One of the best known complexes that fulfil the functions of both the nervous and endocrine systems is the hypothalamic-pituitary-adrenal axis. It is this axis that plays a key role during stress, which, as an organism’s response to unfavourable factors, provokes a number of physiological changes. Thus, the trigger signal when a potential danger is detected is the activation of the hormonal system, namely the secretion of the key stress hormones corticoliberin and cortisol, which together with another hormone oxytocin and the parasympathetic (“inhibitory”) nervous system trigger the “calming of thoughts”. Together with the next hormone, corticotropin, oxytocin promotes concentration, and vasopressin, secreted a little later, activates memory mechanisms. The same hormones together with growth hormone stimulate the activation of the sympathetic (“excitatory”) nervous system, mobilising the adaptive capabilities of the organism.

As we know, inflammation is one of the body’s defences: when an infection occurs in the body or when its own cells are destroyed, the immune cells near the site of damage release a variety of signalling molecules – cytokines. And this happens under the direct control of the nervous system: for example, it has been shown that mental instability (namely, the inability to concentrate and general anxiety) increases the level of the inflammatory cytokine interleukin-6, which can easily lead to a violation of carbohydrate metabolism. In the opposite direction, immunoregulatory effects on the brain are mediated by the autonomic nervous system via the sympathetic and vagus nerves. Neurons are able to respond to pro-inflammatory substances secreted by immune cells: through activation of nerve reflex circuits, they are able to regulate acute and chronic immune responses.

In general, hormones and cytokines have a broad spectrum of action: the same signalling molecules can act on different targets and consequently fulfil different functions. For example, leptin, which acts as a hormone controlling energy metabolism and appetite, at the same time plays an important role in the formation of immunity: it enhances phagocytosis (destruction of pathogenic cells) and production of anti-inflammatory substances. Leptin, by the way, promotes wound healing, as evidenced by its increased production in the process of inflammation. And the hormone adrenaline (“stress hormone”) and the neurotransmitter dopamine (“pleasure hormone”) can regulate cell growth and the synthesis of antibodies by immune cells. The neuroendocrine system is undoubtedly one of the key factors in triggering and regulating the sleep-wake cycle. The change of phases of the cycle depends not only on the level of light, but also on the level of secretion of hormones and cytokines. Thus, the hormone corticoliberin during drowsiness activates serotoninergic neurons causing inhibition of internal organs and skeletal muscles, while immune cell cytokines (interleukin-1 and interferon-alpha) in the slow-wave phase of sleep are produced with the same intensity as the most important hormones – melatonin and delta sleep peptide, ensuring immune homeostasis (i.e. stability).

It is clear that there is a complex relationship between the immune, endocrine and nervous system. This interaction is explained by the production of molecules: cytokines, hormones and neurotransmitters. The nervous system, with the help of neuropeptides, influences the immune system, which in turn is able to produce a number of hormones. The endocrine system also influences the immune system and does so through hormones of the hypothalamic-pituitary-adrenal axis. In addition, the nervous system regulates the endocrine system by acting on the appropriate glands that produce hormones, and the neurons themselves are able to directly produce some of the cytokines involved in immune response reactions. Thus, these three regulatory systems have the common goal of maintaining the constancy of the internal environment, thus forming a unified system of control of our body.