What are the two brains of the human body?

Modern neuroscience recognizes two interconnected neural centers: the central nervous system (CNS) in the skull and the enteric nervous system (ENS) in the gut. The CNS—composed of the brain and spinal cord—coordinates higher-order cognition, voluntary movement, and complex behavior. The ENS, often called the “second brain,” is a dense network of roughly 100 million neurons embedded in the gastrointestinal wall that regulates digestion and communicates bidirectionally with the CNS.

How the two systems communicate

Communication between these two systems occurs via neural, hormonal, and immune pathways collectively known as the gut-brain axis. The vagus nerve is a major neural highway conveying sensory signals from the gut to brainstem nuclei, while endocrine mechanisms like the hypothalamic-pituitary-adrenal (HPA) axis mediate stress responses. Immune signaling and microbial metabolites such as short-chain fatty acids (SCFAs) further modulate neural function and blood-brain barrier integrity.

Role of the microbiome

Trillions of microorganisms in the gut influence both ENS and CNS function. Certain bacteria synthesize or modulate neurotransmitters—serotonin precursors, GABA, and dopamine—and produce metabolites (butyrate, propionate, acetate) that affect neuroinflammation and neuroplasticity. Shifts in microbial composition (dysbiosis) are associated with mood disorders, cognitive symptoms like brain fog, and systemic inflammation linked to neurodegeneration.

Clinical and practical implications

Testing the gut microbiome can reveal patterns of diversity and functional potential that relate to immune status, neurotransmitter production, and SCFA synthesis. For readers seeking more context on testing options, see microbiome testing resources. Understanding microbial signatures supports evidence-based interventions—dietary fiber to bolster SCFA producers, targeted probiotics for specific pathways, and behavioral approaches that enhance vagal tone through breathing and movement.

Lateralization and functional nuance

Neural hemispheres display specialization: left-hemisphere functions often emphasize language and analytical processing, while the right hemisphere plays a larger role in emotion and social cognition. Emerging research suggests that microbiome-driven signals may differentially influence hemisphere-specific networks, shaping symptom patterns such as anxiety, attentional change, or executive dysfunction. For practical insights into factors that can disrupt the gut ecosystem, see this review on what harms the gut microbiome.

Targeted microbial considerations

Species-level differences matter: for example, some Lactobacillus and Bifidobacterium strains contribute to GABA and serotonin pathways, while butyrate-producers like Faecalibacterium prausnitzii support intestinal and neural health. Readers interested in strain-specific information can consult an accessible overview of Lactobacillus acidophilus benefits at Lactobacillus acidophilus benefits and a related summary at an external Lactobacillus acidophilus article.

Conclusion

Framing the body as having two brains highlights a systems-level perspective: cognition and emotion arise not only from neural circuits in the skull but also from microbial-neural interactions in the gut. Integrative research and diagnostic tools increasingly allow for refined assessment of how gut ecology shapes brain health; for an in-depth discussion of this relationship, see the primary article on what the two brains are and how they interact.