Thermoregulation refers to that the thermoreceptors receive the stimulation of the internal and external environmental temperatures, and correspondingly change the activities of endocrine glands, skeletal muscles, skin blood vessels and sweat glands through the activities of the thermoregulation center, thus regulating the heat generation and heat dissipation process of the body and keeping the body temperature at a relatively constant level.
Thermoregulation is an automatic control system (Figure 9-5). The ultimate goal of control is the deep temperature, which is represented by the heart and lungs in Figure 9-5. The internal and external environment of the body is constantly changing, and many factors will interfere with the stability of deep temperature. At this time, the feedback system will transmit the interference information to the thermoregulation center, and then adjust the activities of the controlled system through its integration, so as to achieve a new body heat balance on a new basis and achieve the effect of stabilizing the body temperature.
First, the temperature sensor
(1) Peripheral temperature sensor
Thermoreceptors on skin and some mucous membranes can be divided into two types: cold receptors and heat receptors. They transmit the temperature changes of the skin and the external environment to the thermoregulatory center. In real life, people will feel cold when the skin temperature is 30℃, but will feel warm when the skin temperature is about 35℃. The temperature sensor of abdominal viscera can be called deep temperature sensor, which can sense the change of visceral temperature and then transmit it to the thermoregulation center.
(2) Central temperature receptor
There are temperature-sensitive neurons in hypothalamus, brainstem reticular structure and spinal cord: those whose impulse frequency increases when the temperature rises are called temperature-sensitive neurons; Those whose pulse frequency increases when the temperature drops are called cold-sensitive neurons. There are many temperature-sensitive neurons in the anterior hypothalamus and preoptic area, while cold-sensitive neurons are mainly in the reticular brainstem structure, but both neurons often coexist. The central temperature receptor directly senses the temperature changes of blood flowing through the brain and spinal cord, and transmits impulses to the hypothalamic temperature regulation center through certain nerve connections.
Second, the thermoregulatory center.
(A) the location of the thermoregulatory center
According to the experiments on the brains of many warm-blooded animals, it is proved that after the cerebral cortex and some subcortical structures are removed, as long as the neural structure below the hypothalamus is kept intact, the animals may have some shortcomings in behavior, but they still have the ability to maintain a constant body temperature. If the hypothalamus is further destroyed, animals can no longer maintain a relatively constant body temperature. The above experiments show that the main center for regulating body temperature is located in the hypothalamus. It is generally believed that it should include the anterior hypothalamus and the posterior hypothalamus. As mentioned above, there are many heat-sensitive neurons and a few cold-sensitive neurons in the anterior hypothalamus. Experiments also prove that stimulating this area can cause heat generation and heat dissipation reaction: when this area is heated, heat-sensitive neurons are excited and promote heat dissipation reaction; If cooled, cold-sensitive neurons will be stimulated to promote thermogenic response. If the above temperature is used to stimulate the posterior hypothalamus, the effect is not significant, but electrical stimulation of the posterior hypothalamus can enhance the tension of skeletal muscle and increase heat production. Therefore, it is now considered that the preoptic area-the anterior hypothalamus, after receiving temperature stimulation, transmits information to the posterior hypothalamus for integration, regulates the process of heat generation and heat dissipation, and keeps the body temperature relatively stable.
(B) Fixed-point theory
The mechanism of body temperature regulation, that is, how to maintain the body temperature at 37℃, is generally explained by the set point theory. According to this theory, the body temperature of humans and advanced warm-blooded animals is similar to a thermostat. The set point is equivalent to a thermostat and is the benchmark for adjusting the temperature. Temperature-sensitive neurons and cold-sensitive neurons in the preoptic area of hypothalamus play the role of regulatory points. The intensity of these two types of neurons changes according to the temperature of hypothalamus, and its change is characterized by a bell-shaped curve, as shown in Figure 9-6. The intersection of these two curves is the temperature reference point that has been debugged, which is called the set point. The normal temperature at this time is set to 37℃. If the temperature of blood flowing here exceeds 37℃, the discharge frequency of temperature-sensitive neurons will increase, thus strengthening the heat dissipation process and weakening the heat generation process. If the temperature of blood flowing here is lower than 37℃, it will cause the opposite change. The afferent information of skin temperature receptors can also affect the activity of set points through central integration.
In general, the range of regulation points is very narrow, but it can also be changed due to physiological activities or pathological reactions. If bacterial infection causes fever, pyrogen can change the activity of temperature-sensitive and cold-sensitive neurons and make the regulation point move up (for example, 38℃). After the set point is moved up, the heat generation and heat dissipation process will reach a balance at a high level (38℃). The mechanism of antipyretic and analgesic drugs is to lower the set point, so as to restore the body temperature to normal level.
Thirdly, the effector and feedback effect of thermoregulation.
When determining the set point of body temperature, the hypothalamic thermoregulation center sends out outward signals, so that the process of heat generation and heat dissipation can reach a balance at this temperature. When the body temperature rises slightly and exceeds the set point, the skeletal muscle tension decreases, the secretion of thyroid gland and adrenal gland decreases, the blood vessels dilate, the skin blood flow increases, the sweat gland secretion increases, and the heat dissipation increases, so that the body temperature returns to the normal set point level. When the temperature is slightly lower than the set point, blood vessels contract, skin blood flow decreases, sweat glands stop secreting, skeletal muscle tension increases, resulting in chills and other reactions, thyroid hormone secretion also increases, metabolism improves, heat production increases, and body temperature returns to the normal set point level.
Fourthly, the role of cerebral cortex and behavioral thermoregulation.
Although the body temperature of animals with cerebral cortex resection can still be kept normal, their response to cold and hot stimuli in the environment is obviously slow. This shows that the cerebral cortex plays an important role in thermoregulation. The body can regulate body temperature through conditioned reflex. Visual and auditory stimuli related to cold or extreme heat can improve the metabolic level of the body. For people who work in high or low temperature places, cold or hot stimulation in the environment can be combined with working time and place many times to form conditioned reflex and make the body adapt to the environment.
In addition, human body temperature has behavioral regulation. The body can regulate body temperature through conscious activities. For another example, humans can also create an artificial climate to make the temperature more comfortable.
Verb (abbreviation for verb) abnormal body temperature
The human body's ability to regulate body temperature is limited. If the environmental temperature changes drastically for a long time, or the body temperature regulation mechanism fails, and the heat generation process and heat dissipation process cannot be kept in relative balance, abnormal body temperature will occur.
Heatstroke and fever
When a person is in a high temperature environment or under the scorching sun in summer, the heat generated in the body can not be dissipated in time, resulting in excessive accumulation of body heat and disorder of body temperature, and heatstroke will occur. Its outstanding performance is the increase of body temperature, which can reach above 40℃ in severe cases, and symptoms such as headache, dizziness, weak pulse, decreased blood pressure and even loss of consciousness may occur. Long-term high fever may cause functional failure of thermoregulatory center, resulting in serious consequences.
Fever is a symptom of many diseases. For example, when pyrogens such as bacterial toxins enter the body, the set point will rise and the body temperature can reach above 38℃. Fever can cause discomfort, consume physical strength and increase the burden on the heart.
(2) Hypothermia
In the low temperature environment, if the regulation of the body temperature center makes the heat generation insufficient to compensate for the heat dissipation, the normal body temperature cannot be maintained and gradually decreases.
Due to the appropriate reduction of body temperature, the metabolic rate of the body can be reduced, and the oxygen consumption of the tissue can also be reduced, which can eliminate or alleviate the damage of hypoxia to cells. Therefore, artificial hypothermia anesthesia can be used for large-scale surgery, as well as for preserving tissues and organs, and for clinical organ transplantation.