The Gut Brain
The human digestive tract has its own semi-autonomous brain, with 500 million nerve cells: as many as there are in a cat’s brain. (Alas, in their ignorance, neurobiologists don’t count astrocytes.)
The gut brain controls muscular contractions and secretions from glands and cells. (The gut brain is more properly termed gut mind-brain. Whenever there is brain activity there is a mind behind it.) The gut brain gives the head brain frequent status reports: begging when empty and strutting satisfaction with satiety.
Stomach rumbles are a communication ricochet: the gut brain complaining to the head brain of low blood sugar, which then invokes gastric grumbles.
When food enters, the stomach stretches under gut brain control. Digestion is an extremely intricate process, with the gut brain conducting the orchestra of organs involved.
The gut brain knows when there are nutrients available in the digestive tract. The gut brain feeds the body by stimulating the release of digestive outputs into the blood.
The human gut brain reflects hundreds of millions of years of evolution. The mind-brain system of earlier-evolved organisms is more gut brain than head brain, though little research has been done on the different mind-brain systems in other life forms, as it is and exceedingly difficult study.
In evolutionary time, sophistication developed in the head and gut brains along with adjustments in mental processing and digestive choice. Cooking made foods more digestible, improving nutrient absorption. The evolution of the hominin gut brain reflected this lifestyle choice in conducting its digestive affairs, and in its communications with the brain upstairs.