In recent years, mindfulness meditation has been gaining traction as a form of therapy to address various health-related issues. In a previous blog post, I discussed how meditation is a promising technique to alleviate anxiety and depression. Previously, I examined the effects of mindfulness meditation through a psychological perspective. However, I find that the science of meditation is a hard sell for many people; people who come from a more scientific background often need to understand the underlying physiologic mechanisms of mindfulness meditation to be convinced of its efficacy. This is understandable — I seek these kinds of answers as well. The science of meditation is still very new, and no one has a clear understanding of why it works. However, my research has led me to discover a few promising theories from a bottom-up point of view. How does the practice of meditation change the body, and do these changes then influence the state of the mind?
Meditation appears beneficial to the body, in part, because it increases parasympathetic nervous system (PNS) activity, and decreases sympathetic nervous system (SNS) activity. The SNS is activated during times of danger, and is often called the “fight or flight” response. This response is healthy in moments of danger but becomes detrimental when it is chronically activated. Modern life is full of stressors that keeps the SNS activated, and this chronic SNS activity is poor for our health. Meditation helps us decrease the activation though eliciting the ancillary PNS. But how? The two theories I will outline below all revolve around the effect slow breathing has on increasing parasympathetic activity through vagal tone. More specifically, these theories outline how breathing patterns affect the activation of baroreceptors within blood vessels, and mechanoreceptors within the lungs (Gerritsen & Band, 2018).
Baroreceptors are pressure sensors found within our body. These are important detectors within blood vessels that help relay sensory information to the brain for autonomic control, which helps maintain balance within the cardiovascular systems. For instance, inhalation causes an increase in heart rate, while exhalation causes the heart rate to slow. This is called respiratory sinus arrhythmia (RSA). The detection and signaling of this change, in part, is sensed by baroreceptors within the veins (Gary G. Berntson, John T. Cacioppo, 1993; Karemaker, 2009). The baroreceptors are activated when blood pressure increases in the aorta during exhalation due to increased intra-thoracic pressure. The activation signals a decrease in heart rate that causes a reduction in blood pressure, and vice versa (Gerritsen & Band, 2018; Lehrer & Gevirtz, 2014; Vaschillo et al., 2002). This is one example of the many methods in which balance is maintained within the cardiovascular system via the vagus nerve. Keep in mind that the sensitivity and responsiveness of these receptors can change, which thereby changes the frequency (or tone) of vagus nerve activation. It has been shown that decreasing one’s breath to 0.1 Hz (about 10 seconds per breath) increases the sensitivity of heart rate change given changes in blood pressure (Bernardi et al., 2001; Lehrer et al., 2003). This suggests that the baroreceptors become more sensitive during deep/slow breathing, and the vagus nerve is activated more often. Thus, breathing at this rate increases the sensitivity of these receptors, thereby increasing vagal tone.
Other research has also confirmed that slow breathing rates result in increased heart rate variability (HRV)(Song & Lehrer, 2003). As the name suggests, heart rate variability represents the variability of time intervals between consecutive heartbeats (Makivić et al., 2013). The heart changes speed given the demand of the body. This flexibility of the heart to change its speed quickly is known as heart rate variability. If the speed at which the heart beats changes is fast, the HRV is said to increase. This observation that slower breathing increases HRV makes sense because it is commonly used as an indicator of autonomic nervous system balance (Ernst, 2017) and is increased with increased vagal tone (Laborde et al., 2017). Consequently, because modern day stress causes us to go into “fight or flight” mode often, increasing SNS activity and decreasing PNS activity, decreased HRV can be used as an indicator of stress (Kim et al., 2018). This is important because this breathing frequency of 0.1Hz is the same breathing rate observed in novice Zen meditators (Cysarz & Büssing, 2005). Since meditation results in calmed breathing, it thereby increases vagal tone, and decreases the physiologic processes of stress.
In more simplistic terms, the pace of slowed and consistent breathing increases baroreceptor sensitivity which increases vagal tone because this nerve is what sends the signals of the receptors to the brain (Lehrer et al., 2003). Vagus nerve tone is a major driver of parasympathetic nervous system activity. Due to the vagus nerve innervating all major systems of the body, when activation is increased in one area, activation is increased in the rest. This counteracts the SNS activity caused by stress, which theoretically should balance the body and improve health.
The above theory describes how breathing patterns influence baroreceptors, which influence vagus nerve tone. Another theory argues that it is the receptors within the lungs that cause the physiologic changes(Noble & Hochman, 2019). Within the lungs, there are two pulmonary stretch receptors worth noting: rapidly-adapting receptors (RARs) and slowly-adapting receptors (SARs). RARs are typically activated throughout normal breathing(Noble & Hochman, 2019), while slower breathing additionally activates SARs(Jerath et al., 2006; Schelegle, 2003). SARs are important receptors as they are involved in the Hering-Breur Reflex(Schelegle, 2003), which is a reflex that induces immediate exhalation of the lungs after the detection of an excessively large inhale(Moore, 1927). This reflex likely exists to protect the lungs from overexpansion, but also highlights the importance of the role of these receptors in the nervous system’s control over the lungs.
SARs send signals to a region of the brain called the nuclear tractus solataris (NTS) via the vagus nerve(Schelegle, 2003). The NTS is a relay station for all vagal afferents(Noble & Hochman, 2019), and sends neuronal input to the hypothalamic paraventricular nucleus and the central nucleus of the amygdala(Petrov et al., 1993). The amygdala is highly involved in emotional regulation and processing(Desbordes et al., 2012). Increased amygdala activity is also associated with perceived stress(Taren et al., 2015). SARs activation either directly, or indirectly inhibits amygdala activation by means of the NTS(Noble & Hochman, 2019). This possibly explains how deep breathing can provide stress-reducing effects(Noble & Hochman, 2019).
These two ideas are not mutually exclusive theories. Both of the mechanisms may play a role in the benefits of mindfulness meditation and similar practices. At the same time, you may be noticing that there is a “chicken or egg” question here: are these changes happening because our mind decided to calm down first, or is the mind calming down because of the physical activity of mindfulness? This is surely a difficult question – however, perhaps it is frivolous. It is of my current opinion that both occur simultaneously. The mind and body are one. The practice of meditation induces both mental and physical changes, which in turn cause a positive feedback loop of relaxation. This parasympathetic activity has the potential to create resilience towards our modern, stressful environment. So, like the Yin Yang, these two ideas are not in opposition to each other, but rather represent a larger truth — a truth that challenges us to become more considerate of our body, and work to bring it more calmness.
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