The human nervous system carries out complex analytical and synthetic processes that ensure rapid adaptation of organs and systems to changes in the external and internal environment. The perception of stimuli from the outside world occurs due to the structure, which includes the processes of afferent neurons containing oligodendrocyte glial cells, or lemmocytes. They turn external or internal stimuli into bioelectric phenomena called excitation or nerve impulse. Such structures are called receptors. In this article, we will study the structure and functions of the receptors of various human sensory systems.
Types of nerve endings
In anatomy, there are several systems for their classification. The most common divides receptors into simple (consist of processes of one neuron) and complex (a group of neurocytes and auxiliary glial cells as part of a highly specialized sensory organ). Based on the structure of the sensory processes.they are divided into primary and secondary endings of the centripetal neurocyte. These include various skin receptors: nociceptors, mechanoreceptors, baroreceptors, thermoreceptors, as well as nerve processes innervating internal organs. Secondary are derivatives of the epithelium that create an action potential in response to irritation (taste, hearing, balance receptors). The rods and cones of the light-sensitive membrane of the eye - the retina - occupy an intermediate position between the primary and secondary sensitive nerve endings.
Another classification system is based on such a difference as the type of stimulus. If the irritation comes from the external environment, then it is perceived by exteroreceptors (for example, sounds, smells). And irritation by factors of the internal environment is analyzed by interoreceptors: visceral, proprioreceptors, hair cells of the vestibular apparatus. Thus, the functions of the receptors of sensory systems are determined by their structure and location in the sense organs.
The concept of analyzers
In order to differentiate and distinguish between environmental conditions and adapt to it, a person has special anatomical and physiological structures called analyzers, or sensory systems. The Russian scientist I. P. Pavlov proposed the following scheme for their structure. The first section was called peripheral (receptor). The second is conductive, and the third is central, or cortical.
For example, the visual sensory system includes sensitiveretinal cells - rods and cones, two optic nerves, as well as a zone of the cerebral cortex located in its occipital part.
Some analyzers, such as the already mentioned visual and auditory ones, include a pre-receptor level - certain anatomical structures that improve the perception of adequate stimuli. For the auditory system, this is the outer and middle ear, for the visual system, the light-refracting part of the eye, including the sclera, the aqueous humor of the anterior chamber of the eye, the lens, and the vitreous body. We will focus on the peripheral part of the analyzer and answer the question of what is the function of the receptors included in it.
How cells perceive stimuli
In their membranes (or in the cytosol) there are special molecules consisting of proteins, as well as complex complexes - glycoproteins. Under the influence of environmental factors, these substances change their spatial configuration, which serves as a signal for the cell itself and forces it to respond adequately.
Some chemicals, called ligands, can act on the sensory processes of the cell, resulting in transmembrane ion currents in it. Plasmalemma proteins with receptive properties, together with carbohydrate molecules (i.e. receptors), perform the functions of anten - they perceive and differentiate ligands.
Ionotropic channels
Another type of cellular receptors - ionotropic channels located in the membrane, capable of opening or blocking under the influence ofsignaling chemicals, such as H-cholinergic receptor, vasopressin and insulin receptors.
Intracellular sensing structures are transcription factors that bind to a ligand and then enter the nucleus. Their compounds with DNA are formed, which enhance or inhibit the transcription of one or more genes. Thus, the main functions of cell receptors are the perception of environmental signals and the regulation of plastic metabolism reactions.
Stands and cones: structure and functions
These retinal receptors respond to light stimuli - photons, which cause the process of excitation in the nerve endings. They contain special pigments: iodopsin (cones) and rhodopsin (rods). Rods are irritated by twilight light and are not able to distinguish colors. Cones are responsible for color vision and are divided into three types, each of which contains a separate photopigment. Thus, the function of the eye receptor depends on which light-sensitive proteins it contains. Rods are responsible for visual perception in low light, while cones are responsible for visual acuity and color perception.
Skin is a sense organ
Nerve endings of neurons entering the dermis differ in their structure and react to various environmental stimuli: temperature, pressure, surface shape. The functions of skin receptors are to perceive and transform stimuli into electrical impulses (the process of excitation). Pressure receptors include Meissner bodies located in the middle layer of the skin - the dermis, capable of thindiscrimination of stimuli (have a low threshold of sensitivity).
Pacini bodies belong to baroreceptors. They are located in the subcutaneous fat. The functions of the receptor - pain nociceptor - is protection from pathogenic stimuli. In addition to the skin, such nerve endings are located in all internal organs and look like branching afferent processes. Thermoreceptors can be found both in the skin and in internal organs - blood vessels, parts of the central nervous system. They are classified into heat and cold.
The activity of these sensory endings can increase and depends on in which direction and at what speed the temperature of the skin surface changes. Therefore, the functions of skin receptors are diverse and depend on their structure.
Mechanism of perception of auditory stimuli
Exteroreceptors are hair cells that are highly sensitive to adequate stimuli - sound waves. They are called monomodal and are secondarily sensitive. They are located in the organ of Corti of the inner ear, being part of the cochlea.
The structure of Corti's organ is similar to a harp. Auditory receptors are immersed in the perilymph and have groups of microvilli at their ends. Vibrations of the fluid cause irritation of the hair cells, which turn into bioelectric phenomena - nerve impulses, i.e. the functions of the hearing receptor - this is the perception of signals that have the form of sound waves, and their transformation into a processarousal.
Contact taste buds
Each of us has a preference for food and drink. We perceive the taste range of food products with the help of the organ of taste - the tongue. It contains four types of nerve endings, localized as follows: at the tip of the tongue - taste buds that distinguish between sweet, at its root - bitter, and s alty and sour receptors on the side walls distinguish. Irritants for all types of receptor endings are chemical molecules perceived by the microvilli of taste buds that act as antennas.
The function of the taste receptor is to decode a chemical stimulus and translate it into an electrical impulse that travels along the nerves to the taste zone of the cerebral cortex. It should be noted that the papillae work in tandem with the nerve endings of the olfactory analyzer located in the mucous membrane of the nasal cavity. The joint action of the two sensory systems enhances and enriches the taste sensations of a person.
The Riddle of Smell
Just like the taste, the olfactory analyzer reacts with its nerve endings to the molecules of various chemicals. The very mechanism by which odorous compounds irritate the olfactory bulbs is not yet fully understood. Scientists suggest that odor signaling molecules interact with various sensory neurons in the nasal mucosa. Other researchers attribute the stimulation of olfactory receptors to the fact that signaling molecules have common functional groups (for example, aldehydeor phenolic) with substances included in the sensory neuron.
The functions of the olfactory receptor are in the perception of irritation, its differentiation and translation into the process of excitation. The total number of olfactory bulbs in the mucous membrane of the nasal cavity reaches 60 million, and each of them is equipped with a large number of cilia, due to which the total area of contact of the receptor field with molecules of chemical substances - odors.
Nerve endings of the vestibular apparatus
In the inner ear there is an organ responsible for the coordination and consistency of motor acts, maintaining the body in a state of balance, and also participating in orienting reflexes. It has the form of semicircular canals, is called a labyrinth and is anatomically connected with the organ of Corti. In three bone canals there are nerve endings immersed in the endolymph. When tilting the head and torso, it oscillates, which causes irritation at the ends of the nerve endings.
Vestibular receptors themselves - hair cells - are in contact with the membrane. It consists of small crystals of calcium carbonate - otoliths. Together with the endolymph, they also begin to move, which serves as an irritant for the nerve processes. The main functions of the semicircular canal receptor depend on its location: in the sacs, it responds to gravity and controls the balance of the head and body at rest. Sensory endings located in the ampoules of the organ of balance control changes in the movements of body parts (dynamic gravity).
The role of receptors in the formationreflex arcs
The whole doctrine of reflexes, from the studies of R. Descartes to the fundamental discoveries of I. P. Pavlov and I. M. Sechenov, is based on the idea of nervous activity as an adequate response of the body to the effects of stimuli of the external and internal environment, carried out with the participation of the central nervous system - the brain and spinal cord. Whatever the answer, simple, for example, a knee jerk, or as super complex as speech, memory or thinking, its first link is reception - the perception and discrimination of stimuli by their strength, amplitude, intensity.
Such differentiation is carried out by sensory systems, which IP Pavlov called "tentacles of the brain." In each analyzer, the receptor functions as antennas that capture and probe environmental stimuli: light or sound waves, chemical molecules, and physical factors. The physiologically normal activity of all sensory systems without exception depends on the work of the first section, called the peripheral, or receptor. All reflex arcs (reflexes) without exception originate from it.
Plectrums
These are biologically active substances that carry out the transfer of excitation from one neuron to another in special structures - synapses. They are secreted by the axon of the first neurocyte and, acting as an irritant, cause nerve impulses in the receptor endings of the next nerve cell. Therefore, the structure and functions of mediators and receptors are closely interrelated. Moreover, someneurocytes are able to secrete two or more transmitters, such as glutamic and aspartic acids, adrenaline and GABA.
