Our brain determines what is threatening, and therefore stressful to us. Its stress response involves two-way communication between the brain and the cardiovascular, immune, metabolic, and other systems via the nervous system, endocrine system, and HPA axis.
The optimal homeostasis is a state in which the organism's immediate needs sustain the least possible long-term costs. Allostasis is our adaptive process of maintaining stability during conditions that are outside of our range. So, the allostatic load is the body's cost to maintain this stability, which is often reflected in diseases and pathophysiological conditions. This is especially important for chronic stress and manifests in elevated blood pressure and heart rate, leading to long-term effects like hypertension and cardiac morbidity. Overactivity of stress-responsive systems is associated with increased symptoms in a wide range of diseases:
Psychiatric disorders (anxiety, depression)
Alcohol or substance dependence
Metabolic disorders (diabetes, obesity)
Immune disorders (infection, asthma, cancer)
The “fight-or-flight” response, activated by the amygdala, is responsible for the outward physical reactions most people associate with stress, including increased heart rate, heightened senses, a deeper intake of oxygen, and the rush of adrenaline. Finally, a hormone called cortisol is released, which helps restore the energy lost in the response. Stress induces an imbalance of the autonomic nervous system, decreasing the parasympathetic nervous system and increasing the sympathetic nervous system's activity. It reduces the activity of GABA- the primary inhibitory neurotransmitter, increasing the allostatic load. During periods of high norepinephrine release, GABA and dopamine may be inhibited, influencing our ability to move gracefully (affecting motor control).
Neural circuits that mediate the effects of the ANS and HPA axis converge in the hippocampus, a limbic system component. Embedded in the medial temporal lobe, the amygdala and hippocampus are vital for memory function, emotion processing, and mediation of psychological stress, and modulation of HPA response to stress. The amygdala is responsible for threat perception. Hippocampus is responsible for the experience of stress. It releases a hormone CRH, carried to the anterior pituitary lobe, where it stimulates ACTH's secretion. This induces the adrenal glands to induce cortisol. Chronic stress results in increases in glucocorticoid levels. It provides negative feedback that activates CRH release in the amygdala. It creates fear-based behaviors and defensive reactions. Stress is associated with a volume reduction of the hippocampus and decreased hippocampal GABA levels because of the cortisol increase. Hippocampus has receptors that are sensitive to cortisol. Chronic stress ultimately also changes the brain's chemicals, which modulate cognition and mood, including serotonin. Serotonin, released in the raphe dorsal nucleus, is important for mood regulation and wellbeing. In fact, selective serotonin reuptake inhibitors (SSRIs) are used to restore serotonin's functional activity in the brain in people with depression. 90% of serotonin is produced in the gastrointestinal tract. Sleep and circadian rhythm disruption are a common feature in many psychiatric disorders, including depression and anxiety. Stress hormones, such as cortisol, play a key modulatory role in sleep. Elevated cortisol levels can, therefore, interfere with our sleep. Hypothalamus is responsible for circadian rhythms, and through the hormonal network, it is instrumental in creating the stress response. What's the impact of yoga on our brains? Performing asanas, we deepen the connection between our body and the primary motor cortex. We become more coordinated in our movements and more aware of the space surrounding us. Also, our self-awareness refines with the practice, and we become more aware of the effects of the bodily posture on our mood, getting better and better at detecting signals when we start being out of balance. This is a handy skill in creating resiliency to stress through self-awareness. From the ANS perspective, yoga reduces the allostatic load in stress response to restore homeostasis. Yoga practice is seen to correct the under activity of the PNS and GABA through the stimulation of the vagus nerves- the main peripheral pathway of the PNS. This improves the brain's functions for threat perception, interception, fear processing, emotional regulation, and defensive reactions. What’s more, yoga can modulate the pain perception through the challenging sensations while maintaining asanas for longer periods (influence the anterior cingulate cortex- ACC and insula). Long-time yoga practitioners have higher pain tolerance and thicker ACCs. This certainly changes our perception of a stressful situation, as we know we can handle it, and it will pass, not allowing it to affect us as a whole being, keeping strong and healthy boundaries. Yoga is also seen to increase heart rate variability, especially throughout the mix of energizing and calming asanas and breathing practices (ujjayi/ kapalabhati sandwich). The third and most myelinated vagus, according to Porges, promotes calm states consistent with the metabolic demands of growth, repair, and restoration. It also supports social engagement and prompts feelings of safety. Increasing vagal tone reduces heart rate and blood pressure, and decreasing vagal tone accelerated the heart rate and blood pressure. Vagal control allows for rapid adjustments in the heart’s beat-to-beat intervals. Yogic breathing can influence the ANS and HRV. It stretches the receptors in alveoli, baro-, and chemoreceptors, sending the vagal afferent signals to induce relaxation. An excellent example is coherent breathing, using a fixed rate of 6bpm. Also, Om chanting elongates the exhales, increasing the HRV. The main pathways to influence the PNS are within the vagus nerves. Each vagus is bidirectional, containing efferent and afferent nerves. Various fibers innervate the pharynx, larynx, lungs, heart. Many yogic practices focus on the opening in these areas of the body (backbends, ujjayi, spinal twists). There are neural connections that convey information from the vagus nerves to the structures that mediate interoperceptions and affective states and influence emotional states and thought processes. Yoga practice requires attentional engagement and is associated with positive changes in brain structures, especially in areas connected to awareness, attention, executive functions, and memory. It is seen to enhance the brain cortical thickness in the left prefrontal cortex. The practice of yoga seems to have a neuroprotective effect, thus positively impacting mental health. Finally, yoga practice can also be seen as a type of physical exercise. Exercise tackles inflammation by leading to an anti-inflammatory response. Besides, exercise increases neurogenesis – the production of new brain cells – in important areas, such as the hippocampus. It also improves our mood, cognition, and physical health.