If you’ve ever studied the nervous system, you’ve most likely learned it as a two-fold mechanism - the parasympathetic versus the sympathetic system. This model creates a spectrum from ‘rest and digest’ mode to the ‘fight or flight’ response according to the amount of stress experienced. This theory assumes that we begin in a parasympathetic state and gradually move towards the sympathetic mechanism as stress increases, somewhere along the line crossing a border that puts us in one system or the other. Too much time spent in ‘fight or flight’ mode would trigger a necessity to remove the stressor and return to a state of ‘rest and digest.’
The arch concept, however, paints a different picture of how these two systems function in the body, not as systems at contention with each other, but as antagonistic states in constant checks and balances of one another.
The simple metaphor of an arch is that in order to increase the tension of an arch, you must build at it from both sides. In the muscular system, you’ll notice that the pecs need to engage in a heavy dumbbell row and we know that strong quads mean we also need strong hamstrings. A heavy biceps curl will engage the triceps, too. This balancing act between action and stabilization could be used to describe the nervous system better than the common theory.
In the old model, the act of sex is lost along the spectrum. From an arch perspective, however, we can describe sexual activity more accurately as an increase in tension from either side - building intensity on both on the sympathetic and paraysympathetic side by simultaneously increasing degrees of excitation and relaxation.
According to the arch concept, raising one side of the arch too high will cause a collapse. In humans, too much time spent in a fight or flight response is shown to increase stress-related problems and chronic disease. On the other hand, living only in the comfort of the ‘rest and digest’ system produces it’s own set of issues associated with immobility.
The Polyvagal theory adds another dimension to this new view of the nervous system in which it poses that the nervous system is actually composed of three distinct branches. These three branches help to identify the relationship between human experience and physiological state - our stress, emotions and social behavior have a real, physical impact and we can start to map it.
The newest system, the parasympathetic associated with the ventral vagus nerve, allows us to socialize and maintain rational thinking. The roles connected to the neocortex of the brain involve higher level functions such as sensory perception, spatial reasoning, conscious thought and language, allowing us to function within the world around us and what makes us human. This is also the first part of the brain to shut down under threat.
The second level is the midbrain ‘fight or flight’ sympathetic response and is adaptive to survival, whether as predator or prey, and common to all mammals. These autonomic reactions are often associated to adrenaline response.