Background

What is Anxiety?

Anxiety refers to a mental health disorder characterized by sustained and overwhelming stress and worry. Anxiety disorders interfere with people’s daily life, school, work, and relationships by causing extreme and prolonged worry, stress, and restlessness. Anxiety can come in the form(s) of and aren’t limited to: generalized anxiety disorder, panic disorder, phobia-related disorders, social anxiety, and post traumatic stress disorders. An estimated 31.1% of U.S adults experience an anxiety disorder at some point in their life, and 31.9% of adolescents (ages 13-18) have experienced an anxiety disorder. ⁵ Cognitive behavioral therapy and anti-anxiety medications are two of the most currently used treatment options. Anxiety disorders can arise from a combination of stressful or traumatic experiences, genetics, having other relatives with anxiety, other mental health disorders, drugs, alcohol, or stress from an illness.

Brain circuits involved in Anxiety

Figure A. The anxiety network. Long range connections throughout multiple brain regions are involved in the anxiety circuit. Figure taken from Tovote et al. ²⁴

The neural circuits involved in anxiety are mediated by long connections throughout many different brain regions. Anxiogenic and anxiolytic (anxiety inducing and inhbiting) effects are connected to the basolateral amygdala (BLA) and the anterodorsal bed nucleus of the stria terminalis (adBNST).

Activation of the BLA to the ventral hippocampus (vHC) pathway is anxiogenic, whereas activation of the BLA to the central amygdala is anxiolytic. The circuits involved in anxiety are distinct an can either promote or diminish anxiety-related states. ²⁴

More information on anxiogenic and anxiolytic? Check out the Glossary

What is the Microbiome?

Shortly after birth microbiota colonize the gut, a period of bacterial colonization. From here, a microbial 'factory' stabilizes and forms a highway, a crosstalk between the gut microbe and other organs throughout the host. The microbiota within each of us is unique, although Firmicutes and Bacteroids are the two most prominent bacterial phyla, harboring around three fourths of microbiome. ⁸ The gut microbiota is comprised of bacteria, viruses, funcgi, and archaea that inhabit the gastrointestinal tract, mostly residing in the colon and small intestine. The presence of this commensal bacteria is essential for immune health, nutrient processing, CNS function, and helps regulate many other functions of physiology. The human gut microbial community is a complex system within the digestive tract, and influenced by the environment, through antibiotics, diet, and exercise. Human health and disease are dependent on the diversity and ecology of our microbe.

What is the Gut-Brain Axis?

The gut-brain-axis refers to a bidirectional communication process between the gastrointestinal tract, where our microbiome inhabits the colon and small intestine, and the central nervous system. This communication involves direct and indirect endocrine, immune, and neural pathways. This axis connects the brain and spinal chord, the autonomic peripheral nervous system, the enteric nervous system, and the hypothalamus-pituitary-adrenal axis.

The neural pathways involves the vagus and enteric nervous system. Here, afferent signals in the autonomic nervous system arise from the lumen and are transmitted through the vagus nerve to the brainstem. In the brainstem the signals get picked up and processed by other parts of the brain. Most of the evidence of the influence of the gut microbiota on the brain, airses from studying germ-free animals, which have an altered microbiome.

Other pathways involved in this process include the gut hormones, metabolism, the immune system, neurotransmitters, short-chain fatty acids, and cytokines. In this portfolio I've outline the mechanism of the vagus nerve, for it's direct, and highway-like pathway to the brain.

Figure J. Schematic of the gut-brain-axis. Communication between the microbiome and the brain is bidirectional. The Vagus Neve highlighted as a direct pathway from the brainstem. Created with BioRender.com. Figure modified from Foster (2015). ¹¹

This video by Dr. Dr. Elaine Hsia explains how the microbiota influence health and disease. Video from TedX. ¹⁴

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