A control loop is a control system that can control various processes and functions in the organism. Most functions have their own control loop.
A control loop can either run in the target organ itself or be controlled by a parent organ. Such superior organs are, for example, the central nervous system (CNS) or endocrine glands.
The aim of a control loop is to keep a controlled variable constant or to bring it to a desired setpoint value. This target value is measured by various receptors and compared with the current actual value. The actuator in the control loop then corrects the actual value until it matches the setpoint value. Most control circuits in the human body operate on the principle of negative feedback.
One known regulatory cycle in the human body is the thyroid gated loop that regulates thyroid hormone activity. The thyroid (Glandula thyreoidea) produces the hormones triiodothyronine (T3), thyroxine (T4) and calcitonin. The two iodine-containing hormones T3 and T4 are formed in the follicular epithelial cells of the thyroid gland. They play an important role in energy metabolism and influence the growth of the organism.
The function of the thyroid gland is controlled by the hypothalamus and pituitary gland through the thyrotropic control loop. The pituitary gland secretes the thyroid stimulating hormone (TSH). This passes through the bloodstream to the thyroid cells. On the one hand, TSH promotes the production of T3 and T4 and, on the other hand, stimulates the growth of the thyroid gland. A higher level of T3 and T4 in the blood in turn inhibits the release of TSH. As a result, thyroid levels in the blood are regulated as needed and usually kept relatively constant.
The thyrootropic control loop is an example of negative feedback. The target value of the control loop is not determined by the hypophysis, but by the hypothalamus. This produces thyrotropin releasing hormone (TRH).
Also, the heat balance of the body is regulated by a control loop. The aim of this control loop is to keep the temperature in the body constant at about 37 ° C. The ambient temperature influences the body temperature. Intensive physical activity also has an influence on the temperature, for example.
Temperature gauges are distributed throughout the body. However, the heat sensors are located especially in the spinal cord, in the hypothalamus and in the skin. An important role in the regulation of temperature plays the hypothalamus. Here all actual value information is collected from the body. The hypothalamus is also informed about all physical needs. From these entire inputs, the control center in the hypothalamus now calculates the desired setpoint and the difference between this setpoint and the actual value. Normally, the set point is 36 ° C to 37 ° C.
The setpoint in the body is adjusted, for example, in infections with fever. Even during ovulation in women, the body temperature changes. If both values agree, no regulation is necessary. However, if a difference results in the comparison, the body initiates a reaction. It changes individual actuators in the control loop. A possible actuator in the regulation of temperature, for example, the muscles. When cold, the muscles tremble and generate heat.
Interruptions in the control loop can occur at any point. For example, the target organs, the probes or the actuators may be affected. These changes affect the entire control loop.
Disorders in the thyrotropic control circuit usually lead to either hypothyroidism or hypothyroidism (hyperthyroidism). In a primary hypothyroidism, the cause in the target organ of the control loop, ie in the thyroid gland itself. Cause of such primary hypothyroidism include thyroid surgery, radioiodine therapy, antithyroid drugs or extreme selenium or iodine deficiency.
In secondary hypothyroidism, the cause is found in the pituitary gland. There is too little TSH produced. The control circuit is therefore impaired even before the thyroid gland. The consequences of primary and secondary hyperthyroidism are similar. It comes to tiredness, loss of power, depression, hair loss, constipation, erectile dysfunction and the typical myxedema.
However, thyroid gland disorders can also lead to hyperthyroidism. The causes are often autonomous or autoimmune processes. An example of thyroid disorder in the thyroid gland, which causes thyroid hyperfunction, is Graves' disease. Graves' disease is an autoimmune disease of unknown origin. The body forms antibodies against receptors on the thyroid gland.
These receptors are actually intended for TSH. However, the antibodies bind to the receptors and cause there a similar effect as the TSH. As a result, the thyroid gland is increasingly producing thyroid hormones. However, this happens completely independently of the actual control loop. In Basedow's disease, the TSH level drops to almost 0 because there are too many thyroid hormones in the blood at any one time. Typical symptoms of hyperthyroidism include weight loss, diarrhea, irritability, nervousness, hair loss and heat intolerance.
Pathological control circuits are also called vicious circle or vicious circle. In this case, two disturbed body functions influence each other and thereby reinforce existing diseases or maintain diseases. Pathological control circuits are found in diseases such as heart failure or diabetes mellitus. They are usually based on positive feedback.