Hormone regulation: hormonal control loop |

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As biochemical messengers, hormones influence the most important processes in the body. These are, for example, metabolism, reproduction and growth.

Hormones reach their destinations in the body via the bloodstream, but are only present there in very low concentrations. Even small fluctuations in hormone levels can have serious consequences, which is why good hormone regulation is very important.

The hormone regulation not only has to keep certain parameters of the body (e.g. the blood sugar level) constant, but also react accordingly to changing requirements. But how does this exact system of hormone regulation work? How does a hormonal gland know how many hormones to produce and release when?

Hormone regulation as a control loop

The hormone level in the blood must be checked continuously and corrected if necessary. Hormone regulation therefore follows the principle of a feedback loop.

A control circuit is a self-controlling, closed system.

In a control loop there is

  • a control variable

  • a controller

  • a feeler

  • a variable

  • and actuators.

With regard to hormones, the hormone concentration or another physical parameter represents the control variable. The controller is the endocrine gland or an organ that is superior to the endocrine gland. The sensor is the measuring device of the system. These are receptors that forward the actual value (e.g. the current hormone concentration) to the controller. The controller then compares the actual value with the target value (e.g. hormone concentration to be set). If the two values ​​do not match, the controller takes action to adjust the actual value to the target value. If the actual value exceeds the target value, the hormone release must be slowed down and vice versa. The actuators of a hormonal control circuit are the cells on which the hormone is supposed to take effect and the manipulated variable is usually the hormone secretion rate.

The principle of a control circuit is used to illustrate the example of the thyroid hormones:

In the thyrotropic control loop, the blood level of thyroid hormones can be viewed as the control variable. The actual value is the current concentration of thyroid hormones in the blood, which should be adjusted to the target value using the controller. The target value is given by the hypothalamus. Furthermore, it compares the target value with the actual value. If the actual and target values ​​do not match, the hypothalamus adjusts the concentration of the thyroid hormones. If the hormone concentration is too low, the hypothalamus releases more of the hormone TRH, which indirectly increases the thyroid hormone concentration in the blood as part of a multi-stage system.

Control loops are not uncommon in medicine. They can also be found, for example, in the regulation of breathing and blood pressure.

An important prerequisite within such control circuits, which serve to regulate the hormones, is the presence of feedback mechanisms (feedback mechanisms):

Importance of feedback mechanisms in hormone regulation

Feedback mechanisms enable precise regulation within the framework of endocrine control circuits, because they allow the organs involved to interact with each other.

A distinction is made between negative and positive feedback:

  • negative feedbackIf there is a feedback mechanism within a hormonal control loop, it is usually negative feedback. This means that an increase in the level of a hormone leads to a reduced payout of the same or another hormone.

An example of negative feedback can be found at the thyroid. Here, a high concentration of thyroid hormones leads to a inhibited secretion of TRH (thyrotropin-releasing hormone) in the hypothalamus, which indirectly results in a decrease in thyroid hormones.

The thyroid gland is also referred to in medicine as the thyroid, according to its Latin name. Many terms related to the thyroid gland are derived from this term, for example hyperthyroidism (overactive thyroid gland) or the thyroid hormones thyroxine and triiodothyronine.

  • Positive feedbackBut there are also cases in which an increase in hormones leads to increased payout other hormones.

This is an example of this mechanism estrogen, which, with increased secretion, results in increased LH secretion from the pituitary gland. The positive feedback causes a quick increase in hormone concentration. It comes to a kind signal amplification, which can also quickly derail. That’s because, unlike negative feedback, this system doesn’t self-inhibit, but requires an out-of-loop stop signal.

Hypothalamic-pituitary axis

There are two main ways in which the cells in our body can communicate with each other:

  1. stimulus transmission about neurons of nervous system
  2. signal transmission by the hormones in endocrine system.

In order for complex organisms to be able to function, both systems must also be able to interact with one another. Therefore, there are interfaces between the nervous system and the endocrine system. The most important connection between the nervous system and the endocrine system is the hypothalamus.

The coupling of the nervous and endocrine systems is relevant, for example, for the different release of sex hormones at certain times of the year and thus for the seasonal heat of many animals. Here, the length of daylight is registered by the nervous system and the hormone release is adjusted accordingly.

Found in the hypothalamus neurosecretory cells.

Neurosecretory cells are nerve cells that can receive stimuli from other nerve cells and produce hormones in response. These hormones then act on the pituitary gland, which is why they are also called pituitary hormones designated.

Classic hormones are regulated according to a hierarchical system, with the hypothalamus at the top of this hierarchy. To achieve its control, it produces a variety of control hormones. These control hormones reach the adenohypophysis via a pituitary portal vein system, i.e. via the bloodstream.

portal vein system

Normally, an artery branches into smaller and smaller vessels and ends in a network of capillaries where the oxygen-rich blood delivers its oxygen. The low-oxygen, venous blood then collects again in small venous vessels, which become larger and larger as they progress. In a portal vein system, two capillary networks are connected in series.

The neurohormones produced by the hypothalamus are divided into two categories:

  • control hormonesThe control hormones control the hormone production of other endocrine glands.
  • effector hormonesThe effector hormones are stored in the neurohypophysis and after their secretion act directly on their target cells. They do not reach the pituitary gland via the pituitary portal system, but via the nerve fibers.

The control hormones are further differentiated into liberins and statins. Liberine or Releasing Hormones (RH for short) have a stimulating effect on the formation of other hormones. Statins or inhibiting hormones, on the other hand, have an inhibiting effect on the formation of other hormones.

Here is an overview of some important hormones of the hypothalamus:

Type of hormone Hormone name Function LiberineTRH (thyrotropin-releasing hormone) stimulates the release of THSGnRH (gonadotropin-releasing hormone) stimulates the release of LH and FSHGHRH (growth hormone-releasing hormone or somatoliberin) stimulates the release of GH or STHCRH (corticotropin-releasing hormone) stimulates the release of ACTHStatinGHIH (growth hormone inhibiting hormones or somatostatin) inhibits the release of TSH and GH or STHPIH (prolactin inhibiting hormones) inhibits the release of prolactin effector hormones ADH (antidiuretic hormone or vasopressin) reabsorption of water in the kidneys oxytocin «cuddle hormone» , induces labor during pregnancy, lactation

Immediately subordinate to the hypothalamus is the pituitary. With its hormones, it in turn influences the peripheral endocrine glands. The pituitary gland can be divided into two areas:

  • neuropituitaryNerve fibers of the nerve cells of the hypothalamus, which produce the effector hormones vasopressin and Òxytcin, end in the neurohypophysis. This part of the pituitary gland does not itself form any hormones, but only releases the effector hormones formed in the hypothalamus and is therefore actual no endocrine gland.
  • adenohypophysisThe adenohypophysis, on the other hand, is one endocrine gland. It also produces control hormones, which then act on peripheral endocrine glands, and effector hormones, which act directly on their target cells.

Figure 1: Structure of the pituitary gland

Here is an overview of the most important hormones of the adenohypophysis:

Type of hormone Hormone nameFunction Control hormonesTSH (thyroid-stimulating hormone)stimulates the release of thyroid hormonesACTH (adrenocorticotropic hormone)stimulates the release of glucocorticoidsLH (luteinizing hormone)FSH (follicle-stimulating hormone)stimulates the gonads or release of sex hormoneseffector hormonesGH or STH (growth hormones, somatotropin)regulation of growthPL (prolactin) milk production

Hormone regulation using the example of the thyroid

Thyroid hormones are produced during the thyrotropic control circuit regulated, which also as Hypothalamic-pituitary-thyroid axis referred to as.

The individual steps of hormone regulation of the thyroid gland:

  1. The hypothalamus processes stimuli such as cold or stress and responds by producing TRH (thyrotropin-releasing hormone).
  2. TRH then travels to the adenohypophysis where it causes increased production of TSH (thyroid stimulating hormone).
  3. TSH reaches the thyroid gland via the bloodstream, which in response releases more thyroid hormones (triiodothyronine (T3) and thyroxine (T4)).
  4. The thyroid hormones then also reach their target cells via the endocrine route, i.e. via the bloodstream.

What do thyroid hormones do?

  • accelerate the heartbeat
  • activate the carbohydrate, protein and fat metabolism
  • increase heat production
  • and much more.

Overall it can be said that the thyroid hormones to a increase in basal metabolism and des energy consumption of the body.

If the blood level of thyroid hormones rises too much, the hypothalamus adjusts its TRH release. As a result, less TSH is produced in the pituitary gland and the hormone level drops. The hormone somatostatin of the hypothalamus also inhibits the release of TSH.

In the opposite case, i.e. when the hormone level is below the target value, the hypothalamus produces more TRH, which increases the TSH concentration and also the thyroid hormone concentration in the blood.

Figure 2: Thyrotropic control circuit

How can hormone regulation be disturbed?

Hormone regulation does not always work properly. The causes of such disturbances in hormone regulation are diverse, for example tumors or medication…