You feel parched so you look for a drink. Your stomach grumbles so you raid the fridge. You're drowsy so you snooze. You feel randy and you start to pay close attention to the contours of the bodies of attractive passers-by. It's probably no mystery to you that your body has signals for thirst, hunger, fatigue, and lust, but did you ever stop to wonder how your brain knows what your body needs?
This is not a mere matter of idle curiosity. Problems in the interpretation and satisfaction of our appetites are a matter of huge public health importance and are a source of distress for people suffering from mental illness. In the next few entries we'll focus on the specific appetites for sleep, food, sex, and other bodily functions, but first a little background about the mechanisms of appetite in general.
A few brain structures sit at the nexus of body and brain. The cell clusters or nuclei of the hypothalamus receive raw data about the state of the body's basic functions and translate this information into states of readiness for action and the shape of the action itself. The hypothalamus receives information in three major forms, which derive from hormones, nervous system input, and the chemical properties of the blood.
These various sources of information represent the state of the body in its particulars. Having an empty stomach is only part of what it is to be hungry. When blood sugar is low, your gut produces hormones that trigger the hypothalamus to release its own set of signals to other parts of the brain. As you eat, and raise the sugar and fat content of your blood, different gut hormones tell the hypothalamus that you've had enough; your appetite has been sated. When your bladder is full, the stretching of the walls triggers nervous impulses to the brain that dissipate as you urinate. Signals of low blood pressure or high saltiness in the blood register in the hypothalamus as the physical origins of thirst. When the pressure rises and the chemistry is corrected, there are no more signals.
So the hypothalamus gets information not only about what we need, but about when we've satisfied that need. Via deep brain structures like the amygdala, these sequences of appetite and satiety form the basis of motivated behavior. Once a sequence of appetite and satiety signals has become linked to a sequence of actions encoded in the brain, the reemergence of the same appetite later engages the same set of action codes. These signals also selectively activate the networks in the cortex encoded with the sights, sounds, tastes, feelings, and smells we have experienced at the same time we have cycled through these appetite and satiety signals in the past. Thus, the appetite guides us to what we need.
The signals constituting appetite arouse a specific set of activities and selective attention to what we hunger for, but the longer we go without satisfaction, the more general becomes the state of arousal. Thus when you're really hungry for something you don't just wait for it to come to you, get find the drive to get up and go look for it.
Sensory and autonomic feedback thus plays a major role in the regulation of drive. If we eat until our bellies are full and sleep only until we've had our 40 winks and love only when the time is ripe, we will have, in the ideal, consumed the optimal amount to correct the physiologic need. So then why is there so much gluttony and sloth and "sexual addiction" in the world? Same goes for our acquired appetites for the chemicals that excite us when we're sluggish and relax us when we're jittery.
The reason there is so much gluttony and sloth and excessive lust and addiction in the world is because we as a civilization have devoted our best minds and efforts to the project. Over the past few centuries we have engineered many of the substances we ingest for food or for pharmacologic properties, such as sugar, fat, tobacco, alcohol, opium, cocaine, cannabis, and coffee, to deliver larger effects in ever-smaller volumes.
Humans in the time before refined sugars and deep fat frying, for example, could rely to a greater degree than at present on a full belly to signal adequate intake of calories, because there wasn't any Haagen Dazs to pack nearly a day's worth of calories into a pint-sized container. Now, by the time your belly is stretched, you may have taken in many times the amount of calories needed for sustenance. Rampant obesity results. By the time a modern person is aware of the subjective, autonomically mediated effects from a bacon cheeseburger, a double espresso, a martini, a cigarette, crack cocaine, or heroin, enough may have been taken in to do damage or fuel addiction.