Start Here: Understanding the Gut-Brain Connection

The gut and brain are not isolated systems. They communicate through an interconnected network involving the nervous system, immune system, hormones, intestinal environment and chemical substances produced through normal biological and microbial activity.

Scientists commonly refer to these interactions as the gut-brain axis. When the microorganisms living in and around the gastrointestinal system are considered as part of this communication network, the term microbiota-gut-brain axis is often used.

Understanding this communication system provides an important foundation for exploring many of the questions researchers are asking about digestive health, metabolism, the nervous system and neurodevelopment.

What Is the Gut-Brain Axis?

The gut-brain axis describes two-way communication between the gastrointestinal system and the brain.

The gastrointestinal tract contains its own extensive network of nerve cells, known as the enteric nervous system. The enteric nervous system interacts with the central nervous system, which includes the brain and spinal cord.

Communication between these systems occurs through several overlapping biological pathways. Signals can involve nerves, hormones, immune-system molecules, microbial metabolites and other biologically active substances.

The microorganisms that inhabit the gastrointestinal tract add another layer to this communication network because their activity can interact with the intestinal environment, metabolism and immune system.

Communication Goes Both Ways

One of the most important concepts to understand is that gut-brain communication is bidirectional. Information can travel gut → brain and brain → gut.

Signals traveling from the gastrointestinal system toward the brain are sometimes described as afferent signals. Signals traveling from the brain toward the gastrointestinal system are sometimes described as efferent signals.

Gut-to-brain communication can involve sensory nerves, immune signals, metabolites, hormones and neuroactive molecules. Brain-to-gut communication can influence functions such as gastrointestinal motility, secretion, stress responses and other aspects of digestive physiology through autonomic and neuroendocrine pathways.

This means the gut-brain axis should not be thought of as a one-way pathway. It is an ongoing biological conversation.

How Do the Gut and Brain Communicate?

Researchers study several major pathways.

1. Nervous-System Signaling

The gastrointestinal tract contains the enteric nervous system, a complex network of neurons involved in digestive function. It communicates with the central nervous system through neural pathways.

2. The Vagus Nerve

The vagus nerve is an important communication route between internal organs and the brain. Researchers study how vagal signaling participates in communication between the gastrointestinal system and the central nervous system.

3. Immune Signaling

The gastrointestinal tract has extensive interaction with the immune system. Immune cells and signaling molecules can respond to events occurring within the intestinal environment and participate in communication among different body systems.

4. Hormonal and Neuroendocrine Signaling

The brain and gastrointestinal system also communicate through hormonal pathways. One area researchers study is the hypothalamic-pituitary-adrenal, or HPA, axis, which is involved in the body's response to stress.

5. Microbial Metabolites

Microorganisms in the gastrointestinal tract produce a wide variety of metabolic products. Some of these compounds interact with intestinal cells, metabolism, immune pathways and other biological systems. Researchers are investigating how these interactions may participate in gut-brain communication.

Why Are Researchers Interested in This?

The gut-brain axis brings together several biological systems that were once frequently studied separately. Researchers now investigate interactions among:

• the gastrointestinal system
• the nervous system
• the microbiome
• immune signaling
• metabolism
• hormones
• microbial metabolites
• nutrition
• neurodevelopment

This interconnected view has created important research questions. Scientists are examining how changes or differences within one part of this network may be associated with changes elsewhere.

For families, understanding the gut-brain axis provides useful context for following emerging research without assuming that every association represents a cause or a treatment opportunity.