Neurons throughout the digestive tract respond to the mechanical size and chemistry of food.

“All along that tube there are neurons. Some of the neurons are responding to the mechanical size of whatever portion of the digestive tract it happens to be. So for instance, how distended, or empty, or full, rather, it doesn't have to be distended, it depends on how much you ate, but how full or empty your gut happens to be, whether or not something you just ate is temperature hot, you know, is hot in the sense of hot to the touch, or whether or not it's spicy hot, whether or not it's soothing, whether or not it's kind of hard to swallow, this kind of thing. So you have neurons all along your gut that are responding to the mechanics related to food and digestion, and that are related to the chemistry of food and digestion.”