Figure 1.

Central nervous system stimulation as part of the functioning of the gut–brain axis may occur through 3 afferent pathways: (1) production of endocrine or paracrine cytokines by lymphocytes in contact with the microbiota, (2) activation of neuron terminals by intestinal peptides secreted by enteroendocrine cells, and (3) exerting an endocrine or paracrine effect in intestinal epithelial cells by neurotransmitters or their precursors produced by the intestinal microbiota. After the activation of the central nervous system, the signal reaches the brain stem (e.g., the nucleus of the solitary strand), and it is then transferred to a separate neural network consisting of the amygdala and the cortex of the island, whose task is to integrate information from internal organs. In response to the signal, the activation of the hypothalamic–pituitary axis of the adrenal gland and the secretion of corticosteroids, as well as the stimulation of efferent neurons, may occur, resulting in the activation of the cholinergic anti-inflammatory pathway and/or the sympathetic nervous system, causing the release of classical neurotransmitters, according to [15,16,17].