A complex network of neurotransmitters and receptors that work together in signaling and transmitting information throughout the body, modulating essential neuroprotective functions and helping the maintenance of homeostasis. The fact that endocannabinoids are released by post-synaptic neurons and act as retrograde synaptic signals, which regulate neurotransmission was first published by Wilson and Niccol in 2001.  THC, CBD, CBN, and the other phytocannabinoids act through the endocannabinoid system because of the existence of the CB1 and CB2 receptors, and other structures in the ECS.

Photo Credit: https://medium.com/@bloomcityclub/do-you-know-facts-about-your-endocannabinoid-system-f3a74fb1124f 

Endocannabinoid System (ECS)