The human brain constantly receives and processes signals coming from the outside world with the help of special systems called analyzers. Their structure and features of work were studied in detail by the brilliant Russian scientist I. P. Pavlov. It turned out that all sensory systems include three structures: peripheral, conductive and cortical.
For example, in the analyzer that perceives smells, the first part is represented by olfactory receptors, then nerves follow, and finally, the last part includes a region in the cerebral cortex. Nerve cells that are the first to perceive stimuli (various odors) are located in the mucous membrane of the nasal cavity, and taste receptors are located on the surface of the mucous membrane of the mouth and tongue. Moreover, we feel bitter, sweet, s alty and sour taste in their various parts.
In our article we will find out what taste and olfactory receptors are, and also determine the physiological mechanismoccurrence of corresponding sensations in the human body.
What is a receptor?
This term, used in the physiology of higher nervous activity, starting with the studies of P. Erlich and P. Anokhin, has several meanings. The most informative is the following: a receptor is an element of the nervous or endocrine system, capable of attaching and binding biological substances-mediators of a chemical or neurogenic nature. According to the theory of nerve endings, this formation spatially coincides with the molecule of an odorous or gustatory substance, like a key and a lock. This is a signal for the emergence of an excitation process in the olfactory receptors located in the peripheral section of the analyzer. It is transmitted further to the following parts of the olfactory perceiving system, in which the analysis of the received information takes place.
Structure of a nerve cell
Neurocyte has not only a body, but also two types of processes. Axon is a very long ending, which serves to transmit nerve impulses that have already arisen in short branches (dendrites). Their complex with supporting cells of epithelial origin and intercellular substance, glia, will look like a receptor formation. The principle of operation of their various types, for example, nerve endings that perceive chemicals, which include olfactory receptors, ultimately comes down to the transfer of excitation to the cortical region of the brain. Consider it further.
Mechanism of receptor activity
It can be presented in the following form: first, there is a perception of stimuli and a change under their action of the polarization of its membrane. Modification of the spatial configuration of signal proteins located on the surface of dendrites is also possible. All this causes the generation of action potentials and, as a result, the appearance of nerve impulses in the neuron. As it turned out, olfactory receptors are able to capture a tiny amount of molecules of various gaseous substances, that is, they have a low sensitivity threshold. How does the perception of these compounds affect the state of our body?
World of smells
In V. Pikul's work "The Fragrant Symphony of Life", the poor perfumer unsuccessfully sought the hand and heart of the main character. To annoy his rival (a famous singer), he came up with the following. The young man brought to the concert a large basket of fragrant violets and placed it on top of the piano. The artist failed to hit a single high note, and his premiere failed. The perfumer, it turns out, knew for sure that the human olfactory receptors, catching the smell of violets, affect the vocal cords, disrupting their work.
Indeed, the olfactory analyzer is one of the most sensitive and understudied types of sensory systems. Its activity is closely related to the perception of taste and strongly affects the emotional and physical state of the human body. On this property of smell, such a branch of medicine as aromatherapy arose. It is known that the smells of lavender and rosemary, whichperceive olfactory receptors, calm the nervous system and relieve stress. The aroma of lemon promotes concentration, while eucalyptus and jasmine increase efficiency.
Chemoreceptor sensory systems
The olfactory analyzer transforms irritations caused by chemical particles into odor sensations. It helps a person to trap toxic, hazardous compounds in the air or identify unsuitable foods. This is vital and is a protective adaptive property of the body. Thus, the olfactory receptor perceives the caustic, irritating mucous membranes of the respiratory tract and lungs with the smell of ammonia in a dose of only 70 molecules per 1 ml of water. Being a chemoreceptor, it transmits excitation to the olfactory nerve. From there, nerve impulses enter the depth of the temporal lobe of the cerebral cortex, where the olfactory zone is localized. Note also that the villi of odor receptors are able to respond to minimal concentrations of chemicals: from 2 to 8 molecules in 1 ml of air.
Nose as an organ of smell
In the mucous membrane of the upper and partially middle nasal passages, on an area of 2.6 to 5 cm2, there are neurocytes, in groups of 8-10 cells. They are associated with supporting cellular elements and have hairs containing fibrils inside. Olfactory cells contain a large number of RNA molecules in the cytoplasm. This is due to a high metabolism and actively occurring reactions of protein biosynthesis. Processes-dendritesdirectly contact with the molecules of odorous gaseous substances. These are olfactory receptors. Chemical compounds play the role of stimuli, under the influence of which the membranes of nerve cells depolarize. This process can be slowed down due to inflammatory reactions resulting from respiratory or allergic diseases of the upper respiratory tract. The epithelial lining of the nose swells, secreting excess mucus. This leads to a decrease in the sensitivity of nerve endings and a deterioration in odor discrimination, up to a complete loss of olfactory and taste sensations.
What determines the sensitivity of receptors?
Olfactory receptors are located in the mucous membrane of the upper respiratory tract, so the occurrence of certain odor sensations is primarily affected by the concentration of the odorous substance in the inhaled air. So, thick oil squeezed out of rose petals has an unpleasant, difficult to detect smell. The delicate aroma of roses appears only when the oil concentrate is strongly diluted.
Specialists identify six basic sensations. These include odors: resinous, floral, spicy, putrid, fruity, burnt. According to the physiological characteristics of perception, clean, irritating and mixed odors are detected. The sensitivity of nerve endings to them decreases if a person is a smoker or abuses alcohol.
Scientific theories of the origin of smell
Among scientists there is no single view on the essence of the mechanismodor perception. The most recognized can be considered the stereochemical theory, according to which the main role in determining the chemical stimulus belongs to the nerve endings of neurons. Olfactory receptors are a kind of antennas that capture odor molecules and change the structure of their own membrane proteins in accordance with the spatial configuration of particles of chemical compounds. As a result of this process, the neuron membrane is polarized, and a nerve impulse occurs, i.e., the occurrence of odor has a dual nature: chemical and neurogenic.
We also note that scientists use the concept of olfactory pigment to explain the appearance of smell. This substance has the same principle of action as rhodopsin and iodopsin - compounds that are part of the visual receptors of the retina: rods and cones. The active molecules of the olfactory pigment contain electrons in an excited state, since odorous substances also cause the transition of charged particles to higher energy levels. Returning to stationary orbits, electrons emit a quantum of energy, which ensures the occurrence of excitation in the nerve ending of the olfactory neuron.
Methods for determining the acuity of smell
Some professions (for example, a perfumer or a taster) require increased sensitivity of the senses of smell and taste. The strong sensitivity of the olfactory analyzer receptors to odors is often an innate property of the human body, but it can also develop after prolongedworkouts. There is a test that is carried out with a device - an olfactometer. It defines the perceptual threshold: the minimum amount of a substance that can cause a corresponding olfactory sensation.
It is used in the diagnosis of anosmia, to calculate the maximum allowable concentrations of toxic volatile substances in industrial emissions. It is necessary to use olfactometry in the work of sanitary and epidemiological laboratories to establish the causes of mass poisoning at enterprises, in places of public catering, and in schools.
