Where are tactile receptors located in humans? Tactile receptors

The skin has a variety of poorly differentiated receptors, which are divided into: 1) tactile, irritation of which causes sensations of touch and pressure; 2) thermoreceptors - heat and cold; 3) painful.

Absolute specificity, i.e. the ability to respond only to one type of irritation is characteristic only of some receptor formations of the skin. Many of them react to stimuli of different modalities. The occurrence of various sensations depends not only on which receptor formation of the skin has been irritated, but also on the nature of the impulse coming from this receptor to the central nervous system.

The perception of mechanical stimuli (touch, pressure, vibration, stretching) is called tactile reception. Tactile receptors are located on the surface of the skin and mucous membranes of the mouth and nose. They become aroused when touched or pressed.

Tactile receptors include Meissner's corpuscles and Merkel's discs, which are found in large numbers on the fingertips and lips. Pressure receptors include Pacinian corpuscles, which are concentrated in the deep layers of the skin, tendons, ligaments, peritoneum, and intestinal mesentery. Nerve impulses originating in tactile receptors travel through sensory fibers to the posterior central gyrus of the cerebral cortex.

In different places of the skin, tactile sensitivity manifests itself to varying degrees. It is highest on the surface of the lips and nose, and is less pronounced on the back, sole, and abdomen. It has been shown that simultaneous touching of two points of the skin is not always accompanied by the appearance of a sensation of two impacts. If these points lie very close to each other, then a sensation of one touch occurs. The smallest distance between points of the skin, upon irritation of which a sensation of two touches occurs, is called the threshold of space. Space thresholds are not the same in different places of the skin: they are minimal on the tips of the fingers, lips and tongue and maximum on the thigh, shoulder, and back.

Ambient temperature is exciting thermoreceptors, concentrated in the skin, on the cornea of ​​the eye, in the mucous membranes. Changing the temperature of the internal environment of the body leads to excitation of temperature receptors located in the hypothalamus.

Temperature receptors are very important in maintaining a constant body temperature, without which the vital functions of our body would be impossible.

There are two types of temperature receptors: those that perceive cold and heat. Warm receptors are represented by Ruffini corpuscles, cold receptors are represented by Krause cones. The bare endings of afferent nerve fibers can also function as cold and heat receptors.

Thermoreceptors in the skin are located at different depths: cold receptors are closer to the surface, and heat receptors are deeper. As a result, the reaction time to cold stimulation is shorter than to thermal stimulation. Thermoreceptors are grouped at certain points on the surface of the human body, with significantly more cold points than warm ones. The severity of the sensation of heat and cold depends on the location of the irritation, the size of the irritated surface and the surrounding temperature.

Painful sensations occur when exposed to any irritant of excessive force. The sensation of pain is of great importance for the preservation of life as a danger signal, causing defensive reflexes of skeletal muscles and internal organs. However, damaging or prolonged stimulation of pain receptors distorts defensive reflexes, making them non-adaptive.

Pain is less localized than other types of skin sensitivity, since the excitation that occurs when pain receptors are irritated is widely distributed throughout the nervous system. Painful sensations also occur when a critical level of irritation of tactile receptors and thermoreceptors is reached. Simultaneous irritation of the receptors of vision, hearing, smell and taste reduces the sensation of pain.

It is assumed that the occurrence of pain is associated with irritation of the endings of special nerve fibers. Data have been obtained indicating that the formation of histamine in the nerve endings plays a role in the formation of pain. The occurrence of pain is also associated with other substances formed in tissues at the site of injury - bradykinin, blood coagulation factor XII (Hageman factor), etc.

Conducting pathways and the cortical end of the skin analyzer. Excitation from the receptors of the skin analyzer is sent to the central nervous system through fibers of different diameters. Fibers of small diameter (with an excitation speed of 30 m/s) switch to the second neuron in the spinal cord. The axons of these neurons, as part of the anterior and lateral ascending tracts, are directed, partially crossing, to the visual thalamus, where the third neuron of the cutaneous sensitivity pathway is located. The processes of these neurons reach the somatosensory zone of the pre- and postcentral gyrus of the cortex.

Thicker fibers (with a conduction speed of 30 to 80 m/s) pass without interruption to the medulla oblongata, where the switch to the second neuron occurs. There, the excitation coming from the receptors of the scalp is transmitted to the second neuron. The axons of the neurons of the medulla oblongata completely intersect at the level of the medulla oblongata and are directed to the visual thalamus. Along the axons of the neurons of the visual thalamus, excitation is transmitted to the somatosensory area of ​​the cortex.

In the visual thalamus, the skin surface of the head and face is represented in the posteromedial zone of the posterior ventral nucleus, and the upper and lower limbs and torso are represented in its anterolateral part. There is a certain organization in the vertical arrangement of neurons that receive information from different parts of the skin surface. The neurons that receive information from the skin surface of the legs are located highest, somewhat lower - from the torso, and even lower - from the arms, neck, and head. The same arrangement is typical for the cortical section of the skin analyzer. Neurons that transmit information from the skin surface are divided into mono-, di- and polymodal. Monomodal neurons perform a discrimination function, while di- and polymodal neurons perform an integrative function.

Age-related characteristics of the skin analyzer. At the 8th week of intrauterine development, bundles of unmyelinated nerve fibers are detected in the skin, which terminate freely in it. At this time, a motor reaction to touching the skin in the mouth area appears. At the 3rd month of development, lamellar body type receptors appear. The nerve elements of the skin analyzer appear first in the skin of the lips, then in the pads of the fingers and toes, then in the skin of the forehead, cheeks, and nose. Then, almost simultaneously, the formation of receptors occurs in the skin of the neck, chest, nipple, shoulder, forearm, and armpit.

The early development of receptor formations in the skin of the lips ensures the occurrence of the sucking act under the influence of tactile stimulation. In the sixth month of intrauterine development, the sucking reflex is dominant in relation to the various fetal movements occurring at this time. It entails the occurrence of various facial movements.

In a newborn, the skin is abundantly supplied with receptor formations and the nature of their distribution on its surface is the same as in an adult. In newborns and infants, the skin around the mouth, eyes, forehead, palms of the hands and soles of the feet is most sensitive to touch. The skin of the forearm and lower leg is less sensitive, and the skin of the shoulders, abdomen, back and thighs is even less sensitive - it corresponds to the tactile sensitivity of the skin of adults.

Newborns react to cold and heat over a much longer period than adults, and the skin of the face is most sensitive to heat to cold more than to heat. The sensation of pain in newborns is presented without localizing its source. The skin of the face is most sensitive to painful irritations. Localization of pain caused by irritation of interoreceptors is absent even in children 2-3 years old. There is no exact localization of all skin irritations in the first months or first year of life. By the end of the first year of life, children easily distinguish between mechanical and thermal stimulation.

An intense increase in encapsulated receptors occurs in the first years after birth. At the same time, their number increases especially strongly in areas exposed to pressure: for example, with the start of walking, the number of receptors on the plantar surface of the foot increases.

Since the hand becomes increasingly important in a person’s life with age, the role of its receptor formations in the analysis and assessment of objects in the surrounding world, in the assessment of movements performed, increases. On the palmar surface of the hand and fingers, the number of polyaxon receptors increases, which are characterized by the fact that many fibers grow into one flask. In this case, one receptor formation transmits information to the central nervous system along many afferent pathways and, therefore, has a large area of ​​​​representation in the cortex. An increase in the number of skin receptors can also occur in an adult, for example, after loss of vision and the need to navigate by touch.

During the first year of life, qualitative transformations of skin receptors occur, and only towards the end of the year do all receptor formations of the skin approach the adult state in their morphofunctional characteristics. Over the years, the excitability of tactile receptors increases, especially in the prepubertal and pubertal periods and reaches a maximum by 17-25 years. Over the course of life, temporary connections are formed between the zone of musculocutaneous sensitivity and other perceptive zones, which make it possible to clearly localize the resulting skin irritation.

Mental fatigue leads to a sharp decrease in the tactile sensitivity of the skin; for example, after five general education lessons it can be halved. Training exercises can increase skin sensitivity.

Tactile receptors, or touch and pressure receptors, are located on the surface of the skin.

Touch receptors are Meissner's corpuscles, located in the skin papillae, and Merkel's discs, located especially in large numbers on the fingertips and lips. Hair on skin that is covered with hair is highly sensitive to touch. This is explained by the fact that the root of the hair is wrapped around a nerve plexus and any touch to the hair is transmitted to this plexus, causing its stimulation. Shaving your hair greatly reduces the skin's sensitivity to touch. Pressure receptors are Pacinian corpuscles.

Thick myelin fibers serve as conductors of tactile reception. Electrophysiological recording of action potentials showed that even with a very short stimulation tactile receptors not one impulse arises in them, but a whole series of discharges.

Adaptation of tactile receptors. Tactile receptors way to quickly adapt, so only the change in pressure is felt, and not the pressure itself. If you place a weight on the plantar pad of a cat’s paw, nerve impulses arise in the receptor, the frequency of which can reach 250-350 impulses/sec. This impulse lasts several seconds and stops due to the onset of adaptation. In humans, a decrease in the frequency of impulses is accompanied by a decrease in the strength of sensation.
The speed of adaptation of different skin receptors is not the same. The receptors located at the hair roots and the Pacinian corpuscles adapt most quickly.
Due to adaptation, a person feels the pressure of clothing only at the moment when it is put on or when the clothing rubs against the skin when moving.

Localization of tactile sensations. A person very accurately attributes all sensations of touch and pressure to a specific place on the skin. The localization of tactile sensations is developed through experience under the control of other senses, mainly vision and muscle sense. To prove this, we can cite Aristotle’s famous experiment: touching a small ball with the crossed index and middle fingers gives the sensation of touching two balls, since ordinary experience teaches that only two separate balls can touch the inside of the index finger and the outside of the middle finger at the same time.

Tactile Sensitivity Measurement. Tactile sensitivity is developed very differently in different areas of the skin. Tactile sensitivity is measured with a Frey esthesiometer, which is used to determine the force of pressure necessary to irritate the receptors and produce sensation.

The irritation threshold for the most sensitive areas of the skin is 50 mg, the least sensitive - 10 g. The sensitivity of the lips, nose, tongue is the highest, the sensitivity of the back, sole of the foot, and abdomen is the least.

Thresholds of space. When two points of the skin are touched simultaneously, two touches are not always felt: if these two points lie close to each other, then only one touch may be felt. The smallest distance between two points of the skin, upon irritation of which the sensation of two touches arises, is called the threshold of space.

The thresholds of space are measured using a compass, or Weber aesthesiometer, which is a compass with a scale indicating the distance between its legs in millimeters.

The thresholds of space are very different at different places on the skin, that is, the sensation of two touches occurs at different distances of the legs of the compass ( rice. 194). Space thresholds are minimal at the fingertips, lips and tongue, where they are 1-2.5 mm, and maximal at the hip, shoulder and back (over 00 mm). Spatial thresholds depend in part on how much afferent nerve fibers branch in the periphery and how many receptors one nerve fiber transmits impulses from. According to electrophysiological observations, the area of ​​the skin surface innervated by one afferent fiber varies in different parts of the body and ranges from several square millimeters to 2-3 cm2 more. Rice. 194. The magnitude of space thresholds in different parts of the human body.

The somatosensory system includes the skin sensitivity system and the sensitive system of the musculoskeletal system, the main role in which belongs to proprioception.

1) Receiving information from receptors.

2) Processing information about various stimuli.

Mechanoreceptors

Nocireceptors

Thermoreceptors

Propriorectors

Rapidly adapting: hair follicle receptors, Pacinian corpuscles, Meissner corpuscles, Krause cones, Free n. endings of type Aδ.

Slowly adapting: Merkel discs, Taurus Rufini, free n. endings like C.

Hair follicle receptors

Location: in the inner layer of the skin, surrounding the hair follicle

Adaptation: fast. The discharge stops 50-500 ms after the stimulus is turned on

Reception: for movement, twitching of hairs, but not for the degree of their displacement

Innervation: one nerve fiber can serve several hundred follicles, and each follicle can be innervated by many receptors

Pacinian corpuscles (lamellar corpuscle, Vater-Pacini corpuscle)

Structure: has the structure of an onion or matryoshka doll. Enclosed in mucous membranes of connective tissue. Inside there is an elliptical nerve window.

Size: 0.5 – 0.7 mm in diameter and about 1-2 mm in length

Location: both in hairy and smooth skin, deep in the skin (in the adipose tissue of the subcutaneous layers, deeper than other solutions), little in the lips, fingertips

Reception: strong and sudden changes in pressure on the skin. They do not respond to constant pressure. Vibration: respond to vibration from 70 to 1000 Hz. However, the greatest sensitivity is at a vibration frequency of 200-400 Hz; in this case, they are able to respond to skin deformation of only 1 micron.

Pacinian corpuscles do not turn off during local anesthesia

Meissner's corpuscles

Located in the superficial layers of smooth skin (papillary dermis) and on the mucous membranes. Most of them are on the lips, palms, fingers, soles

They are analogues of hair follicle receptors for smooth skin.

Structure: oval-shaped connective tissue capsule (length 40-180 µm, width 30-60 µm)

The nerve endings form a spiral inside the capsule, the branches of which are isolated from each other by the membranes of Schwann cells.

The capsule is attached to the overlying layers of the epithelium by collagen fibers (which increases the mechanical connection between it and the surface of the skin)

Reception: responds to touch or pressure

Quickly adaptable. The discharge stops 50-500 ms after the stimulus is turned on

They respond to low-frequency vibration of 10-200 Hz, with a maximum frequency of 30 Hz.

Have small receptive fields

Krause cones (Terminal Krause flasks, Krause bulbs)

Location: smooth skin epidermis and mucous membranes. Only found in non-primate mammals (not in humans)

Structure: similar to Meissner's corpuscles. Lamellar capsule containing a spiral or rod-shaped nerve ending inside

Reception: for a long time it was believed that these were cold solutions, but this is not so. Krause flasks respond to low frequency vibration of 10-100 Hz.

Slowly adapting tactile receptors

1) Merkel discs

2) Taurus Ruffini

3) Free nerve endings (type C)

Illusion of sensory contrast

Merkel discs

Location: on areas of smooth skin they are located in small groups in the lowest layers of the epidermis, from where they are directed to the papillae of the dermis. In hairy areas they are located in special tactile discs (Pincus-Iggo bodies) - small elevations of the skin.

Structure: capsules with large, irregularly shaped nuclei and microvilli

Innervation: three tactile discs may have one nerve fiber, and inside the tactile discs, all Merkel discs (30-50 pcs) are served by the 1st nerve branch.

Reception: respond to touch or pressure. Stimulus – flexion of the epidermis under the action of a mechanical stimulus. Slowly adapt.r-ry. Continue to generate potentials even when pressure is maintained for a long time. They have small receptive fields.

Ruffini corpuscles (Ruffini cylinders, Ruffini endings)

Location: lower layer of dermis and mucous membrane

Reception: They were thought to respond to heat, but this is not the case. React to prolonged skin displacement and pressure.

Slowly adapt, continue to generate potentials even when pressure is maintained for a long time.

They have large receptor fields.

Free nerve endings

Location: in the epidermis and dermis, the most common solutions. Found in almost all areas of the skin.

Structure: do not have specialized detector cells. Type A delta (myelinated) or type C (unmyelinated) fibers extend from the endings.

Reception: excited by very weak, near-threshold stimulation. They react only to 1 gradation of stimulus (yes-no). Can detect weak mechanical stimuli, movement on the skin (crawling insect)

Adaptation: type A delta fibers... ?

Anterior spinothalamic tract - see photo

· The first neuron - axons in the dorsal roots enter the dorsal horn of the SPM, the body is in the SPM ganglion, the dendrite ends in mechanoreceptors of the skin

· The second neuron - axons move to the other side of the SPM and form the anterior spinothamic tract, body and dendrites in the cells of the gelatinous substance (posterior horn of the SPM)

· Third neuron - axons: part in the postcentral gyrus, part in the superior parietal lobule, body and dendrites in the posterior ventrolateral nuclei of the thalamus

Proprioception.

Proprioception is the perception of the posture and movement of our own body. Posture is determined by the angle of the bones at each joint, set either passively (by external forces) or actively (by muscle contraction). Their work combines signals from the vestibular organ, which makes it possible to determine the position of the body in the field of gravity. Proprioceptors are also involved in our conscious and unconscious motor activity. The afferent and efferent systems combine to create conscious proprioceptive sensations. If a sensation, for example, movement in a joint, persists after one of the components of the system is eliminated, it does not necessarily follow that it does not normally participate in the formation of this sensation. This corresponds to the principle of redundancy of the nervous system. Afferent information can be modulated at synapses by descending inhibition.
At the synapses through which the activity of afferents is transmitted to the central somatosensory neuron, it can change the magnitude of the receptive field of that neuron if afferents coming from the peripheral part of the receptive field are inhibited.

Types of proprioceptors

Mammals:

1) Muscle spindles

Mammalian muscle fibers

1) Extrafusal. Do all the work of muscle contraction

2) Intrafusal. They lack actin and myosin. They are designed to detect tension using solutions called muscle spindles

· Static. They react with constant muscle tension. Contraction force is detected

· Dynamic. They respond to on-off muscle stretching. Contraction speed is detected

2) Golgi tendon organs

IN tendons- part of the muscle, which is a connective tissue formation, through which the muscle is attached to the bone.

SOG – cluster-shaped sensory endings (2-3 mm in length and 1-1.5 mm in width). They are excited by muscle contraction due to tension in the tendons.

3) Joint receptors

· In articular capsules: endings like Ruffini's corpuscles. Slow to adapt. Each has its own “excitation angle”

· In articular ligaments: endings like Golgi corpuscle and Pacinian corpuscle. They are activated when the joint moves to extreme positions or when its rotation is outside the normal range.

Nerve pathways

1) Cortical proprioceptive pathway– precisely localized conscious proprioceptive sensations

· Burdach's path

· Gaulle's path

His defeats:

1. Loss of sense of position and locomotion. With eyes closed, the patient cannot determine the position of his limbs

2. Astereognosis. With eyes closed, the patient can recognize and describe an object by touch.

2) Cerebellar pathways– unconscious coordination of movements

Flexing Path

Govers Way

Lesions of these pathways: movement coordination disorder. It becomes impossible to perform even the simplest movements without visual control without making gross mistakes. For example, touch the tip of your nose.

Body diagram

Body diagram– unconscious ideas about the position of one’s own body and its parts in space, about its boundaries and dynamic characteristics.

Properties of the body diagram (according to Haggard and Wolpert)

1) Spatial coding

3-dimensional spatial coordinates of the body and objects around it. The idea of ​​the boundaries of the body may not correspond to its real boundaries (tennis - the representation of the body as the end of a racket).

2) Modularity

The body diagram is not represented in any single region of the brain. Different parts of the body are in different areas of the cortex.

3) Adaptability

Ideas about one's own body diagram develop over the course of life.

Somatosensory plasticity

4) Update while moving

After performing the movements, the body diagram changes according to the new body position

5) Interpersonality

St with mirror neurons.

6) Supramodality

Oliver Sacks. “The Man Who Fell Out of Bed” The body schema is not related to the primary sensory modality. It includes proprioception, vision, tactile information, etc. Sensory information is recoded into an abstract, supramodal form.

7) Coherence

When forming a body diagram, information from different senses is integrated.

Related information.

(touch)

After I have described the structure and structure of the nervous system, it is time to think about how this system works. It is very easy to see that in order for the nervous system to control the actions of the organism for the benefit of the latter, it must constantly evaluate the details of the environment. It is useless to quickly lower your head if it is not in danger of colliding with some object. On the other hand, it is very dangerous not to do this if such a threat exists.

In order to have an idea of ​​the state of the environment, it is necessary to sense or perceive it. The body senses the environment through the interaction of specialized nerve endings with certain environmental factors. The interaction is interpreted by the central nervous system in ways that differ depending on the nature of the receiving nerve endings. Each form of interaction and interpretation is distinguished as a special type of sensory (sensory) perception.

In everyday speech we usually distinguish between five senses - sight, hearing, taste, smell and tactility, or the sensation of touch. We have separate organs, each of which is responsible for one type of perception. We perceive images through the eyes, auditory stimuli through the ears, smells reach our consciousness through the nose, and tastes with the tongue. We can group these sensations into one class and call them specialized sensations, since each of them requires the participation of a special (that is, special) organ.

No special organ is required to perceive tactile sensations. Nerve endings that sense touch are scattered throughout the body. Touch is an example of a general sensation.

We are quite poor at differentiating sensations, the perception of which does not require the participation of special organs, and therefore we speak of touch as the only sensation that we perceive through the skin. For example, we often say that something is “hot to the touch,” when in fact touch and temperature are sensed by different nerve endings. The ability to perceive touch, pressure, heat, cold and pain is united by the general term - cutaneous sensitivity, since the nerve endings with which we perceive these irritations are located in the skin. These nerve endings are also called exteroceptors (from the Latin word “extra,” meaning “outside”). Exteroception also exists inside the body, since the endings located in the wall of the gastrointestinal tract are essentially exteroceptors, since this tract communicates with the environment through the mouth and anus. One could consider the sensations resulting from irritation of these endings to be a type of external sensitivity, but it is distinguished into a special type called interoception (from the Latin word “intra” - “inside”), or visceral sensitivity.

Finally, there are nerve endings that transmit signals from the organs of the body itself - from muscles, tendons, ligaments, joints, and the like. This sensitivity is called proprioceptive (“proprius” in Latin means “own”). We are least aware of proprioceptive sensitivity, taking the results of its work for granted. Proprioceptive sensitivity is realized by specific nerve endings located in various organs. For clarity, we can mention the nerve endings located in the muscles, in the so-called specialized muscle fibers. When these fibers stretch or contract, impulses arise in the nerve endings, which are transmitted along the nerves to the spinal cord, and then, along the ascending tracts, to the brain stem. The greater the degree of stretching or contraction of the fiber, the more impulses are generated per unit time. Other nerve endings respond to pressure in the feet when standing or in the gluteal muscles when sitting. There are other types of nerve endings that respond to the degree of tension in the ligaments, to the angle of relative position of the bones connected in the joints, and so on.

The lower parts of the brain process incoming signals from all parts of the body and use this information to coordinate and organize muscle movements designed to maintain balance, change awkward body positions, and adapt to external conditions. Although the normal work of the body in coordinating movements while standing, sitting, walking or running eludes our awareness, certain sensations sometimes reach the cerebral cortex, and thanks to them we are aware at any time of the relative position of the parts of our body. Without looking, we know exactly where and how our elbow or big toe is located, and with our eyes closed we can touch any part of the body that is named to us. If someone bends our arm at the elbow, we know exactly what position our limb is in without having to look at it. In order to do this, we must constantly interpret the countless combinations of nerve impulses entering the brain from stretched or bent muscles, ligaments and tendons.

Various proprioceptive perceptions are sometimes united under the general name of positional sense, or sense of position. This sense is often called kinesthetic (from the Greek words meaning “sense of movement”). It is unknown to what extent this feeling depends on the interaction of the forces developed by the muscles with the force of gravity. This issue has become especially relevant for biologists recently, in connection with the development of astronautics. During long-term space flights, astronauts spend a long time in a state of weightlessness, when proprioceptive sensitivity is deprived of signals about the usual effects of gravity.

As for exteroceptive sensitivity, which perceives modalities such as touch, pressure, heat, cold and pain, it is mediated by nerve impulses that are generated in nerve endings of a certain type for each type of sensitivity. To perceive all types of stimuli, except pain, nerve endings have certain structures, which are named after the scientists who first described these structures.

Thus, tactile receptors (that is, structures that perceive touch) often end in Meissner's corpuscles, which were described by the German anatomist Georg Meissner in 1853. The receptors that perceive cold are called Krause cones, named after the German anatomist Wilhelm Krause who first described these structures in 1860. Thermal receptors are called Ruffini end organs, named after the Italian anatomist Angelo Ruffini, who described them in 1898. The pressure receptors are called Pacinian corpuscles, named after the Italian anatomist Filippo Pacini, who described them in 1830. Each of these receptors is easily distinguished from other receptors by its morphological structure. (However, pain receptors are simply exposed nerve fiber endings, devoid of any structural features.)

Specialized nerve endings of each type are adapted to perceive only one type of irritation. A light touch to the skin in the immediate vicinity of a tactile receptor will cause an impulse to occur in it, but will not cause any reaction in other receptors. If you touch the skin with a warm object, the heat receptor will react to this, but the others will not respond with any reaction. In each case, the nerve impulses themselves are identical in any of these nerves (indeed, the impulses are identical in all nerves), but their interpretation in the central nervous system depends on which nerve transmitted a particular impulse. For example, an impulse from a heat receptor will produce a sensation of warmth regardless of the nature of the stimulus. When stimulating other receptors, specific sensations also arise that are characteristic only of this type of receptor and do not depend on the nature of the stimulus.

(This is also true for specialized sense organs. It is a well-known fact that when a person receives a blow to the eye, “sparks fly” from it, that is, the brain interprets any irritation of the optic nerve as light. Sharp pressure on the eye will also cause a sensation of light. Then the same thing happens when the tongue is stimulated with a weak electric current. With such irritation, a person develops a certain taste sensation.)

Cutaneous receptors are not located in every area of ​​the skin, and where one type of receptor is present, other types of receptors may not be present. The skin can be mapped according to different types of sensitivity. If we use a fine hair to touch different areas of the skin, we will find that in some places the person perceives the touch and in others it does not. With a little more work, we can similarly map the skin for heat and cold sensitivity. The gaps between receptors are small, and therefore in everyday life we ​​almost always respond to stimuli that irritate our skin. In total, the skin contains 200,000 nerve endings that respond to temperature, half a million receptors that respond to touch and pressure, and about three million pain receptors.

As one would expect, tactile receptors are most densely located in the tongue and in the fingertips, that is, in those places that by nature are intended for exploring the properties of the surrounding world. The tongue and fingertips are hairless, but in other areas of the skin tactile receptors are associated with hair. Hair is a dead structure, completely devoid of sensitivity, but we all know well that a person feels any, even the slightest touch to the hair. The obvious paradox can be explained very simply if we understand that when we touch a hair, it bends and, like a lever, exerts pressure on the area of ​​skin located next to it. Thus, stimulation of tactile receptors located in close proximity to the hair root occurs.

This is a very useful property, as it allows us to feel touch without direct skin contact with a foreign object. At night, we can locate an inanimate object (that we cannot see, hear or smell) if we touch it with our hair. (There is also the ability to echolocate, which we will discuss shortly.)

Some nocturnal animals perfect their “hair sensitivity.” The most familiar example is the cat family, which includes the well-known domestic cats. These animals have whiskers, which zoologists call vibrissae. These are long hairs, they touch objects at a fairly large distance from the surface of the body. Hair is quite stiff, so the physical impact is transmitted to the skin without attenuation, that is, with minimal loss. Vibrissae are located near the mouth, where the concentration of tactile receptors is very high. Thus, the dead structures, insensitive in themselves, became extremely subtle organs of perception of tactile stimuli.

If the touch becomes more intense, it begins to stimulate the Pacinian corpuscles in the nerve endings that perceive pressure. Unlike tactile receptors located on the surface of the skin, pressure sensing organs are localized in the subcutaneous tissues. There is a fairly thick layer of tissue between these nerve endings and the environment, and the impact must be greater to overcome the cushioning effect of this protective cushion.

On the other hand, if touch continues long enough, the nerve endings of the tactile receptors become less and less sensitive and eventually stop responding to touch. That is, you are aware of the touch at the very beginning, but if its intensity remains unchanged, then the sensation of touch disappears. This is a reasonable decision, because otherwise we would constantly feel the touch of clothes and many other objects on our skin, and these sensations would load our brain with a lot of unnecessary and useless information. In this regard, temperature receptors behave in a similar way. For example, the water in the bathtub seems very hot to us when we lie down in it, but then, as we “get used” to it, it becomes pleasantly warm. Likewise, the cold lake water becomes pleasantly cool some time after we dive into it. The activating reticular formation blocks the flow of impulses that carry useless or insignificant information, freeing the brain for more important and pressing matters.

In order for the sensation of touch to be perceived for a long time, it is necessary that its characteristics constantly change over time and that new receptors are constantly involved in it. Thus, touching turns into tickling or caressing. The thalamus is capable of localizing such sensations to some extent, but to accurately determine the location of the touch, the cerebral cortex must come into play. This fine discrimination is performed in the sensory cortex. So, when a mosquito lands on our skin, an accurate strike follows immediately, without even looking at the unfortunate insect. The accuracy of spatial discrimination varies depending on the location on the skin. We perceive as separate touches on two points on the tongue, separated from each other by a distance of 1.1 mm. In order for two touches to be perceived as separate, the distance between the stimulated points on the fingers must be at least 2.3 mm. In the nose, this distance reaches 6.6 mm. However, it is worth comparing these data with data obtained for the skin of the back. There, two touches are perceived as separate if the distance between them exceeds 67 mm.

When interpreting sensations, the central nervous system does not simply differentiate one type of sensation from another or one place of stimulation from another. It also determines the intensity of irritation. For example, we can easily determine which of two objects is heavier if we hold one in each hand, even if the objects are similar in volume and shape. A heavier object puts more pressure on the skin, more strongly excites pressure receptors, which in response are discharged with more frequent volleys of impulses. We can also weigh these objects by moving them up and down alternately. A heavier object requires more muscle effort to overcome gravity for movements of the same amplitude, and our proprioceptive sense will tell us which hand develops more force when lifting its object. (The same applies to other senses. We distinguish the degree of heat or cold, the intensity of pain, the brightness of light, the volume of sound, and the strength of smell or taste.)

Obviously, there is a certain threshold of discrimination. If one object weighs 9 ounces and another 18 ounces, then we can easily determine this difference even with our eyes closed, simply by weighing these objects on the palms of our hands. If one object weighs 9 ounces and another 10 ounces, then we will have to “jig” the objects in our hands, but in the end the correct answer will still be found. However, if one item weighs 9 ounces and the other weighs 9.5 ounces, you probably won't be able to tell the difference. The person will hesitate, and his answer may be equally likely to be correct or incorrect. The ability to distinguish the strength of stimuli lies not in their absolute difference, but in their relative one. It is the difference of 10% that plays a role in distinguishing between objects weighing 9 and 10 ounces, respectively, rather than an absolute difference of one ounce. For example, we will not be able to tell the difference between objects weighing 90 ounces and 91 ounces, although the difference in weight is the same one ounce. But we can easily tell the difference between objects weighing 90 and 100 ounces. However, it will be quite easy for us to determine the difference between the weights of objects if one of them weighs one ounce, and the other one ounce and a quarter, although the difference between these quantities is much less than one ounce.

In another way, the same thing can be said this way: the body evaluates the difference in the intensity of any sensory stimuli on a logarithmic scale. This law is called the Weber-Fechner law, after the names of two German scientists - Ernst Heinrich Weber and Gustav Theodor Fechner, who discovered it. By functioning in this way, the senses are able to process a larger range of stimulus intensities than would be possible with linear perception. Suppose, for example, that some nerve ending can discharge twenty times more often under maximum impact than under minimal impact. (At a level of stimulation above the maximum, nerve damage occurs, and at a level below the minimum there is simply no response.) If the nerve ending responded to stimulation on a linear scale, then the maximum stimulus might be only twenty times stronger than the minimum. When using a logarithmic scale - even if we take 2 as the base of the logarithm - the maximum frequency of discharges from the nerve ending will be achieved if the maximum stimulus is two to the twentieth power of times higher than the minimum. This number is approximately one million.

It is thanks to the fact that the nervous system works according to the Weber-Fechner law that we are able to hear thunder and rustle of leaves, see the sun and barely noticeable stars.

Skin receptors are responsible for our ability to sense touch, heat, cold and pain. Receptors are modified nerve endings that can be either free, unspecialized or encapsulated complex structures that are responsible for a certain type of sensitivity. Receptors play a signaling role, so they are necessary for humans to interact effectively and safely with the external environment.

Main types of skin receptors and their functions

All types of receptors can be divided into three groups. The first group of receptors is responsible for tactile sensitivity. These include Pacinian, Meissner, Merkel and Ruffini corpuscles. The second group is
thermoreceptors: Krause flasks and free nerve endings. The third group includes pain receptors.

The palms and fingers are more sensitive to vibration: due to the large number of Pacinian receptors in these areas.

All types of receptors have different zones of sensitivity, depending on the function they perform.

Skin receptors:
. skin receptors responsible for tactile sensitivity;
. skin receptors that respond to temperature changes;
. nociceptors: skin receptors responsible for pain sensitivity.

Skin receptors responsible for tactile sensitivity

There are several types of receptors responsible for tactile sensations:
. Pacinian corpuscles are receptors that quickly adapt to changes in pressure and have wide receptive fields. These receptors are located in the subcutaneous fat and are responsible for gross sensitivity;
. Meissner's corpuscles are located in the dermis and have narrow reception fields, which determines their perception of fine sensitivity;
. Merkel bodies - adapt slowly and have narrow receptor fields, and therefore their main function is the sensation of surface structure;
. Ruffini's corpuscles are responsible for sensations of constant pressure and are located mainly in the area of ​​​​the soles of the feet.

Also separately identified are receptors located inside the hair follicle, which signal the deviation of the hair from its original position.

Skin receptors that respond to temperature changes

According to some theories, there are different types of receptors for the perception of heat and cold. Krause flasks are responsible for the perception of cold, and free nerve endings are responsible for hot. Other theories of thermoreception claim that it is free nerve endings that are designed to sense temperature. In this case, thermal stimulation is analyzed by deep nerve fibers, and cold stimulation by superficial ones. Between themselves, temperature sensitivity receptors form a “mosaic” consisting of cold and heat spots.

Nociceptors: skin receptors responsible for pain sensitivity

At this stage, there is no final opinion regarding the presence or absence of pain receptors. Some theories are based on the fact that free nerve endings located in the skin are responsible for the perception of pain.

Prolonged and severe painful stimulation stimulates the emergence of a stream of outgoing impulses, and therefore adaptation to pain slows down.

Other theories deny the presence of separate nociceptors. It is assumed that tactile and temperature receptors have a certain threshold of irritation, above which pain occurs.