| Mechanism | Description | Role in the gut-brain connection | Potential implications for neurological and psychiatric disorders |
| Vagus nerve | A bidirectional nerve that connects the gut to the brain and can transmit signals in both directions. | The vagus nerve plays a key role in regulating gut-brain communication. It can transmit signals from the gut to the brain about the gut microbiome, food intake, and inflammation. The vagus nerve can also transmit signals from the brain to the gut to control gut motility and secretion. | Dysfunction of the vagus nerve has been implicated in a number of neurological and psychiatric disorders, including depression, anxiety, and autism [42] . |
| Short-chain fatty acids (SCFAs) | Produced by the fermentation of fiber by gut bacteria, SCFAs can cross the blood-brain barrier and interact with receptors in the brain. | SCFAs have a number of effects on the brain, including reducing inflammation, improving cognitive function, and regulating mood. | Alterations in SCFA levels have been observed in a number of neurological and psychiatric disorders, including depression, anxiety, and Alzheimer’s disease [8] . |
| Cytokines | Signalling molecules that are produced by the immune system, cytokines produced in the gut can communicate with the brain through the vagus nerve and other pathways. | Cytokines play a role in regulating gut inflammation and immune function. They can also communicate with the brain to regulate mood and behavior. | Elevated levels of cytokines have been observed in a number of neurological and psychiatric disorders, including depression, anxiety, and schizophrenia [43] . |
| Neurotransmitters | Chemical messengers that are used by neurons to communicate with each other, neurotransmitters produced in the gut can communicate with the brain through the vagus nerve and other pathways. | Neurotransmitters such as serotonin, dopamine, and GABA play a key role in regulating mood, cognition, and behavior. | Alterations in neurotransmitter levels have been observed in a number of neurological and psychiatric disorders, including depression, anxiety, and Parkinson’s disease [44] . |