The stay of the human body in the external environment requires a continuous analysis of the environmental impact of the same. The information received is transmitted to the nerve centers. Thanks to analyzers, which serve as unique biological devices, the brain is constantly aware of the state of internal organs. The physiology of the sensory system is based on a number of principles that ensure the perception of signals of physical and chemical energy by appropriate receptors, with its subsequent transformation into nerve impulses and transmission to the brain through neural circuits.
Organization of sensory perception in humans
The subject of physiology of sensory systems is the study of the relationship of the three main departments that are found in any neural-receptor structure:
- Peripheral - consists of nerve endings that perceive the effect, and organs that contribute to the performance of receptors of their functions. This group includes eyes, ears, skin, etc.
- Conductor - a complex of pathways and subcortical nerve centers.
- Cortical - these are areas of the cerebral hemispheres to which incoming impulses are addressed.
Nerve pathways bind receptors to cortical cells. Basically, they consist of four neurons:
- the first, located outside the central nervous system (in the nodes of the spinal cord and cranial nerves, including the spiral snail and vestibular);
- the second, present in the spinal, oblong or midbrain;
- the third, related to the diencephalon (thalamus nucleus);
- the fourth, which is a cortical cell of the cerebral hemispheres.
The role of stimuli and receptors
Speaking about the physiology of sensory systems and GNI (higher nervous activity) of a person, one cannot ignore the question of their purpose, the answer to which lies in the description of the main functions:
- Obtaining and processing information about the external environment of the organism and the state of internal organs.
- Maintaining feedback with nerve centers through reporting on performance.
- Ensuring the health of the brain.
The ability to distinguish and analyze external and internal stimuli is the main advantage of receptor analyzers. The physiology of the sensory system implies the ability to refine and hone skills. For example, in order to improve the technique of performed movements or any sports exercises, the central nervous system must constantly receive information about the duration and intensity of muscle contractions, speed, points of movement of the body, change of pace, etc.
In the end, due to the physiology of sensory systems, a significant contribution is made to the regulation of the working state of the body. Impulses coming from various receptors to the cerebral cortex along the nerve pathways are necessary to maintain its normal functionality. In the framework of experiments conducted on animals, scientists were able to prove that in the event of a forced shutdown of the senses, there is a sudden decrease in the tone of the cerebral cortex, it goes into "sleep mode". The experimental creature woke up exclusively for feeding and when urged to empty the bladder or intestines.
Adaptation of the sensory systems of the body
In his lectures on the physiology of sensory systems, the Russian scientist I.P. Pavlov pays special attention to the properties of receptors. In his opinion, the main one is selective sensitivity to external and internal stimuli. However, most analyzers are configured to perceive only one modality of pathogens - light, sound, taste, pain, or others. The degree of receptivity of receptors will be much higher for specific and unusual stimuli.
Another property of receptors is the establishment of low sensitivity thresholds for adequate external influences. For example, the physiology of the visual sensory system determines the degree of excitation of photoreceptors, which depends primarily on the effect of light energy. The start-up of the analyzers also occurs under the influence of inadequate stimuli (for example, the reflection of light from mechanical or electrical effects). In these cases, the excitation thresholds are an order of magnitude higher.
Since any living organism within the framework of its own physiology has the ability to adapt, that is, adapt to uncomfortable and unusual conditions, the habituation processes affect not only the functions of the receptors, but also all parts of the sensory system. It is possible to get used to various peripheral elements due to the variability of the excitation thresholds of the analyzers. As soon as they increase, receptor sensitivity decreases, and adaptation to prolonged monotonous effects occurs. For example, imagine a ticking clock. Upon arrival at the room, a person immediately draws attention to an extraneous sound that, to put it mildly, is not to his liking, but after some time he will no longer notice a continuously acting stimulus.
When considering the physiology of human sensory systems, the speed of adaptation to long-term irritation of receptors is not ignored. The nerve endings can get used to the action of the stimulus quickly (such receptors are called phase receptors) and slowly. Those receptors that do not immediately adapt to the pathogen are called tonic. The former respond to the stimulus only at the beginning and end of its action, notifying the central nervous system of two to three impulses. Tonic receptors send stable, unremitting signals to the cerebral cortex.
According to the teachings of I.P. Pavlov on the physiology of the sensory system, adaptation can be accompanied by an increase and decrease in the threshold of receptor excitability. For example, moving from a well-lit room to twilight, a person gradually adapts to the need to distinguish objects in the dark. The excitability of the receptors is high. However, if a person returns to a bright room, during the transition he will feel a sharp discomfort and the effect of short-term blindness. Under such conditions, photoreceptor excitability instantly decreases, and the sensory system adapts to the external environment.
As already noted, external information is transmitted from the nerve endings to the higher parts of the brain in specific and non-specific ways. These include conductors of the visual, auditory, motor and other sensory systems. The physiology of each of them involves participation in the non-specific part of the brain, which does not have a direct connection with peripheral receptors.
Features of visual sensory
This system of perception and analysis of light stimuli is designed to transmit most of the information about the external environment. Thanks to her, we perceive light rays exclusively in the accessible part of the spectrum.
The structure and physiology of the visual sensory system is predetermined by the organization principles described above, therefore it consists of peripheral (visual organs), conductor (optic nerves) and cortical (four groups of neurons) departments.
The eyeball is a spherical chamber, the radius of which reaches 12.5 mm. Inside it contains elements that conduct light - the cornea, the moisture of the anterior chamber, the lens and the vitreous. The latter is a gel-like liquid, the purpose of which is to refract light rays and focus them towards the retina and the receptors located on it.
The eyeball is surrounded by three shells, each of which plays a role in sensory perception:
- Outdoor . This is an opaque membrane (sclera) that passes into the cornea.
- Medium . It is located in front of the eyeball and is involved in the formation of the ciliary body and the iris, which determines the color of the eyes. In the middle of the iris there is a pupil - a hole that controls the amount of transmitted light rays through its own narrowing or expansion.
- Inner . It is a retina that contains photoreceptors of the eye. Thanks to this shell, light energy is converted into nervous excitement. A clear image on it is provided by the light-refracting media of the eye (cornea and lens).
The refraction of rays arriving at right angles to the retina through the center of the main optical axis, i.e., the middle of the light-refracting bodies, is impossible. The remaining rays, arriving not perpendicularly, converge inside the eye chamber at a single point - the focus. Ophthalmologists call the ability of the eye to see objects of different distances as accommodation. With age, the visual near point moves away, as accommodation and lens elasticity deteriorate. If at 7 years a person sees clearly at a distance of 7 cm, then in old age this becomes impossible. The near point is getting farther, senile hyperopia develops.
How is our hearing
To perceive the sound vibrations coming from the external environment, a person is endowed with an auditory sensory system. The physiology of auditory analyzers is important for maintaining communication and verbal communication in society. In addition, the sensory hearing system is important for assessing the pace and rhythm of movement.
The peripheral part is represented by the outer, middle and inner ear. This is a complex body that transfers information from the outside to the conductor area. The first neuron, which receives excitation from the receptors of the inner ear, is located in a spiral snail node, after which information passes through its fibers to the second neuron in the medulla oblongata. The next neuron receives part of the impulses in the posterior region of the midbrain, and the rest goes to the nuclei of the intermediate internal cranked body. In the cortical department, in particular, in the area of the projection auditory field, where the fourth neuron is located, a complex process of processing the received sound information takes place.
Considering the physiology of the sensory system of hearing, you should pay attention to the process of perception of sound. Information enters the auricle, after which the sounds are separated by the frequency and place of their maximum effect on the membrane. The next stage is the conversion of mechanical vibrations by receptors with further excitation of neurons.
In the process of studying the physiology of higher nervous activity and the sensory system of hearing, in particular, more attention is paid to changes provoked by membrane displacement due to fluctuations. The decisive role here belongs to the pitch. The maximum displacement of the membrane is observed in a person whose hearing is focused on the adoption of high frequencies - they give the greatest effect, while low frequencies reach only the top of the cochlea. At different sound frequencies, hair cells and various nerve fibers are excited. Against the background of an increase in sound power, the intensity of membrane vibrations also increases.
The organs of sense of balance and body position in space
In the general physiology of sensory systems, there is a section devoted to the study of the functions and relationships of the vestibular apparatus with the cerebral cortex. Having become acquainted with it, you can get a detailed idea of how external and internal impulses help the body maintain the balance of the body. The fully functioning vestibular sensory system enables a person to regulate and maintain a certain position of the body, to organize movements in space. Its peripheral part is located in the inner ear, and the cavities and channels in the temporal region form a bone labyrinth, partially filled with membranous formations. In fact, this is what is called the vestibular apparatus. Between the bony and membranous sections there is a liquid perilymph.
The vestibular sensory system is interconnected with the centers of the spinal cord and brain, which are responsible for a number of somatic and autonomic reflexes (changes in muscle tone, eye movement, nystagmus). In addition to the analytic function necessary for controlling posture and movements, the vestibular system affects the functioning of the visual organs. By the way, it does not always positively affect individual body functions that arise as a result of irradiating excitation on accessible nerve centers. Irritation of the vestibular apparatus leads to a decrease in the excitability of the visual and skin sensory, which ultimately provokes a decrease in control over the accuracy of movements. Violations of coordination and gait, heart rhythm failure, sudden changes in blood pressure, nausea and vomiting - this is not the whole list of adverse reactions to which vestibular irritations can lead.
Other sensor systems
In the skin and internal organs, receptors are also present that are excited by exposure to stimuli. According to the physiology of GNI and sensory systems, they include the following types of reception:
- skin;
- visceroceptive;
- olfactory;
- flavoring.
Nerve endings in the skin
The human epidermis is represented by tactile, pain and temperature reception. So, for 1 square. cm skin tissue accounts for about 100 pain points, about 15 of those that respond to lower or higher temperatures, and 25 tactile. Each of them has its own characteristics.
So, the physiology of the pain sensory system, according to most experts, is primitive in comparison with tactile. As such, there are no pain receptors - any mechanical irritation is perceived by the nearest nerve endings. A receptive reaction also occurs with temperature exposure.
Thanks to tactile sensing, a person is able to analyze the degree of pressure on the skin and feel touch. In addition to free nerve endings, the bodies of Paccini and Meissner respond to external irritation - these are complex formations, the nerve endings of which are contained in a kind of capsule. Tactile receptors are present in the skin vessels, upper and lower epidermal layers, as well as in the hair follicles. Especially a lot of them on the fingers, palms, lips and soles.
The temperature sensory system is represented in the skin by cold and thermal receptors. If the epidermis cools down to +31 ° C, the thermal nerve endings lose activity, and the cold ones, on the contrary, are included in the work. They completely stop responding to any stimuli when the skin temperature drops to +12 ° C.
Sensitivity of internal organs
The visceral sensory system is a complex of receptors found in the internal organs. These include vascular baroreceptors, chemoreceptors, thermoreceptors. Thanks to the visceral nerve endings, the central nervous system is quickly informed of any changes that occur in the body. The signals enter the diencephalon and other areas of the cerebral cortex. The activity of the visceral sensory system is in no way felt during the normal functioning of organs and systems, but it makes itself felt with severe irritation, the development of diseases.
Physiology of olfactory and gustatory reception
The processes of transmitting information about smell and taste are among the most studied in the field of physiology of sensory systems. The olfactory and gustatory endings are intended for the analysis and perception of irritants of a chemical nature. Chemoreceptors, which are hairy bipolar cells, are located in the upper epithelium of the nasal passages and are responsible for the perception of odors from the environment. They transmit information to the cells of the olfactory bulb of the brain. At the same time, chemoreceptors react differently to molecules of aromatic substances.
In many ways, the taste sensory system is similar to the olfactory one. Its physiology has some differences. For example, chemoreceptors are not located on the mucous membrane of the nasal passages, but in the epithelium of the tongue, soft palate and posterior wall of the larynx. With age, the number of taste buds decreases. In children, microvilli that respond to incoming substances are several times more than in adults.
Interestingly, the receptors of different parts of the language perceive only four tastes: bitter, sour, sweet and salty. Against the background of pregnancy or some diseases, taste sensations can change. Information entering the brain from this sensory system is crucial for organizing eating behavior and optimal gastrointestinal tract function.