The Parasympathetic Nervous System (PNS) and the Sympathetic Nervous System (SNS) is part of a larger Autonomic Nervous System (ANS). Your ANS is a control system that encompasses all unconscious bodily functions and rates, including your heart rate, respiratory functions and also your digestion amongst others.
Parasympathetic Nervous System
The PNS is a large and slow functioning system that typically involves the “Rest and Digest” response in your body. When at rest, the body involves the PNS homeostatic response (regulating internal chemical conditions) to restore optimal levels of function and recovery as need be. The PNS has larger pathway system throughout the body thus having a slower response to stimulus. It is typically responsible for the musculoskeletal system (Eg Muscles) to relax, and causing constrictions in your bronchial tubes (within your Lungs), in which will slow down your heart rate making you more calm and relaxed. Your salivary glands will increase saliva production as well as your digestion as the gastrointestinal system is fired due to relaxation — hence “Rest and Digest”.
Sympathetic Nervous System
The SNS however exudes a different response, The “Fight or Flight” response in which the body comes onto a high alert condition sensing a perceived threat to the body. It is a shorter and faster system as it runs through the synapses and short neurons within your overall nervous system. The Fight or Flight response typically causes the gastrointestinals and salivary production to slow down, contrasting the PNS, as the body transfers focus to the survival of organs that are critical, for example the lungs and the heart. In this, the heart rate increases due to rapid conversion of glycogen to glucose for muscular energy (survival response), increased adrenaline from your adrenal gland, making your muscles easier to contract, hence your breathing rate increases as well due to the bronchial tubes (tubes in the lungs) dilating.
Parasympathetic vs. Sympathetic
The main differences between these two systems are apparent, in a “fight or flight” response to a “rest and digest” response. These two systems typically counter-balances each other, especially when one has gone through a SNS phase, the PNS will come after to balance the system for regulation of the body. Understanding the two systems can be imperative for one to be aware when going through either of the systems to adapt to a situation. As there are many ways where one can induce, for example the PNS through breathwork exercises or supplementation, to balance a SNS inducing trauma and accident or an injury.