The receptors are structures in the body surface, body cavity or tissues of animals that can receive internal and external environmental stimuli and convert them into neural processes. Classification I. According to the receptors (sensory receptor) in the body distribution of the parts and the source of receiving stimuli can be distinguished as: 1, endoreceptors: including cardiovascular wall of mechanical and chemical receptors, gastrointestinal tract, ureter, bladder, body cavity wall and mesenteric roots of various types of receptors. 2.External receptors: including photoreceptors, auditory receptors, taste receptors, olfactory receptors and distribution of skin, mucous membrane (including olfactory mucosa, taste buds), visual apparatus, hearing instruments and other places. 3, proprioceptors: distributed in the skeletal muscle muscle belly, tendons, joint capsule, ligaments and inner ear taste apparatus, etc., to receive the body movement and balance when the stimulus generated. Second, according to the characteristics of the stimulus received can be divided into receptors: ① mechanoreceptors: including touch, pressure receptors located in the skin, the root of the mesentery, lips, external genitalia and other parts of the touch, pressure receptors located in the cardiovascular wall, alveoli and bronchial walls, the cavities of the visceral wall of the tension (or pull) receptors. ②Temperature receptors: including both thermoreceptors and cold receptors, located in the skin and mucous membranes of the oral cavity and genital organs. Acoustic receptors: in most higher animals have developed into a complex structure of the auditory organ, its components in addition to the inner ear spiral to receive acoustic oscillations, and enhance the sound pressure of the middle ear and the outer ear to collect sound. ④Photoreceptors: The most important receptors in animals (and even some plants), even protozoa, such as eye worms have light-sensitive eye spots. The primary component of its photoreceptors is the photoreceptor cell, and the vast majority of animals also have a multi-layered structure of the retina. ⑤ chemoreceptors: mainly located in the nasal mucosa, oral mucosa, urethral mucosa, ocular conjunctiva, etc., mainly feel the chemical stimuli contained in the air and water, such as Na, H and some volatile oils. (6) Balance receptors: such as the lateral lines on both sides of the body in fish, and the highly developed inner ear balance organs in birds and mammals. (7) pain receptors: also called receptors for injurious stimuli, widely distributed in the free nerve endings of the skin, cornea, conjunctiva, oral mucosa, etc., and nerve endings distributed in the pleura, peritoneum and periosteum, etc., mostly without special structures. It is very sensitive to the change of osmotic pressure in body fluid. When the plasma osmotic pressure decreases, the anti-diuretic hormone secreted by it decreases, and vice versa increases, thus regulating the water excreted in urine and maintaining the normal osmotic pressure of body fluid. Physiological mechanism If the impulses from the receptors reach only the lower parts of the central nervous system, they can only cause some simple reflex activities, such as spinal reflexes. If the stimulus is stronger and the frequency of the incoming impulse is higher, it can then be uploaded to the higher centers or spread to their lower centers via the lower nerve centers, and the response that occurs then is more complex and can even cause subjective sensations. In the anesthetized state, the subjective sensation disappears, but the reflex activity still exists. Therefore, after the receptors receive a stimulus, they do not necessarily cause sensation; true sensation requires the participation of complex centers, especially the activity of the cerebral cortex. Features All types of receptors in the body have the following common features in function: 1. All types of receptors have their own suitable stimuli. Suitable stimulus means that only a very small intensity of a stimulus is needed to cause excitation of the receptor, and this form of stimulus is called the suitable stimulus for the receptor. The minimum suitable stimulus intensity to cause receptor excitation is called the sensory threshold of the receptor. 2, all types of receptors have a transduction effect, that is, they can act on their various forms of stimulus energy into the corresponding afferent nerve fibers on the action potential, to the corresponding parts of the central nervous system. The central nervous system gets the afferent signal from each receptor through many afferent nerve fibers. 3, the receptors convert external stimuli into neural action potentials, not only in the form of energy conversion, but more importantly, the information about the environmental changes contained in the stimulus is also transferred to the new electrical signal system, which is called the encoding role. The question of why the quality and quantity of external stimuli and other properties are encoded in the nerve-specific electrical signals is very complex and is not yet clear. It is only known that the elicitation of different sensations is determined not only by the nature of the stimulus and the receptor being stimulated. It is also determined by the site where the afferent impulse reaches its end point in the cerebral cortex. For example, if a patient’s optic nerve is stimulated with an electric current, the impulse reaches the occipital cortex and produces the sensation of light. Another clinical example is tinnitus, which occurs when a tumor or other lesion compresses the auditory nerve. This is due to the stimulation of the auditory nerve impulses to the cortical auditory center. It follows that the nature of sensation is determined by the site where the afferent impulses reach the higher centers. As for the question of how to encode the intensity (or amount) of a stimulus within the same sensory type, it is now believed that receptors can respond to the intensity of a stimulus by changing the frequency of action potentials on the corresponding afferent nerve fibers. When the stimulus is reinforced, more than one receptor and afferent nerve can also be made to issue impulses to the center. 4, all types of receptors have the phenomenon of adaptation. The so-called adaptation phenomenon refers to the stimulation of receptors when the stimulus is still present, and the sensation gradually disappears. This phenomenon is also often reflected in life, such as “into the room of the orchid, long and do not smell its fragrance”. That is, the phenomenon of adaptation of the olfactory sense to the stimulus. Experiments have also shown that the frequency of action potentials on afferent nerve fibers decreases when the stimulus continues to act on the receptor, which proves that the receptor has the phenomenon of adaptation. Appropriate stimulus The energy (sensory threshold) stimulus form or type is called the appropriate stimulus for the receptor. Each receptor has only one suitable stimulus. Other forms of energy stimuli do not respond, or are very unresponsive. For example, skin temperature receptors are about 2000 times more sensitive to heat radiation than nociceptive receptors. The various forms of change that occur in the internal and external environment of the body always act on the receptors that correspond to them first, as a result of biological evolution. The receptors can transform the stimuli acting on them into corresponding nerve impulses to the nerve center to cause sensation or perception. It is generally believed that the mechanical deformation of the nerve cell membrane causes an increase in the permeability of the nerve endings to Na+, resulting in an inward flow of Na+ to the receptor potential. Encoding The afferent impulse from any receptor is an action potential that is basically the same in terms of waveform and generation principle, while different types of sensation are realized through the encoding of receptors. Experiments have shown that the elicitation of different types of sensations is determined not only by the nature of the stimulus and the stimulated receptor, but also by the terminal part of the cerebral cortex where the afferent impulses reach. For example, using electrical stimulation to act on the optic nerve to artificially produce afferent impulses to the occipital cortex or directly stimulating the occipital cortex to produce excitation will cause the sensation of light. This suggests that the nature of the sensation is determined by the higher site reached by the afferent impulse rather than by the properties of the action potential itself. In other words, the process of differentiation of the type of stimulus in the overall situation is the evolutionary process of differentiation of the receptor apparatus so that one of the receptors becomes particularly sensitive to a stimulus of a certain nature and discriminates the type of stimulus. Adaptation phenomenon When a stimulus acts on a receptor, the stimulus continues to act, but the frequency of the afferent impulse has begun to decrease, this phenomenon is called the receptor adaptation phenomenon. Sensory adaptation is not only related to the receptor, but also to the characteristics of the center that produces the sensation. The speed of adaptation varies greatly from receptor to receptor, and each has its own significance: ① Fast adaptation receptors, such as skin tactile receptors. Fast adaptation can be thought of as a form of information closure that aims to avoid the nervous system being overwhelmed by stimuli that no longer provide valid information. For example, the role of touch is generally to explore novel objects or obstacles, and its fast adaptation facilitates the receptors to receive new stimuli again. ② Slow-adapting receptors, such as myosin, nociceptive, and carotid sinus pressure receptors. Slow adaptation facilitates lasting regulation of certain body functions such as posture and maintains a high level of vigilance for those stimuli that are particularly important. Adaptation is not fatigue, because after adaptation to a stimulus, increasing the intensity of this stimulus can in turn cause an increase in afferent impulses. Sensory thresholds and receptor potentials The excitation of sensory nerves requires the use of appropriate stimuli to stimulate the corresponding receptors, if the stimulus intensity is too weak, the afferent nerve will not appear action potential, this type of stimulus is called subthreshold stimulation, the stimulus intensity is not too weak, but the action time is too short will also appear similar situation. Electrical stimulators are commonly used to determine the thresholds of certain receptors. When determining the threshold of a tissue, the appearance of an action potential on the afferent nerve fiber, or the onset of a non-diffusive potential change in the basal potential, known as the generator potential, is often used as an indicator. If the action potential of the afferent nerve and the potential change of the receptor (receptor with certain structure) are recorded simultaneously when the receptor is stimulated, it can be seen that before the afferent impulse occurs, a non-diffusive potential change occurs inside the receptor first, starting with a local potential decrease, and with the increase of the stimulus intensity, the potential decrease is gradually obvious until it is strong enough to affect the nerve endings inside the receptor and make it occur The potential decrease becomes more pronounced with increasing stimulus intensity until it is strong enough to affect the nerve endings in the receptor and cause an action potential to occur. If the intensity of the stimulus used is not large, the local potential can subside with the cessation of the stimulus, and this potential change is called the generator potential, also called the receptor potential. The greater the intensity of the stimulus used, the faster the rate of growth of the receptor potential, and therefore the higher the frequency of afferent impulses on the peripheral nerve fibers caused by it. Some receptors are themselves nerve endings, such as pain receptors. In this case, the receptor potential is equal to the generator potential. In some receptors, the receptor cell itself does not have axons, but the nerve net surrounding the base of the cell generates the afferent impulses, in which case the receptor potential is first generated by the receptor cell. The receptor potential then excites the nerve endings, causing local depolarization. In turn, action potentials are induced. Excitation of receptors and physiological responses If the impulse from the receptor reaches only the lower parts of the central nervous system, it can only cause some simple reflex activities, such as spinal reflexes. If the stimulus is stronger and the frequency of the incoming impulse is higher, through the lower nerve centers, it can then be uploaded to the higher centers or spread to other centers, and then the response appears to be more complex and can even cause subjective sensation. However, this does not mean that the stimulus that causes subjective sensation needs to be strong, but depends on which kind of receptor is stimulated. Shining a weak light on the human eye can cause the pupil to narrow and also cause the sensation of the light spot. Here there is both reflex activity and subjective sensation. In the anesthetized state, the subjective sensation disappears, but the reflex activity still exists. Therefore, after the receptors receive stimuli, they do not necessarily cause sensation, and true sensation requires the participation of complex centers, especially the activity of the cerebral cortex. Research significance The study of the functional activity of receptors not only provides an understanding of how changes in the external and internal environment are transformed into information that is transferred to the central nervous system to form our senses, but also has practical significance. For example, the principles of the beautiful sights we see every day and the music we hear are based on the study of receptor activity patterns. The study of receptors is also important for the development of bionics and clinical medicine.