The adrenaline response during stress needs to be switched off once stress has ended. This protects against cardiovascular disease. Scientists have found how the sympathetic nervous system is turned off, and how this may help to stimulate the parasympathetic nervous system to calm you down.
The sympathetic nervous system is active during stress
The sympathetic nervous system is one of two major regulators of stress responses in the body. It stimulates the production of the stress hormone adrenaline (US: epinephrine) from the adrenal glands, which enables the "fight-or-flight" reactions to prepare the body for immediate action. Adrenaline accelerates heartbeat and respiration and increases blood pressure for example to supply muscles with more oxygen. It further liberates glucose from the liver to increase energy supply to the muscles and other organs as well. This enables one to either actively deal with the stressor (the thing that causes stress) by attacking it or by avoiding or escaping from it.
These are all very useful adaptations to the stressor, and can even make the difference between life and death. However, if adrenaline levels remain too high for a longer period of time, they may cause cardiovascular and other diseases. It is therefore necessary to shut off the activity of the sympathetic nervous system on time once the stressor is no longer posing a problem. There appears indeed to be a mechanism in the body that switches off the sympathetic nervous system and adrenaline release into the blood, but this has been difficult to find.
A difference between cortisol and adrenaline
Switching off stress responses has been studied mostly for the HPA-axis. This is because the stress hormone cortisol, the end product of the HPA-axis, can pass into the brain by crossing the blood-brain-barrier. The blood-brain-barrier is a protective shield that keeps the brain isolated from potentially harmful substances in the blood. Most substances that act in the brain need to be transported across the blood-brain-barrier by special molecular pumps. Cortisol can cross the blood-brain-barrier by itself and has therefore easy access to the hypothalamus, prefrontal cortex and hippocampus to signal that HPA-axis activity should stop. Cortisol therefore can inhibit its own production through a negative feedback mechanism.
Adrenaline cannot cross the blood-brain-barrier by itself. Scientists found it therefore hard to imagine that adrenaline can signal back to the brain as cortisol does. Could there be another way that adrenaline uses to terminate the activity of the sympathetic nervous system?
The vagus nerve and the parasympathetic nervous system
It is well known that the sympathetic (for the fight-or-flight, active responses) and parasympathetic nervous system (breed-and-feed, calming things down) can work in opposite ways of each other. They can also inhibit each other, so that when the sympathetic nervous system is highly active, the parasympathetic nervous system is inhibited. And vice versa.
The vagus nerve is part of the parasympathetic nervous system. It wanders to many organs in the body, giving instructions from the brain to the organs, and giving feedback from the organs back to the brain. Two-way traffic between the body and the brain.
One of the organs that the vagus nerve visits is the adrenal gland. More specifically, it enters the central part of the adrenals, the medulla, where adrenaline is produced and released. Scientists have indications that the vagus nerve endings in the medulla can sense adrenaline through specialized receptors. These receptors are like antennas that can capture adrenaline. These receptors give a signal through the vagus nerve all the way up to the brain. As the vagus nerve is part of the parasympathetic nervous system, this negative feedback signal will activate the parasympathetic nervous system in the brain, and by doing so, inactivate the sympathetic nervous system.
Adrenaline activates thus a negative feedback mechanism, just like cortisol does. But unlike cortisol, it does not do so in the brain, but on the endings of a nerve descending from the brain.
Take a deep breath to activate your parasympathetic nervous system!
It thus appears that activating the parasympathetic nervous system can help to deactivate the sympathetic nervous system and stop the adrenergic stress response. What could you do to activate the parasympathetic nervous system yourself while you are feeling stressed?
The easiest way to get your parasympathetic nervous system going is by doing breathing exercises. Breathing deeply and slowly counteracts the fast and superficial breathing induced by sympathetic activation. You are more or less forcing the parasympathetic nervous system to be active (if not you cannot breathe deeply and slowly), and as a consequence, the parasympathetic nervous system will inhibit the sympathetic nervous system in the brain.
Breathing exercises work in addition to the feedback mechanism by adrenaline. They do not enhance negative feedback. They help to give the parasympathetic nervous system an extra boost, so that it can reduce the activity of the sympathetic nervous system more effectively.
Breathing exercises are an integral part of yoga and mindfulness. This is because of the calming effect they have. For rapid and short-term stress relief, they are certainly useful. So take a deep breath before going into a difficult discussion with your boss or before having to speak in public. It will help to calm you down!
We have collected a number of efficient breathing exercises for you, which you can read here. Put your parasympathetic nervous system to work!
Reference
Mravec, Role of catecholamine-induced activation of vagal afferent pathways in regulation of sympathoadrenal system activity: negative feedback loop of stress response. Endocrine Regulations 45, 37-41 (2011).
