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 the work were studied in detail by the brilliant Russian scientist I.P. Pavlov. It turned out that the composition of all sensory systems includes three structures: the peripheral part, the conductor and cortical.
For example, in an analyzer that perceives odors, the first part is represented by the olfactory receptors, then the nerves follow, and finally the last part includes a section in the cerebral cortex. Nerve cells, the first to perceive irritants (various smells), are located in the mucous membrane of the nasal cavity, and taste-discriminating receptors are located on the surface of the mucous membrane of the mouth and tongue. Moreover, we feel the bitter, sweet, salty and sour taste in their various areas.
In our article, we will find out what taste and olfactory receptors are, and we will also determine the physiological mechanism of the occurrence of the 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 that can attach and bind biological substances-mediators of a chemical or neurogenic nature. According to the theory of nerve endings, this formation spatially coincides with a molecule of odorous or flavoring substance, like a key and a lock. This is a signal for the initiation process in the olfactory receptors located in the peripheral part 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.
Nerve cell structure
A neurocyte has not only a body, but also two types of processes. Axon is a very long ending that 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 have the form of a receptor formation. The principle of action of their various species, for example, nerve endings that perceive chemicals, which include olfactory receptors, is ultimately reduced to the transfer of excitation to the cortical part of the brain. Let's consider it further.
Receptor activity mechanism
It can be represented in the following form: at first, the perception of stimuli and the polarization of their membrane change under their influence. A modification of the spatial configuration of signaling 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 a neuron. As it turned out, olfactory receptors are capable of capturing a scanty number of molecules of various gaseous substances, i.e., they have a low threshold of sensitivity. How does the perception of these compounds affect the state of our body?
World of smells
In the work of V. Pikul, The Fragrant Symphony of Life, the poor perfumer unsuccessfully sought the arms and hearts of the main character. To annoy his rival (famous singer), he came up with the following. The young man brought to the concert a large basket of fragrant violets and put it on the piano lid. The artist failed to pick up a single high note, and his premiere failed. The perfumer, it turns out, knew for sure that the olfactory receptors of a person, trapping the smell of violets, affect the vocal cords, disrupting their work.
Indeed, the olfactory analyzer is one of the most sensitive and insufficiently studied types of sensory systems. His activity is closely related to the perception of taste and greatly affects the emotional and physical state of the human body. On this property of smell arose a branch of medicine such as aromatherapy. It is known that the smells of lavender and rosemary, which perceive olfactory receptors, soothe the nervous system and relieve stress. The aroma of lemon promotes concentration, and eucalyptus and jasmine increase efficiency.
Chemoreceptor sensory systems
The olfactory analyzer transforms irritations caused by particles of chemicals into sensations of smell. It helps a person to capture toxic compounds that are harmful to health in the air or to identify foods that are unsuitable for food. This is vital and is a protective adaptive property of the body. So, the smell of ammonia, perturbing the mucous membranes of the respiratory tract and lungs, perceives the olfactory receptor at a dose of only 70 molecules in 1 ml of water. Being a chemoreceptor, it transfers 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. We also note that the villi of odor receptors are capable of responding to minimum 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 cm 2 , 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 high metabolism and actively occurring protein biosynthesis reactions. The dendritic processes directly 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 are depolarized. This process may be slowed down due to inflammatory reactions resulting from respiratory or allergic diseases of the upper respiratory tract. The epithelial membrane of the nose swells, secreting excess mucus. This leads to a decrease in the sensitivity of nerve endings and a deterioration in the discrimination of smells, up to a complete loss of olfactory as well as taste sensations.
What determines the sensitivity of receptors?
The olfactory receptors are located in the mucous membrane of the upper respiratory tract, so the appearance of certain odor sensations is affected primarily by the concentration of odorous substance in the inhaled air. So, a thick oil squeezed from rose petals has an unpleasant, difficult to detect smell. A delicate aroma of roses appears only with a strong dilution of the oil concentrate.
Experts identify six basic sensations. These include odors: tarry, floral, spicy, putrid, fruity, burnt. The physiological characteristics of perception reveal clean, irritating and mixed odors. The sensitivity of nerve endings to them is reduced if a person is a smoker or abuse alcohol.
Scientific theories of smell
Among scientists, there is no single view of the essence of the mechanism of perception of odors. The stereochemical theory can be considered the most recognized, according to which the main role in determining the chemical stimulus belongs to the nerve endings of neurons. Olfactory receptors are unique antennas that capture odor molecules and alter 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 membrane of a neuron is polarized, and a nerve impulse arises, i.e., the appearance of a smell has a dual nature: chemical and neurogenic.
We also note that in the explanation of the occurrence of odors, scientists use the concept of olfactory pigment. This substance has the same principle of action as rhodopsin and iodopsin - compounds that are part of the visual receptors of the retina of the eye: 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, the electrons emit a quantum of energy, which provides the occurrence of excitation in the nerve ending of the olfactory neuron.
Methods for determining the severity of smell
Some professions (for example, perfumers or tasters) require increased sensitivity of the sense of smell and taste. The strong sensitivity of olfactory analyzer receptors to odors is often an innate property of the human body, but it can develop after prolonged training. There is a test that is performed with an olfactometer device. It determines the threshold of perception: the minimum amount of 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-epidemiological laboratories to establish the causes of mass poisoning in enterprises, in public catering, in schools.