What is Parasympathetic Nervous System?
The parasympathetic nervous system (PNS) is one of the two main branches of the autonomic nervous system. It is often summed up as "rest and digest": through the vagus nerve it slows the heart, and it supports digestion and recovery. At rest, its influence on the heart predominates.
Key Effects
- •Slower heart rate (via the vagus nerve)
- •Stimulated digestion and gut movement (peristalsis)
- •More saliva and tears; narrower pupils
Not a switch
The parasympathetic and sympathetic branches are not two ends of one dial. They can be active at the same time and can change independently, so "more calm" does not simply mean "less stress" — "autonomic balance" is a shorthand, not something a single number measures. Cortisol is not controlled by this branch: it is regulated by the hormonal hypothalamic–pituitary–adrenal axis.
Polyvagal theory adds ideas such as a "social engagement system" linked to the vagus nerve. These are terms of the theory, which is debated, not established physiology.
In ONDA Life
Slow breathing is associated with higher vagally mediated HRV; whether a longer exhale adds anything beyond slowing the breath is still debated. ONDA's practices use slow, guided breathing; they do not measure parasympathetic activity directly. Vagal tone cannot be measured directly; HRV measures such as RMSSD reflect vagally mediated changes in heart rate.
Scientific Basis
See The Autonomic Nervous System for the full picture and sources.