‘My heart stopped, then began to beat rapidly, my muscles stiffened, the back of my neck grew cold, my mouth dried up, goose bumps ran down my back, and cold sweat formed on my forehead.’ ‘My breath caught, my heart pounded, my hands and feet trembled, my cheeks flushed, heat spread across my chest and stomach, and my eyes sparkled.’ Even if we don’t name the feelings that these manifestations are characteristic of, we can still guess that we are talking about fear and attraction, because in most healthy, mentally normal people, the physical expressions of feelings, especially strong ones, are very similar. This is the basis for the effects of ‘recognising oneself’ in the characters of books and films. What is the basis for such reactions? Many would say nerves. But in fact, the nerve impulse is only the trigger that activates the storm, which is controlled by hormones. The activities of the hormonal (endocrine) and nervous systems are closely intertwined. Therefore, the effects that these systems have on the body together are called neuroendocrine regulation. The hormonal (endocrine) system has a clear hierarchy: there are ‘controlling’ organs and there are executive glands. The highest place in this hierarchy is occupied by the hypothalamus.

The hypothalamus is often referred to as the emperor or conductor of the endocrine system. This unique structure, located in a part of the brain called the diencephalon, combines the properties of nervous tissue and glands. Its cells are called neurosecretory cells. In response to incoming nerve impulses, they become excited (similar to cells in the nervous system) and begin to synthesise hormones (like glandular cells). The hypothalamus is a huge computer connected to all structures of the brain and spinal cord. It receives information about the state of the internal environment of the body (body temperature, energy balance, water and salt metabolism, blood pressure, etc.). And the ‘wires’ coming from the sensory organs inform the hypothalamus about events happening “outside”. It is here that nerve impulses are ‘translated’ into the language of hormones. Having received information on the basis of which it is necessary to adjust the internal environment or behaviour, the hypothalamus ‘turns’ to its assistant – the pituitary gland.

Unlike the hypothalamus, the pituitary gland does not claim to be the brain, but is content with the title of gland. However, it is not just any gland, but the most important one. In essence, the pituitary gland is a conduit that conveys the ‘decisions of the emperor’ to the executive glands. The hypothalamus produces two types of hormones: liberins and statins. The former stimulate the production of hormonal substances in the pituitary gland, while the latter inhibit it. Through small vessels called the pituitary portal system, they enter the pituitary gland and activate the production of hormones that affect the body. A total of about 15 hormones are synthesised in the pituitary gland. Some of them act specifically on a particular gland. For example, thyroid-stimulating hormone stimulates the production of thyroid hormones, and adrenocorticotropic hormone stimulates the synthesis of adrenal hormones. Others have a systemic effect on the entire body. For example, the somatotropic hormone of the pituitary gland activates bone growth in children and adolescents by affecting the growth plates in the tubular (long) bones; its deficiency is the cause of dwarfism. Somatotropic hormone is also one of the main builders of the body, stimulating protein synthesis in connective tissue and muscles, promoting recovery after physical exertion, helping to burn fat faster, etc. The pituitary gland and hypothalamus together form the hypothalamic-pituitary system. The total mass of these organs is barely six grams. Nevertheless, this is the control centre not only for the functioning of our body, but also for our emotions, instinctive reactions, potential for longevity, etc.

Hormones synthesized by the pituitary gland under the control of the hypothalamus are carried by the bloodstream to the endocrine glands: the thyroid, thymus, pancreas, as well as the ovaries, testicles, and adrenal glands. In response to these ‘hormonal messages,’ the glands activate (or, conversely, block) the production of their own hormones, which causes the corresponding physiological effects. The sequence of transmission of hormonal signals from the hypothalamus to the pituitary gland and the target gland is commonly referred to as the regulatory axis. Thus, the hypothalamic-pituitary-adrenal axis is responsible for regulating the production of adrenal hormones (e.g., adrenaline or noradrenaline). The hypothalamic-pituitary-thyroid axis is responsible for the production of thyroid hormones, and the hypothalamic-pituitary-gonadal axis is responsible for the synthesis of sex hormones. At the same time, some pituitary hormones (e.g., somatotropic hormone) do not use other glands as levers to achieve the desired effects, but cause the necessary physiological reactions by directly affecting tissues.

In addition to the endocrine cells that make up the ‘official’ glands, there are cells with the same function – hormone production – but scattered throughout the body. Scientists have identified more than 60 types of apudocytes, as endocrine-like cells located in various organs are called. Scientists have named cells that have hormonal activity but are not part of the ‘official’ endocrine system the diffuse (i.e., pervasive) neuroendocrine system. The largest concentration of hormone-producing cells is found in the gastrointestinal tract: in the stomach, pancreas, large intestine and small intestine. Some hormones, such as gastrin, produced by the glandular cells of the intestine, are primarily involved in regulating digestion. Cholecystokinin, synthesized in the duodenum, affects both food digestion and human behavior, preventing the development of depressive states. The lion’s share of serotonin, the hormone responsible for good mood, is produced by the secretory cells of the intestine (60 to 80% of the total amount of serotonin in the body). And the pancreas produces an exact analogue of the pituitary somatotropic hormone, which has the same effect on metabolic processes. The kidneys have a hormonal ‘hobby’ of producing renin, which raises blood pressure, and erythropoietin, which stimulates the formation of blood cells. The heart is also involved in hormonal activity: it synthesises atrial natriuretic hormone, which causes the kidneys to excrete sodium more quickly, and with it, water. Even a seemingly inert substance such as adipose tissue also exhibits hormonal activity by producing leptin. This hormone increases the sensitivity of cells to insulin (a hormone produced by the pancreas that facilitates the penetration of glucose into cells). And in large doses (in cases of severe obesity), leptin suppresses insulin production, leading to the development of type II diabetes mellitus.

The pineal gland (or pineal body) stands apart from the other organs of the endocrine system. It is called the mystery gland because this tiny organ, weighing less than one gram and located in the centre of the brain, has been associated with human clairvoyance since ancient times. Modern scientists jokingly refer to the pineal gland as the ‘grey cardinal’. As far as researchers know, this tiny gland does not obey the orders of the ‘hormonal government’ – the hypothalamic-pituitary system – but, on the contrary, influences its activity. The substances produced by the pineal gland block the formation of hormonal signals in the pituitary and pineal glands that activate the sex organs. When its function is impaired, early puberty occurs, with premature and excessive development of secondary sexual characteristics. It is also known that the pineal gland, which is considered to be analogous to the unpaired pineal eye in some animals, reacts to light and regulates daily biorhythms. Studies show that sleep and wakefulness patterns depend on the hormone melatonin, which is synthesised in the pineal gland at night. Due to the isolation of the pineal gland and its independence from the influences of the hypothalamic-pituitary system, scientists classify the pineal gland as an organ of the diffuse neuroendocrine system, just like the hormonal cells of the intestines, kidneys, heart, etc.

A cascade of hormonal reactions can be triggered by external influences, such as irritation of nerve endings. For example, we are walking in the forest and see a bear (impulse from the visual analyser) or we do not see it, but hear the rustling of bushes (impulse from the auditory analyser). We may smell something that the brain associates with potential danger (olfactory analyser) or feel the touch of a paw (irritation of tactile receptors). The impulse from the nerve endings rushes to the cerebral cortex, and from there to the hypothalamus, activating the areas responsible for communication with the adrenal glands (the hypothalamic-pituitary-adrenal axis). The hypothalamus secretes the hormone corticoliberin, which enters the pituitary gland, which in turn synthesises the hormone adrenocorticotropic hormone. The latter is carried by the bloodstream to the adrenal glands and activates the production of stress hormones – adrenaline and cortisol. Adrenaline is carried by the blood throughout the body, stimulating nerve endings sensitive to it, called adrenoreceptors. Their stimulation leads to the appearance of the familiar signs of intense fear. This whole process happens very quickly: from the moment the receptors (visual, auditory, etc.) are stimulated to the appearance of signs of fear, only a fraction of a second passes. This is enough time for the body to instantly put us in a state of ‘combat readiness’ when faced with danger. The synthesis of many hormones is practically independent of external influences. For example, the intensity of thyroid hormone production (triiodothyronine, thyroxine and calcitonin, which affect metabolism, growth and tissue development) varies depending on the increase or decrease in the concentration of these hormones in the blood. If the content of thyroid hormones in the bloodstream decreases, the hypothalamus receives a signal about the need to stimulate the process of their formation (hypothalamic-pituitary-thyroid axis). The hypothalamus cells synthesise thyrotropin-releasing hormone (TRH), which stimulates the pituitary cells to produce thyrotropin. This hormone enters the bloodstream, reaches the thyroid cells and activates hormone production. Conversely, high levels of thyroid hormones in the blood ‘block’ the production of thyroid-stimulating hormone in the pituitary gland. As a result, the synthesis of new hormones in the thyroid gland slows down.

Why are the feelings that each of us experiences in life so similar? Because they are based on physiological reactions caused by a certain ratio and concentration of the same hormones. Scientists have been able to ‘break down’ virtually all known emotions into hormones. Some sensations are the result of a single hormone. For example, fear is the domain of adrenaline. This hormone, entering the bloodstream from the adrenal glands, can give us all the attributes of intense fear: heart palpitations, cold sweat, increased muscle tone, enabling us to fight the source of fear… or run away. A whole cocktail of hormones is needed to produce other feelings. For example, love is a real ‘hormonal smoothie,’ the composition of which depends on the phase of the relationship. In the initial stage, dopamine prevails, driving us to pursue the desired object, and adrenaline gives the lover an excess of strength. But the level of serotonin, the hormone of joy, fluctuates, causing you to soar in the heavens one moment and fall into depression the next. When meeting the object of passion (as well as when thinking about them), both men and women experience a surge of testosterone: by acting on receptors, it leads to the appearance of signs of sexual attraction. When passion in a relationship is replaced by stability, the concentration of the hormone oxytocin, which is synthesised in the pituitary gland, increases in the body. By affecting the brain, it promotes feelings of attachment, tenderness and trust.

The close connection between the nervous and endocrine systems is the basis for both beneficial effects that improve life and, often, the basis for the formation of harmful habits. For example, the decision to take up sport for an untrained person in the early stages may be accompanied by the release of stress hormones. They imagine an unfamiliar environment and discomfort – such thoughts lead to the production of the hormone cortisol. As a result, their heart rate increases and their blood pressure rises — they are already experiencing the whole range of negative feelings that they believe they will have to endure in the future. This link between thoughts and hormones often becomes an obstacle to any change in life. But if a person overcomes their fear and goes to the gym, copes with the initial negative feelings that are natural when stepping outside their comfort zone, their body soon begins to receive ‘hormonal bonuses.’ Endorphins, which are produced during physical activity, improve well-being and give a feeling of joy. And now, thoughts about training do not lead to the release of stress hormones, but to the formation of endorphins, which increases motivation to continue exercising. This ability of the body to change its hormonal balance not only when faced with real situations, but also when thinking about them, is the basis of self-programming. The problem is that the body gets used to the state in which it finds itself most of the time, even if that state is negative. For example, in response to a situation that causes feelings of resentment, the hormones cortisol, noradrenaline and melatonin are released. The release of these hormones into the bloodstream triggers a chain of chemical reactions that irritate certain nerve endings. As a result, we feel fear, anger and sadness, which together form resentment. If the situation is successfully resolved, the hormones that provoke negative feelings cease to be produced. However, if a person continues to think about the resentment, the hormones continue to be synthesised: the endocrine system does not distinguish between the actual event and thoughts about it. When such thoughts (and, accordingly, hormonal reactions) persist day after day, the body becomes accustomed to this state on a chemical level. And now the feeling of resentment (anger, guilt, etc.) becomes a comfort zone, while the feeling of joy and the corresponding hormonal background become a departure from the usual framework. It is precisely this ‘negative biochemistry’ of the body that is the reason why many correct endeavours based on correct actions do not bring positive results. Therefore, many practices aimed at improving life primarily involve working with thoughts. It is recommended to first experience positive events at the level of the mind, accustoming the body to ‘good hormones,’ and only then take actions that reinforce the positive result and allow you to form positive dynamics at both the biochemical and physical levels.