Hunger is a complex intrinsic internal bodily response associated with the nervous system in response to an absence of food and a necessity for metabolic precursors to sustain metabolism and subsequent life.
The stomach is connected to the brain by the vagus and splenic nerves. When it is too full and is overly stretched, it sends the brain signals that one has eaten too much and is sated. When glucose, a byproduct of sufficient metabolism, is detected in the duodenum, axons alert the hypothalamus that one is sated and should no longer eat.
The primary controller of hunger in the human brain is the hypothalamus, controller of all visceral functions. There are three primary parts of the hypothalamus involved with hunger: the ventromedial, paraventricular, and ventricular regions. If the ventromedial or paraventricular areas of the hypothalamus are damaged, persons will eat many small meals and gain weight, suggesting that activation of the ventromedial or paraventricular areas leads to increased hunger. If the medial area of the hypothalamus is damaged, the stomach clears fluids quickly and therefore have the stomach relatively tenser and more hungry, suggesting that the medial area's stimulation leads to increased hunger.