07/08/2019
It is a popular belief that inverted Yoga poses (like Sirsasana) increase the blood supply to the brain to promote relaxation and calmness. This is not true.
Then, what makes inversions so effective for mental health? Let’s see from a neuroscience perspective.
Cerebral Blood Flow (CBF) is the blood supply to the brain in a given period of time. In an adult, CBF is typically 0.75 liters per minute. It is a popular belief that inversions increase the CBF which in turn promote mental health – which is not true. In fact, the human system is highly complex. It keeps tight control of the homeostatic conditions of its most complex part – the brain, which requires a constant CBF in order to function properly. Too much blood (a condition known as hyperemia) can raise the pressure inside the skull (called intracranial pressure - ICP), which can compress and damage delicate brain tissues. And, too little blood flow (ischemia) can potentially result in damage or even death of brain cells.
The gravity plays an important role in determining how much the heart has to work to meet the brain’s demand for the blood. For example, while standing, the heart needs to be more active than while someone is sleeping or inverted. Also, it is imperative that the heart responds to any sudden changes in arterial pressure, which can occur, for example, when a person suddenly stands up from a sleeping position, or falls down while standing, or for some reason becomes inverted (like inverted Yoga poses). To handles such a complex work, a negative feedback system is present in the body that consists of pressure sensors (called Baroreceptors) and the autonomic nervous system (consisting of sympathetic and parasympathetic). (NOTE: we covered autonomic nervous system in one of our previous posts).
In modern medical terms, this negative feedback system is called the “Baroreflex” or “Baroreceptor reflex”. It is one of the body's homeostatic mechanisms to help maintain blood pressure at nearly constant levels. The Baroreflex provides a rapid negative feedback loop in which an elevated blood pressure towards the head (e.g when you get into Sirsasana) reflexively causes the heart rate to decrease and also causes blood pressure to decrease. Similarly, any decrease in blood pressure (e.g. when you stand up) triggers the Baroreflex activation and causes the heart rate to increase, and to restore blood pressure levels.
Hence the brain needs to know the difference between the pressure at the head and at the heart, so that it can activate a proper autonomic response. To sense the pressure, the Baroreceptors are present in the neck in Carotid sinus, which is located at the original of Carotid arteries that deliver blood to your brain and head. The Baroreceptor are also located in the aortic arch to sense the pressure at the heart.
These Baroreceptors report the in pressure through the cranial nerve to the brain. The brain, in turn, modulates the sympathetic and parasympathetic activities based this pressure information from baroreceptors. The brain stimulates sympathetic activity when the pressure reported by the Baroreceptors at the neck becomes lower (when standing) and increases parasympathetic activity when the reported pressure is high (in inversions). The idea is to bring back the homeostatic balance in such a way that the requirement of blood supply and pressure at the brain is met. (NOTE: Moreover, factors other than the Baroreceptor pressure also may work to alter sympathetic vs parasympathetic response. For example, increased body temperature will act to increase sympathetic activity, as will feelings such as fear, anger, and anxiety)
The stimulation of Baroreceptors triggering parasympathetic response, as in inversions, also calms down Reticular Formation - an area in the Brainstem that filters the sensory signals. This lowers the level of excitation impulses from the sensory nerves - giving the state of inwardness. Reticular formation and its influence in Yogasana will be covered separately in a future post.
So, in all the inverted Yoga postures, the Baroreflex plays an important role in mental calmness and relaxation - as the parasympathetic nervous system gets stimulated and the heart is relaxed. This is a natural activation of the Baroreflex as the Cartoid sinus Baroreceptors sense higher pressure because of the gravity pull. And, it is not because of the increase in CBF – as wrongly perceived.
The Baroreflex also plays a role in enhancing the relaxation the poses that use Jalandhara Bandha (also called Chin Lock). In Jalandhar Bandha, the chin is dropped into the sternal notch. This cervical flexion generates a mild local pressure on the Carotid sinuses that throttles the flow of blood through the carotid arteries - triggering Baroreflex.
The inverted poses that have Jalandhar Bandha Sarvangasana, Setubandh Sarvangasana, Halasana and Karnapidasana, produce profound relaxation and calmness because of enhanced Baroreflex caused by inversion as well as chin lock. When these poses are supported using props (as in Iyengar Yoga), they are highly restorative in nature. These supported poses are highly recommended because of their therapeutic nature to calm down the mind and the nervous system.
These inverted postures with Jalandhar Bandh are typically performed towards the end of the Yoga practice so as to promote relaxation response and cooling down. We have covered the need for “Cooling Down” in one of our previous posts.
In this post, we have not covered how inversions promote Diaphragmatic Breathing and how gravity helps lengthen the exhalations - which increase the calming effect on the mind. This is a topic worth a separate post.
It is advised to perform Yogasana (particularly inversions) mentioned in this post under the guidance of an experienced teacher or practitioner.
