Why do I always feel hot?

Thermoregulation is an example of a biological automatic control system. Hypothalamic thermoregulation center, including set-point neurons, belongs to control system. The information from it controls the activities of the controlled system of heat-producing organs such as liver and skeletal muscle and heat-dissipating organs such as skin, blood vessels and sweat glands. And keep the controlled object-the deep body temperature at a stable level. However, the output variable body temperature will always be disturbed by internal and external environmental factors (such as body movement or changes in external environmental climate factors, such as temperature, humidity, wind speed, etc.). ). At this time, the interference information is fed back to the set point through temperature detectors-skin and deep temperature receptors (including central temperature receptors). After integrating the temperature regulation center and adjusting the activities of the controlled system, the body heat balance can still be established under the current conditions, and the effect of stabilizing body temperature can be achieved.

(1) temperature sensor

Temperature-sensitive receptors are called temperature receptors and are divided into peripheral temperature receptors and central temperature receptors.

Peripheral thermoreceptors exist in human skin, mucosa and viscera. Thermoreceptors can be divided into cold receptors and heat receptors, both of which are free nerve endings. When the skin temperature rises, the thermal receptor is excited, while when the skin temperature drops, the cold receptor is excited. It can be seen from the records of temperature sensor impulse that the impulse frequency of temperature sensor and cold sensor is the highest at 28℃, while the impulse frequency of temperature sensor is the highest at 43℃. When the skin temperature deviates from these two temperatures, the pulse frequency from the two receptors gradually decreases. In addition, the temperature sensor is more sensitive to the skin temperature change rate.

Visceral organs also have temperature receptors. Someone buried the electric heater in the abdominal cavity of the sheep and heated it to 43-44℃. It was observed that the respiratory frequency and evaporation heat dissipation of sheep increased rapidly. After heating for 3-5 minutes, the animals began to gasp, which lowered the temperature of hypothalamus. It shows that the increase of visceral temperature can cause obvious heat dissipation reaction.

Central thermoreceptors exist in spinal cord, medulla oblongata, reticular structure of brain stem and hypothalamus.

The anterior hypothalamus of anesthetized rabbits, cats or dogs is heated or cooled by a device (temperature change tube) to change the temperature of brain tissue. It was found that the heating of the anterior hypothalamus (PO/AH) in the preoptic area can cause the animals to have heat dissipation reactions such as wheezing and sweating, while the local cooling will increase the heat production, indicating that PO/AH itself can regulate the two opposites of heat dissipation and heat production. Electrophysiological method was used to record the existence of temperature-sensitive neurons and cold-sensitive neurons in PO/AH. The discharge frequency of the former increases with the increase of local temperature, while that of the latter increases with the decrease of brain tissue temperature. Experiments show that when the temperature of local brain tissue changes by 0. 1C, the discharge frequency of these two temperature-sensitive neurons will be reflected, and there is no adaptation phenomenon.

There are also temperature-sensitive neurons in the spinal cord. Cooling the neck, thoracic pulp or thoracic and lumbar pulp of lightly anesthetized dogs will cause skin vasoconstriction, chills and other thermoregulatory reactions. At this time, the vascular reaction and chills will not disappear if the posterior root of the cooling part is cut off or the spinal cord is cut off at a high position. Warming the spinal cord will cause skin vasodilation, fever, wheezing and breathing, and chills will be suppressed. In addition, it is said that the ascending neurons conducting temperature information in the spinal cord run in the anterior fiber cord, which sends information to PO/AH.

There are also temperature sensitive neurons in the medulla oblongata. The afferent temperature information of skin, spinal cord and midbrain will gather in the temperature-sensitive neurons of the medulla oblongata; The medulla oblongata also receives information from PO/AH and transmits information to PO/AH.

The reticular structure of brain stem also has neurons that respond to local temperature changes. These neurons receive the temperature information of skin and spinal cord and transmit it to PO/AH.

(2) Thermoregulation Center

According to the segmented brain resection experiments of many warm-blooded animals, it is found that after the cerebral cortex and some subcortical structures are removed, as long as the neural structure below the hypothalamus is intact, the animals may have some shortcomings in behavior, but they still have the ability to maintain body temperature. If the hypothalamus is further destroyed, animals will no longer be able to maintain a relatively constant body temperature. These facts show that the basic center for regulating body temperature is hypothalamus. Experiments such as local destruction of hypothalamus or electrical stimulation show that when PO/AH is destroyed, the heat dissipation reaction disappears and the body temperature rises. Stimulation will cause heat dissipation reaction, and chills will be suppressed; However, the posterior hypothalamus was destroyed, the body temperature dropped, and the anti-fever was inhibited; Stimulating it will cause chills. Based on this, it is concluded that the front of hypothalamus is the center of heat dissipation and the back of hypothalamus is the center of heat production. However, these two experimental methods are relatively rough, so the conclusions obtained are not consistent with the results observed by the fine experimental method.

As mentioned above, PO/AH has heat-sensitive neurons and cold-sensitive neurons, which regulate the heat dissipation and heat generation reactions respectively. There are two similar neurons in brain cells outside the hypothalamus. There seems to be no clearly defined heating center and cooling center. Thermoregulation involves multi-input temperature information and multi-system outgoing response, so it is an advanced central integration. The preoptic area-the anterior hypothalamus should be the basic part of body temperature regulation. Thermo-sensitive neurons and cold-sensitive neurons in the front of hypothalamus can not only feel the temperature changes in their parts, but also integrate them through the incoming temperature information. Therefore, when the temperature of the external environment changes, the information of temperature change can be transmitted to the nerves along the body through the spinal cord through the stimulation of the skin temperature and cold receptors, and reach the temperature regulation center of the hypothalamus; ② The change of external temperature can cause the change of deep temperature through blood and directly affect the anterior hypothalamus; ③ Central temperature receptors other than spinal cord and hypothalamus also transmit temperature information to the front of hypothalamus. Through the integration of the anterior hypothalamus and other parts of the center, the body temperature is regulated in the following three ways: ① the vasomotor response and sweat gland secretion of the skin are regulated through the sympathetic nervous system; ② Change the activity of skeletal muscle through somatic nerve, such as shivering in cold environment; ③ The metabolic rate of the body is regulated by the changes of medullary hormone secretion in thyroid and adrenal gland. Some people think that the excitation of skin temperature receptors mainly regulates the vasomotor activity and blood flow of skin blood vessels; Deep temperature changes mainly regulate sweating and skeletal muscle activity. Through the above complicated regulation process, the body can maintain a relatively stable body temperature when the external temperature changes.

Set point theory This theory holds that the regulation of body temperature is similar to a thermostat, and there is a set point in PO/AH, that is, a specified value (such as 37℃). If it deviates from the specified value, the feedback system will send deviation information to the control system, and then the control system will make adjustments to keep the body temperature constant. It is generally believed that the temperature-sensitive neurons in PO/AH may play a set-point role in thermoregulation. For example, this theory holds that the fever caused by bacteria is the result of the increase of the threshold of heat-sensitive neurons and the upward adjustment point (such as 39℃) due to the action of pyrogen. Therefore, the fever reaction starts from the heat-generating reactions such as aversion to cold and trembling, and the heat dissipation reaction does not occur until the body temperature rises above 39℃. As long as the heat-causing factors are not eliminated, the two processes of heat production and heat dissipation will continue to maintain balance at this new body temperature level. It should be pointed out that fever does not hinder the function of body temperature regulation, but only because the adjustment point moves up, the body temperature is adjusted to the fever level.

The role of monoamines in thermoregulation The nerve endings related to thermoregulation in mammalian hypothalamus are rich in monoamines. In the early 1960s, experiments with dogs, cats and monkeys proved that perfusion of 5- carboxytryptamine into animal ventricles or microinjection into hypothalamus increased animal body temperature, accompanied by vasoconstriction and chills. However, norrenin can reduce the body temperature of animals by 0.5-2℃, accompanied by peripheral vasodilation. According to this kind of experiment, the monoamine theory of body temperature regulation is put forward, and it is considered that the dynamic balance of 5- carboxytryptamine and norepinephrine can keep the body temperature constant. However, at present, it is considered that these two substances can only regulate the activity of the thermoregulatory center, and have no decisive effect on the constant body temperature level.