The parasympathetic nervous system, the sympathetic nervous system, and the enteric nervous system comprise the autonomic nervous system in the body. The autonomic nervous system is continuously active and is responsible for unconscious regulation of our glands and organs. The parasympathetic nervous system takes care of “rest and repair” activities, such as salivation, tears, sexual arousal, urination, digestion and defecation. These activities are complementary to those of the sympathetic nervous system, which activates processes associated with the “fight or flight” response.
The natural opposition of the sympathetic and parasympathetic nervous systems provides both quick responses when needed, and slower responses for less urgent needs. These systems act something like an accelerator and a brake for our body, and help maintain balance, or “homeostasis”. The parasympathetic system is like a brake and helps you save energy, lower blood pressure, decrease heart rate, and it allows digestion to start. It helps return the body to a state of balance after a period of higher sympathetic nervous system activity. Unlike the sympathetic nervous system, these responses are carried out as needed, rather than all at once.
Chiropractic adjustments have been known to affect the autonomic nervous system by promoting parasympathetic system activity and down regulating the sympathetic nervous system. For example, pain receptors are part of the sympathetic nervous system. A chiropractic adjustment may be able reduce the firing of the pain receptor by decreasing input to the spinal cord, thus reducing the sensation of pain.
The parasympathetic nerves originate in spinal cord and in the medulla, part of the brain. The main parasympathetic nerve is the vagus nerve, which is also known as cranial nerve X. The vagus nerve sends information between the brain stem, hypothalamus, and primary organs and glands.
The main functions of the parasympathetic nerves are to conserve energy, so it promotes passive activities such as dissociation, withdrawal, disengagement, and the immobility response. Physical and emotional withdrawal, as well as the negative emotions of shame, hopelessness, disgust, and despair, is also associated with the parasympathetic system. Interestingly, these functions develop later in life than those of the sympathetic nervous system. This is because the inhibitory responses of the parasympathetic system are not conducive to exploring, which is necessary for young mammals in order to be able to grow and learn effectively.