The neural circuit in the human brain that drives all pleasure and is responsible for what neuroscientists call "Primary Reinforcement". It runs between the frontal cortex and the basal ganglia, driven by the neurotransmitters Dopamine and Seratonin.
I'm not a neuroscientist, or behavioral psychologist - but this is my understanding of how it works. Our subject is hungry. He seeks out food and eats. The reward path fires - his brain floods with Dopamine and Seratonin, he feels really really good. If it's been a long time since he's eaten, the RP fires for a long time. Same as taking a shit, or having sex - the reward path gets activated. All of these things:eating, sleeping, shitting, sex, are fundamental activities and are primary triggers of the RP - thus they are Primary Reinforcers - a term from the behavioral science of operant conditioning.
There are secondary reinforcers as well, and this is how your brain runs the show. The anticipation of security and pleasure that comes with winning a million dollars can trigger the RP. Drugs work almost exclusively off or artificially tripping the reward trigger. Tobacco, nicotine crosses the blood-brain barrier and directly triggers dopamine release. This is why it's such a bitch to quit. Stopping smoking is like stopping eating. The addiction hijacks the bodies natural guidance system. Eating and sexual disorders are the same thing - in Anorexia, the system somehow gets flipped, and the subject finds themselves being rewarded for NOT eating.
It's one of the oldest parts of the brain, evolutionarily speaking. Amazing that our big fancy rational minds are so easily enslaved by the lizard brain wrapped around the top of our spinal cord