Part-2 talked about the bottom-up evolutionary organisation of the nervous system. According to Polyvagal Theory, there are three distinct stages of development of the autonomic nervous system – immobilisation (freeze response we see in reptiles), mobilisation (fight-flight response) and social engagement system (what we see in humans). Vagus nerve plays an important role in regulation of these.
Vagus nerve is a system of afferent (carry signal to the brain) and efferent (carry signal from the brain) nerves that innervate heart, lungs and other visceral organs as well as muscles of the speech. I refer anatomy nerds to this excellent page to learn more about vagus. Vagus is the backbone of the parasympathetic system. When the environment is perceived safe, this system promotes growth and restoration by slowing down heart rate, inhibiting sympathetic nervous system, dampening the cortisol production and reducing inflammation. In other words, it promotes visceral homeostasis.
Porges suggests that, through evolution, the cluster of neurons in the brainstem that regulate visceral homeostasis through the vagus became anatomically linked with the neurons that regulate the muscles of face and head. These muscles are critical for social engagement, for example, through the control of eye lid opening, facial expressions, orientation of head, middle ear muscles (to extract human voice from background voice), laryngeal and pharyngeal muscles (speech). It is also notable that neural mechanisms for eye contact are shared with those needed for listening to human voice. We know that difficulties in eye contact, facial expressions and extraction of human voice are some common challenges faced by individuals on Autism Spectrum.
The linkage of these two neural pathways suggest a possible mechanism through which disorders of visceral states influence social engagement. Deficits in either of these would compromise spontaneous social engagement. This suggests that therapy strategies that improve vagal tone could aid in spontaneous social engagement.
There is evidence that Yoga improves vagal tone, making it a useful daily practice that works on spiritual, somatic, cognitive and behavioural levels. This could be of great help to the souls that struggle with challenging bodies.