Link to the Autonomic nervous system: 

The parasympathetic nervous system is one of three branches of the autonomic nervous system, and the vagus nerve is the main contributor to this nervous system. The system controls specific bodily functions such as digestion, heart rate, and the immune system (Breit et al., 2018)

Together with the sympathetic nervous system, the parasympathetic nervous system regulates vegetative functions. For example, parasympathetic innervation dilates blood vessels and bronchioles and stimulates the salivary glands. (Breit et al., 2018).

The enteric nervous system (ENS) is a branch of the autonomic nervous system. The ENS and the vagus nerve, as a part of the CNS, lead to a bidirectional flow of information. The (ENS) arises primarily from vagal origin and extends across the gastrointestinal tract, regulating the major enteric processes, such as immune response, detection of nutrients, motility, microvascular circulation, and epithelial secretion of fluids, ions, and bioactive peptides (Breit et al., 2018).

The link between the Central and Enteric nervous system: 

The connection between the CNS and the ENS is commonly referred to as the brain-gut axis. 

The brain-gut axis includes:

• brain
• spinal cord
• autonomic nervous system (sympathetic, parasympathetic, and ENS)
• hypothalamic-pituitary-adrenal axis (HPA)

The HPA-axis is responsible for monitoring physiological homeostasis and connecting the emotional and cognitive areas of the brain with peripheral intestinal functions, such as immune activation, intestinal permeability, enteric reflex, and enteroendocrine signaling (Breit et al., 2018). 

The vagal afferent pathways are involved in the activation/regulation of the HPA axis and coordinate the adaptive responses to stressors of any kind.

Both neural (vagus) and hormonal (HPA axis) lines of communication combine to allow the brain to influence the intestinal functional effector cells, such as immune cells, epithelial cells, enteric neurons, smooth muscle cells, interstitial cells of Cajal, and enterochromaffin cells (Breit et al., 2018).

Modulation of intestinal immune homeostasis:

The gastrointestinal tract is highly innervated by vagal fibers, connecting the CNS with the intestinal immune system – making vagus nerve a major component of the neuroendocrine-immune axis. 

This axis is involved in coordinated neural, behavioral, and endocrine responses, important for the first-line defense against inflammation. The connection between the nervous and immune systems functions as an anti-inflammatory mechanism.