The human body is endowed with the ability to perceive both the external and internal world, the impact of which can receive various signals. Such signals in the human body are capable of being perceived by receptors - special nerve endings.
What is a receptor and what is its purpose in the body?
Receptors are a set of nerve fiber endings that are highly sensitive and capable of perceiving many internal factors and external stimuli and converting them into a ready-made impulse for transmission to the brain. In other words, any information received by a person from the outside has the ability to be captured and correctly perceived by the human body precisely thanks to the receptors, of which there are a huge number.
Types of receptors and their classification
For each sensation, scientifically called a stimulus, there is its own type of analyzer that is capable of converting it into an impulse accessible to the nervous system. To better understand what receptors are, you first need to understand their classification.
Receptors can differ in location and type of signals received:
- exteroceptors are taste, visual, auditory and tactile receptors;
- interoreceptors - responsible for the musculoskeletal system and control of internal organs.
Human receptors are also classified depending on the form of manifestation of the stimulus:
- chemoreceptors - receptors of smell, tongue and blood vessels;
- mechanoreceptors - vestibular, tactile, auditory;
- thermoreceptors - skin and internal organs receptors;
- photoreceptors - visual;
- nociceptive (pain) receptors.
Receptors are also distinguished by their ability to transmit quantitative impulses:
- monomodal - capable of transmitting only one type of stimulus (auditory, visual);
- polymodal - can perceive several types (pain receptors).
Principles of receptor functioning
Having considered the above classification, we can conclude that perception is distributed depending on the types of sensations for which there are certain sensory systems in the body that differ in functional features, namely:
- taste system (tongue receptors);
- olfactory system;
- visual system;
- vestibular apparatus (motor skills, movement);
- auditory sensory system (auditory receptors).
Let's look at each of these systems in more detail. This is the only way to fully understand what receptors are.
Taste sensory system
The main organ in this system is the tongue, thanks to the receptors of which the human brain is able to evaluate the quality and taste of food and drinks consumed.
The tongue contains mechanoreceptors that can evaluate the consistency of foods, thermoreceptors that determine the temperature level of food, and chemoreceptors that are directly involved in determining taste. The receptors of the tongue are located in taste buds (buds), which contain a set of proteins that, upon contact with an irritant, change their chemical properties, thereby forming a nerve impulse for transmission to the brain. They are able to distinguish four types of tastes:
- salty - the front part of the tongue (except for the tip);
- bitter - the back of the organ;
- sour - lateral receptors;
- sweet - receptors on the tip of the tongue.
But only in conjunction with the olfactory system is the human brain able to assess the completeness of the sensations transmitted by receptors and, if something happens, protect against unsuitable products for consumption.
Olfactory sensory system
The main organ in this system is the nose. The system got its name due to the content of the olfactory glands in which cells of the same name are formed. When reacting with a stimulus, they form olfactory filaments for transmission to the cavity of the cranium, and then to the brain. The olfactory system consists of:
- perceiver (olfactory organs);
- conduction (olfactory nerve);
- central sections (olfactory bulb).
In other words, the stimulus is captured by olfactory receptors and transmitted along the olfactory nerve to the bulb, which is connected by branches to the subcortex of the forebrain.
Visual sensory system
One of the most significant systems in human life and having a complex structure. The main organs in the visual system are the eyes. Let's look at what eye receptors are. The retina of the eye is a center of nerve endings in which incoming signals are processed and converted into impulses ready for transmission to the brain. Signals are transmitted thanks to special cells with different functions:
- photoreceptors (cones and rods);
- ganglion cells;
- bipolar cells.
Thanks to photosensitive cells, the visual analyzer perceives color images in the daytime and at dusk at a speed of 720 m/s.
Vestibular apparatus
The receptors of this system are secondary sensory cells that do not have their own nerve endings. The transmission of impulses occurs when the position of the head or body changes in relation to the surrounding space. Thanks to the impulses received, the human body is able to maintain the desired body position. An important part of this system is the cerebellum, which senses vestibular afferents.
Auditory sensory system
A system that makes it possible to capture any sound vibrations. The hearing organ contains the following receptors:
- organ of Corti - perceives sound stimuli;
- receptors necessary to maintain body balance.
Auditory receptors are located in the cochlea of the inner ear and perceive sound vibrations with the help of auxiliary structures.
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.
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.
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.
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 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.
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
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 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.
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 in the receptors occurs 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