The scientific explanation behind yoga's effect on stress.
(Updated post with references)
There are two functional parts of the brain that play a key role in stress. These serve the functions of emotion and cognitive function. So I am calling them the 'emotional' brain (the amygdala and its connections) and the 'logical' brain (parts of the prefrontal cortex and parts of the hippocampus).
The emotional brain is able to initiate a 'stress response' via the sympathetic nervous system which culminates in adrenaline and cortisol racing through our circulation.The logical brain is always trying to 'turn-off' this stress response and it is also trying to restrain the emotional brain. The stronger our logical brain, the better it becomes at doing these two things. When the stress response is 'turned off', our parasympathetic nervous system signal is 'turned on'. This signal 'relaxes' the body. So a strong logical brain goes hand in hand with relaxation.
During yoga, there are four things taking place: 1. Raising and lowering of intracranial pressure. 2. Stimulation of baroreceptors i.e. by the carotid arteries, thoracic cavity and the heart, stimulation of the vestibulosympathetic reflex via the vestibular apparatus, stimulation of the sympathetic signal from the exercise component 3. The readjustment of systemic blood pressure to keep cerebral blood flow constant, despite the changing intracranial pressure. This is implemented by the autonomic nervous system's sympathetic and parasympathetic signals. 4. Concentration and focus while we are forced to balance or keep still. The logical brain (parts of the prefrontal cortex) have been shown in MR imaging to be responsible for thought redirection and focus. Parts of the prefrontal cortex also oppose the limbic system's stress response and the generation of the hypothalamic sympathetic signal.
Yoga is training the entire stress circuit at two levels. First, every time we are 'holding' a posture, staying very still to concentrate or trying to balance, our logical brain is being activated. When we are bending forwards and lowering our head, our 'relaxation' signal is being turned on, since bending forwards and lowering our head increases the pressure of the fluid within the cranial cavity. The relaxation signal brings our blood pressure down to compensate for this. So bending forwards and concentrating at the same time is triggering both the logical brain and the relaxation signal at the same time.
Bending backwards can trigger the stress response signal through baroreceptors and through signals from other receptors including those within our vestibular system within the inner ear. Contracting a muscle also triggers the stress response signal. So, when we bend backwards and contract our muscles while still having to stay still and concentrate on balancing, our logical brain is given an extra challenge. It has to overcome the stress response signal being triggered through these ways before we can be still and concentrate during a posture. This 'extra' resistance the logical brain is having to work against, can be seen to be 'training' it like a muscle.
At the end of a series of yoga postures, the logical brain has had a 'workout'. It is buzzing with activity. You feel mentally calm as it is keeping your emotional brain quiet. Training the logical brain in this way for a long time can result in a rewiring of the nerve connections within the logical brain. New circuitry that enables you to find it easier to control your thoughts is formed. You may find it easier to channel your thoughts in the direction you want and not 'dwell' on negative thoughts or experiences. This is partly why yoga seems to have a positive effect on depression and anxiety, where sufferers have a tendency to dwell on negative life events. Stronger connections within the logical brain keeps the lid down on the emotional brain and the stress response. This is why yoga can be so effective at battling stress.
The key thing to do is to attempt yoga postures which are structured in a well-formulated sequence where each posture involves a long hold.
In essence, yoga seems to 'train' the prefrontal cortex, or our 'rational' mind. Greater rationality and restrained emotional reactivity makes our perception of the world around us more accurate. This is the premise behind the origin of the practice of yoga. Many scholars argue that Hinduism is not a religion but a philosophy as made clear by the philosphical content of the texts associated with Hinduism. In fact, Hinduism does not involve one single school of philosophical thought, but a great many, both atheist and theist (this is where scholars point out that a 'religion' that includes atheist components should perhaps not be called a 'religion'). The Sankhya school of philosophy within Hinduism is one example. This developed gradually over a long period of time but became established as a distinct pathway of ideas at around the 5th century BC. The Sankhya school of philosophy is based, like the ideas of Descartes, on the philosophical concept of dualism but it is different from the classical idea of western dualism in that it makes a distinction between consiousness and matter. The Sankhya philosphophical school proposes that we gather knowledge through three steps: pratyaksha (perception), anumana (inference) and sabda (testimony of reliable sources). What this translates to is beautifully exemplified in the title of the great work by the German philosopher Arthur Schopenhauer, 'The world as Will and Representation' or George Berkeley's words, "Esse est Percipi" (to exist is to be perceived). In essence, our perception of the world seems to be all that we have to go by. Our prefrontal cortex is the source of our rational mind. If we keep our prefrontal cortex finely tuned and prevent our emotional prejudices and fears from 'clouding' our minds, we are more likely to gather more accurate information of the world around us. A healthy prefrontal cortex will help solve problems in mathematics and logic which brings yoga back to the heart of Hindu philosophy which is strongly based on acquiring knowledge, in particular, learning the 'truth' about the universe through the use of logic and inference.
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