Autonomic nervous system

Appendix A endnote 6, from How Emotions are Made: The Secret Life of the Brain by Lisa Feldman Barrett.
Some context is:

There are three branches of the autonomic nervous system. The sympathetic nervous system, sometimes called the “fight or flight” system, tells the body to spend its energy resources. It sends information to the sweat glands in your skin, to the smooth muscles that surround your blood vessels, to your internal body organs, to the muscles that dilate your pupils, to the parts of the body that generate your immune cells, and so on. The parasympathetic nervous system, also known as the “rest and digest” system, tells the body to replenish its energy resources. It tells your pupillary muscles to contract, your body to secrete saliva and insulin, and other functions related to digesting food, in part by communicating with the third branch, called the enteric nervous system.

The sympathetic and parasympathetic branches of the autonomic nervous system do not work like a simple on-off switch. Some body organs, like the heart, receive input from both branches. When your heart races, it’s due to some combination of a sympathetic increase and/or a parasympathetic decrease in activation. Other physiological changes, such as orgasm, require the sympathetic and the parasympathetic nervous systems to activate together. In young children, for example, changes in heart rate are more due to parasympathetic withdrawal until around the age of eight, when the sympathetic branch finally kicks in. Some physiological changes are primarily a function of the sympathetic branch alone (perspiration, measured as skin conductance) whereas others are more a function of the parasympathetic branch alone (e.g., breathing, measured as respiratory sinus arrhythmia, which is the naturally occurring ups and downs in heart rate that occur due to inhaling and exhaling).

We don’t really know that much about the enteric nervous system, which carries information between the stomach/intestines and the brain, because it is very difficult to study. Most test subjects are not particularly enthusiastic to be “probed” for science.