- Article Review by Dr. Stephanie Shorter
Yoga research is still in its infancy stage as most studies are concerned with simply describing the positive effects of a yoga practice. For example, different studies show that yoga improves depression, anxiety, post-traumatic stress disorder, diabetes, heart disease, high blood pressure, substance abuse, and chronic pain, among other conditions. Every field of research has its own developmental course where it moves from describing to explaining. As of this month, yoga research has pushed its edge to enter more fully the explanatory stage with a newly published theory that elegantly weaves together behavior, physiology, and anatomy, while unifying observations in yoga asana research and beyond. A new paper1 by Dr. Chris Streeter and colleagues2 is the most substantial paper to date about the neural mechanisms that underlie how yoga heals body and mood.
Let’s examine the pieces of this theory.
Vagus nerves are one pair of the twelve pairs of cranial nerves that originate in the brain. Most of these nerves have either a sensory function (e.g., optic nerves conduct light signals from eye to brain) or a motor function (e.g., oculomotor nerves send commands from brain to eye muscles to shift the gaze around in space). However, a few cranial nerves, including the vagus nerve, have both a sensory and a motor component, which means that they are information highways supplying communication in both directions between brain and body. The name ‘vagus’ comes from the Latin meaning ‘wandering’, and this meandering nerve does wander around! It travels down the neck near the carotid artery to the heart where it plays a critical role in regulating the pace of the heartbeat. Other branches of the vagus nerve wander their curvy routes throughout the torso to innervate the esophagus, lungs, stomach, intestines, and sexual organs. Vagal activity is important in metabolism, detoxification, cell repair, inflammation, and immune function.
The sympathetic nervous system – the so-called fight or flight system – is chronically revved up in many people in our multitasking, high-octane caffeinated culture. In contrast, the parasympathetic nervous system is all about rest and digest. The vagus nerve is one of the dominant carriers of the parasympathetic message to relax, rest, and digest. It tells the heart to slow down and reduce blood pressure. It signals muscles to contract in a peristaltic wave to efficiently move food through the gastrointestinal tract and it guides liver cells to detoxify and regenerate.
The wandering vagus nerve is an anatomical mind-body highway that would allow mindfulness to heal organs and systems while also allowing the body (during asana, let’s say) to soothe the thoughts and mood – sensory feedback and motor commands, mind and body.
Another piece of the theory relates to neurochemistry – specifically, the most prevalent inhibitory neurotransmitter in the brain, gamma-aminobutyric acid (GABA). Streeter has previously shown that yoga increases GABA levels in the brain and that these post-practice GABA surges correlate with self-reported increases in positive mood and decreases in anxiety.
The Weight of Stress
Allostatic load is a term that was coined in the ‘90s to refer to the cellular cost of chronic stress, which causes or accelerates disease. New data show that practicing yoga can lift the allostatic load by stimulating GABA release and increasing activity in the parasympathetic nervous system while decreasing flight-or-flight activity in the sympathetic nervous system.
The hallmark of a theory – what distinguishes it from a hypothesis, which is an educated guess to an unanswered question – is that theory provides the glue that connects multiple, often disparate pieces of already existing evidence into a coherent picture. Toward this end, the paper incorporates non-yoga clinical evidence about severe depression and epilepsy cases that cannot be treated pharmaceutically. Many of these intractable cases can be alleviated with a radical surgical approach: implantation of a stimulating device near the collarbones that generates small electrical pulses to activate the vagus nerve. So we see that practicing yoga helps mood management and the implication is that it is via vagal stimulation.
Putting it into Practice
Yoga is all about finding balance and uniting opposites including bringing sympathetic and parasympathetic neural activity into greater harmony, moment by moment. As yoga practitioners and teachers, how do we increase parasympathetic activity by stimulating the vagus nerve? Stay tuned for future Austin Yoga Hub articles about the neuroscience of yoga.
1 Streeter CC et al. Effects of yoga on the autonomic nervous system, gamma-aminobutyric-acid, and allostasis in epilepsy, depression, and post-traumatic stress disorder. Medical Hypotheses (2012). In press.
2 Co-authors Drs. Patricia Gerbarg and Richard Brown, both psychiatric medical professors and long-time members of the International Association of Yoga Therapists, are experts in using asana and breathwork to heal trauma. Learn more about their work at www.haveahealthymind.com.