As you’ll recall from chapter 5, the autonomic nervous system is our brain’s most elementary survival system, its two branches regulating arousal throughout the body. Roughly speaking, the sympathetic nervous system (SNS) uses chemicals like adrenaline to fuel the body and brain to take action, while the parasympathetic nervous system (PNS) uses acetylcholine to help regulate basic body functions like digestion, wound healing, and sleep and dream cycles.
