An Introduction to the Nervous System
https://www.khanacademy.org/science/health-and-medicine/human-anatomy-and-physiology#nervous-system-introduction
The Difference Between Epinephrine and Acegylcholine
https://forums.studentdoctor.net/threads/epinephrine-and-acetylcholine.405197/
Why Scientists Are Battling Over Pleasure
https://www.nytimes.com/2018/04/10/science/pleasure-art-sex-food-drugs.html
More on the Anatomy of the Circulatory System
https://www.khanacademy.org/science/health-and-medicine/human-anatomy-and-physiology#heart-introduction