
  • Monomodal - responding to only one type of stimulus (for example, photoreceptors to light)
  • Polymodal - responsive to multiple types of stimuli (for example, many pain receptors, as well as some invertebrate receptors that respond simultaneously to mechanical and chemical stimuli)
• By adequate stimulus :
  • Chemoreceptors - perceive the effects of dissolved or volatile chemicals
  • Osmoreceptors - perceive changesosmotic concentration fluids (usually the internal environment)
  • Mechanoreceptors- perceive mechanical stimuli (touch, pressure, stretching, vibrations of water or air, etc.)
  • Photoreceptors - perceive visible and ultraviolet light
  • Thermoreceptors - perceive decreasing (cold) or increasing (heat) stimuli
  • Pain receptors , 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 (chemical, thermal or mechanical) 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.
Skin receptors

• Pain receptors.
• Pacinian Taurus — encapsulated pressure receptors in a round multilayer capsule. They are located in the subcutaneous fat. They are quickly adapting (they react only at the moment the impact begins), that is, they register the force of pressure. They have large receptive fields, that is, they represent gross sensitivity.
• Meissner's corpuscles - pressure receptors located in dermis . They are a layered structure with a nerve ending running between the layers. They are quickly adaptable. They have small receptive fields, that is, they represent subtle sensitivity.
• Merkel discs are unencapsulated pressure receptors. They are slowly adapting (react throughout the entire duration of exposure), that is, they record the duration of pressure. They have small receptive fields.
• Hair follicle receptors - respond to hair deviation.
• Ruffini endings are stretch receptors. They are slow to adapt and have large receptive fields.

Muscle and tendon receptors

• Muscle spindles - muscle stretch receptors are of two types:
  • with nuclear bag
  • with nuclear chain
• Golgi tendon organ - muscle contraction receptors. When a muscle contracts, the tendon stretches and its fibers compress the receptor ending, activating it.

Ligament receptors
They are mostly free nerve endings (Types 1, 3 and 4), with a smaller group being encapsulated (Type 2). Type 1 is similar to Ruffini's endings, Type 2 is similar to Paccini's corpuscles.
Retinal receptors
Retina contains rods ( sticks) and cone ( cones) photosensitive cells that contain light-sensitive pigments . The rods are sensitive to very weak light, they are long and thin cells oriented along the axis of light transmission. All sticks contain the same photosensitive pigment. Cones require much brighter lighting, they are short cone-shaped cells, person cones are divided into three types, each of which contains its own light-sensitive pigment - this is the basiscolor vision .
Under the influence of light, receptors undergo discoloration-visual pigment molecule absorbs photon and turns into another compound that absorbs wave light worse (this wavelength ). In almost all animals (from insects to humans), this pigment consists of a protein to which is attached a small molecule close to vitamin A . This molecule is the part chemically transformed by light. The protein part of the faded visual pigment molecule activates transducin molecules, each of which deactivates hundreds of moleculescyclic guanosine monophosphate involved in the opening of membrane pores for sodium ions , as a result of which the flow of ions stops - the membrane hyperpolarizes.
The sensitivity of the rods is such thatadapted By complete darkness, a person is able to see a flash of light so weak that no receptor can receive more than one photon. At the same time, sticks are unable to react to changes in illumination, when the light is so bright that all sodium pores are already closed.
Literature:

• David Hubel - “Eye, Brain, Vision” translation from English. Ph.D. biol. Sciences O. V. Levashova, Ph.D. biol. Sciences G. A. Sharaeva, ed. Corresponding member USSR Academy of Sciences A. L. Byzova, Moscow “Mir”, 1990
• http://anatomus.ru/articles/rol-retseptorov.html

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! However, 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 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.

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 be changed due to impulses coming from the central nervous system (usually by 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 most primitive receptors are considered to be mechanical, responding to touch and pressure. The difference between these two sensations is quantitative; touch is usually registered by the finest neuron endings located close to the surface of the skin, at the bases of hairs or antennae. There are also specialized organs - Meissner's corpuscles. Pacinian corpuscles, consisting of a single nerve ending surrounded by connective tissue, react to pressure. The impulses are excited due to a change in the permeability of the membrane, which occurs due to its stretching.

The organ of balance in mammals is the 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 - the 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 the 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 a movable cartilaginous auricle. In mammals that have switched to an aquatic lifestyle, the external ear is reduced. In mammals, the main element of the ear, the eardrum, separates the outer ear from the middle ear. Its vibrations, excited by sound waves, are amplified by the 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; the mutual movement of the basilar and tectorial membranes converts the mechanical signal into an electrical signal, which is then sent to the central nervous system. The Eustachian tube, which connects the middle ear to the pharynx, equalizes pressure and prevents damage to the auditory organs when pressure changes.

Diagram of the structure of the human ear

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).

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

Some groups of fish have developed paired electrical organs designed for defense, 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.

Taste zones of the human tongue

The structure of the taste bud

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.

Olfactory organs

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 compound eyes of arthropods consist of numerous individual ocelli - ommatidia. Each ommatidium has a transparent biconvex horny lens and a crystal cone that focus light onto a cluster of light-sensitive cells. The field of view of each ommatidium is very small; together they form an overlapping mosaic image, which does not have very high resolution, but is quite sensitive.

Structure of the human eye

The most advanced eyes - the so-called chamber vision - are possessed by cephalopods and vertebrates (especially birds). Vertebrate eyes 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) and the iris, which serves for accommodation. 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 - the pupil. Special muscles can change the diameter of the pupil, regulating the entry of light rays into the eye. The lens is located behind the iris; contraction of the ciliary body ensures a change in its curvature, that is, precise focusing.

1. Central nervous system

The central nervous system is part of the vertebrate nervous system, represented by a collection of nerve cells that form the brain and spinal cord.

The central nervous system regulates the processes occurring in the body and serves as the control center for all systems. The mechanisms of central nervous system activity are based on the interaction of excitation and inhibition.

Higher nervous activity (HNA)

Higher nervous activity - according to I.P. Pavlov - is a complex form of life activity that ensures individual behavioral adaptation of humans and higher animals to changing environmental conditions.

The basis of higher nervous activity is the interaction of innate unconditioned and conditioned reflexes acquired in the process of ontogenesis, to which a second signaling system is added in humans.

The structural basis of the VND is the cerebral cortex with the subcortical nuclei of the forebrain and some structures of the diencephalon.

2. Higher nervous activity

Higher nervous activity (HNA) is the activity of the higher parts of the central nervous system, ensuring the most perfect adaptation of animals and humans to the environment (behavior). The structural basis of the VNI is the cerebral cortex with the subcortical nuclei of the forebrain and the formations of the diencephalon, however, there is no strict connection of the VNI with brain structures. Lower nervous activity is represented as a function of the central nervous system, aimed at regulating physiological processes in the body itself. The most important feature of GNI is its signaling nature, which allows one to prepare in advance for one or another form of activity (eating, defensive, sexual, etc.)

Characteristics of VND: variability, signaling, adaptability - provide flexibility and adaptability of reactions. The probabilistic nature of the external environment gives relativity to any behavioral reaction and encourages the body to make probabilistic forecasts. The ability to learn highly depends not only on the processes of excitation, but also inhibition. Conditioned inhibition promotes a rapid change in forms of behavior in accordance with conditions and motivations.

The term GNI was introduced by I. P. Pavlov, who considered it equivalent to the concept of “mental activity.” According to I.P. Pavlov, this is a combined reflex (conditioned and unconditioned reflex) function of the cerebral cortex and the nearest subcortex of the brain. He also introduced the concept of “signal systems” as systems of conditioned reflex connections, highlighting the first signaling system common to animals and humans and the second, specific only to humans.

The first signaling system (PSS) - direct sensations and perceptions, forms the basis of the GNI and is reduced to a set of diverse conditioned and unconditioned reflexes to direct stimuli. The human PSS is characterized by a greater speed of propagation and concentration of the nervous process, its mobility, which ensures rapid switching and formation of conditioned reflexes. Animals are better at distinguishing between individual stimuli, and humans are better at distinguishing between their combinations.

The second signaling system was formed in humans on the basis of the first as a system of speech signals (pronounced, audible, visible). The words contain a generalization of the signals of the first signaling system. The process of generalization by word is developed during the formation of conditioned reflexes. Generalized reflection and abstraction are formed only in the process of communication, i.e. determined by biological and social factors.

Receptor - (from Latin recipere - to receive), nerve formations that convert chemical and physical influences from the external or internal environment of the body into nerve impulses; a peripheral specialized part of the analyzer, through which only a certain type of energy is transformed into the process of nervous excitation. Receptors vary widely in the degree of structural complexity and in the level of adaptation to their function. Depending on the energy of the corresponding stimulation, the receptors are divided into mechanoreceptors and chemoreceptors. Mechanoreceptors are found in the ear, vestibular apparatus, muscles, joints, skin and internal organs. Chemoreceptors serve olfactory and taste sensitivity: many of them are located in the brain, responding to changes in the chemical composition of the body fluid. Visual receptors are also essentially chemoreceptors. Depending on their position in the body and the function they perform, receptors are divided into exteroceptors, interoreceptors and proprioceptors. Exteroceptors include distant receptors that receive information at some distance from the source of stimulation (olfactory, auditory, visual, gustatory); interoceptors signal about stimuli from the internal environment, and proprioceptors signal about the state of the body’s motor system. Individual receptors are anatomically connected to each other and form receptive fields that can overlap.

3. Receptor

From Latin Receptum - to take

A receptor is a sensitive nerve ending or specialized cell that converts perceived stimulation into nerve impulses.

All receptors are characterized by the presence of a specific membrane region containing a receptor protein that determines reception processes. Depending on the chosen classification, receptors are divided:

For primary and secondary;

In photo-, phono-, thermo-, electro- and baro-;

On extero- and intero-;

On mechano-, photo- and chemo-;

On nociceptors, heat, cold, tactile, etc.;

For mono- and polyvalent;

For auditory, visual, olfactory, tactile and gustatory;

For contact and remote;

Into phasic, tonic and phase-tonic.

Types of receptors. Adaptation of receptor mechanisms

Adaptation of receptor mechanisms is the process of reducing (reducing) the activity of receptors as a stimulus with constant physical characteristics acts.

The nature of adaptation of receptor mechanisms depends on:

From the properties of the auxiliary apparatus;

From the characteristics of the perceiving structures of the receptor;

From the properties of the regenerative elements of the nerve ending;

For secondary sensory receptors: from the properties of the synaptic contact between the receptor cell and the ending of the sensory neuron.

Pain receptor

Nocireceptor; Nociceptor

A pain receptor is a receptor whose irritation causes pain.

Vestibuloreceptors

Acceleroceptors

Vestibuloreceptors are receptors that perceive changes in the speed and direction of body movement in space. In humans, vestibuloreceptors are represented by hair cells in the membranous labyrinth of the inner ear.

Taste buds

Taste buds are chemoreceptors, the irritation of which causes taste sensations.

Taste buds:

Localized in the oral mucosa;

They react to four types of substances: sour, salty, bitter and sweet.

Secondary sensory receptor

Non-free receptor

A secondary sensory receptor is a receptor that is a specialized cell, the excitation of which is transmitted to the endings of the corresponding afferent neuron.

Glucoreceptors

Glucoreceptors are receptors that are sensitive to changes in the concentration of glucose in the blood.

Distant receptor

Telereceptor

Distant receptor - a receptor that perceives irritations, the source of which is located at some distance from the body.

Visual tuberosities

The visual thalamus is part of the diencephalon; main subcortical centers of sensitivity. Impulses from all receptors of the body enter the visual thalamus along the ascending pathways, and from here to the cerebral cortex.

Interoreceptor

Interoceptor; Visceroreceptor; Internal receptor

From Latin Interior - internally + Capio - to take

Interoreceptor - receptor:

Located in internal organs, tissues or vessels; And

Perceiving mechanical, chemical and other changes in the internal environment of the body.

Skin receptor

Cutaneous receptor - a receptor located in the skin and providing perception of mechanical, temperature and pain stimulation.

Mechanoreceptor

A mechanoreceptor is a sensitive nerve ending that perceives mechanical influences: pressure, acceleration, etc.

Monomodal receptor

Monovalent receptor

Monomodal receptor - a receptor that perceives only one type of stimulation.

Olfactory receptors

Olfactory receptors are chemoreceptors of the mucous membrane of the upper parts of the nasal cavity, the irritation of which causes the sensation of smell.

Primary sensory receptor

Primary sensory receptor - a receptor that is a sensitive nerve ending.

Polymodal receptor

Polyvalent receptor

A polymodal receptor is a receptor that perceives several types of stimuli.

Tissue receptors

Tissue receptors are receptors located in organs and tissues outside specialized reflexogenic zones.

Tonic receptor

Tonic receptor - a thermoreceptor, retinal rod, or other slowly adapting receptor that responds in a more or less constant manner to the absolute magnitude of the stimulus.

Chemoreceptors

Chemoceptors; Chemoreceptors

Chemoreceptors are specialized sensitive cells or cellular structures through which the body of animals and humans perceives chemical stimuli, including changes in metabolism. The effect of chemicals on chemoreceptors leads to the appearance of bioelectric potentials in the chemoreceptors.

Exteroceptor

Exteroceptor; External receptor

From lat.Exter - lat + Recipere - take

Exteroceptor - a receptor localized on the surface of the body and perceiving irritations coming from the external environment. Typically, exteroceptors are specialized nerve epithelial formations.

The receptor is the working organ of the peripheral part of the sensory neuron. The body of the neuron is located in the intervertebral ganglion. The peripheral process of the pseudounipolar ganglion ends in the tissue with a receptor, while the central one enters the spinal cord and is involved in the formation of various sensory pathways.

Sensory nerve fibers are divided into branches, which are directed to different parts of the same tissue or to several different tissues. Nerve endings - receptors - can be located directly on the working structures of surrounding tissues, in such cases they are called free. Others adhere to the surface of special auxiliary cells and form non-free endings. Non-free endings can be enclosed in a more or less complex capsule consisting of auxiliary cells (encapsulated receptors). According to histologists, auxiliary cells perform the functions of supporting tissue and participate in the excitatory process.

From the point of view of functional specialization, it is customary to distinguish extero-, proprio- and interoreceptors. Exteroceptors, as the name suggests, are located on human integumentary tissues and are mostly represented by free endings. Some nerve fibers are strongly branched and form bushes, the branches of which end in fibrillar networks or thickenings among epithelial cells, while others are directed to the free surface of the epithelium without branching and even extend to its surface. The terminal sections of such receptors, together with the desquamating epithelial cells, die and are torn off, which is expressed by the increased regenerative activity of receptors of this structure. Among the specialized receptors of integumentary tissues, one should name non-free endings found in the taste organs (taste buds, bulbs, etc.), tactile Merkel corpuscles, olfactory bulbs, etc. From the point of view of acupuncture, it is important that in practical activities the receptors of the skin and mucous membranes of some parts of the body (nasal septum).

Deeper receptors lie in muscles, fascia, ligaments, periosteum, blood vessels and nerves.

The receptor for striated muscle tissue is a specialized formation of the neuromuscular spindle. It is a part of one or two to three muscle fibers up to several millimeters long, braided with branches of sensitive nerve fiber, which forms a kind of coupling around the muscle fibers. These receptors are free receptors that respond to stretching of muscle tissue.

Myocardial receptors are represented by the aforementioned muscle spindles and “climbing” nerve endings ending in wide fibrillar plates.

In the smooth muscles of various internal organs, only bush-like receptors of various shapes were found.

Receptors of connective tissue and blood vessels are the most diverse. Among them, free, non-free and encapsulated endings are distinguished. More often than others, a variety of bush-like or tree-like receptors of varying degrees of complexity are detected in the connective tissue. The characteristic form of connective tissue receptors are nerve endings in the form of “glomeruli”. The most loose “glomeruli” are penetrated by connective tissue fibers and are stretch receptors, others are relatively isolated from the surrounding tissues, acting as pressure receptors. There are also more complexly arranged nerve endings in the form of Vater-Paccini corpuscles, Krause flasks, Golgi-Mazzoni corpuscles, and Meissner corpuscles. It has been established that Vater - Paccini corpuscles are receptors for mechanical pressure, Krause flasks for temperature, Golgi - Mazzoni pressure and stretching, and Meissner tactile stimuli.

Vascular receptors are no less diverse. The vessels have abundant sensory innervation all the way from the heart to the intraorgan capillaries. The main form of receptors are bush-like endings, which can be free or non-free. They record the state of stretching of the vascular wall, the amount of blood pressure in the vessels, and the chemical composition of the blood. A characteristic feature of the receptors of intraorgan vessels is that they cover with their branches the area of ​​​​the surrounding tissue (vascular tissue receptors). Receptors of lymphatic vessels have been studied to a lesser extent; they are represented by ordinary connective tissue receptors.

Receptors of the peripheral nervous system and autonomic ganglia are varied in shape and perform the functions of general reception.

The nerve impulse generated in the receptors by the sensory fiber action potential reaches the first relay station for processing (perception) of the afferent flow in the central nervous system. The spinal cord (medulla spinalis) in adults is a cord 41–45 cm long, somewhat flattened from front to back. It has two thickenings corresponding to the nerve roots of the upper and lower extremities. Of these thickenings, the lumbar one is larger, but the cervical one is more differentiated, which is associated with the complexly organized motor skills of the hand. In functional terms, it should be emphasized that the organization of sensory complexes at the level of the cervical segments is subordinated to this basic function.

Receptors (Latin receptor - receiving, from recipio - accepting, receiving), special sensitive formations that perceive and transform irritations from the external or internal environment of the body and transmit information about the active agent to the nervous system, receptor. characterized by diversity in structural and functional terms. They can be represented by free endings of nerve fibers, endings covered with a special capsule, as well as specialized cells in complexly organized formations, such as the retina, organ of Corti, etc., consisting of many receptors.