The HumanNervous System

Transport yourself to a remarkable meal, in your memory or imagination. Imagine music playing. Friends, family, and strangers who share your love of this food are seated around you. The air is crisp, and aromas waft in from the kitchen. Imagine the food placed in front of you. The exact food you wanted. Take a deep breath as all your senses converge on this experience.

Before you eat, atomized molecules of the food’s ingredients waft through the air and reach your nose, lighting up a chain of neural activity. The Olfactory Nerves (designated as Cranial Nerve I) dangle receptors into your nose and report the presence of various odorant molecules directly into your brain. All the other cranial nerves synapse in the thalamus or brainstem. Our sense of smell is ancient and is wired directly into our cerebral hemispheres, intimately connected to our memory. The stream of neural information carrying olfactory information splits as it goes into the brain. One part of this stream goes into the Primary Olfactory Cortex. The other part of the stream goes into the amygdala and then to the hippocampus and the circuit of Papez (our memory/emotion circuit). We discussed these structures in the last post on sex and violence.

The Primary Olfactory Cortex, the part of the brain dedicated to smell, is on the inferior surfaces of the frontal lobes. It receives input from the Olfactory Nerve and neurons fire in anticipation of flavor, texture, warmth, and maybe even comfort. The food, the company, the ambience, and your memory all contribute to the expereince of the food. Autonomic cranial nerves activate saliva glands to wet the mouth and start the early stages of digestion. Now it is time to put the food in your mouth.

Taste buds on the tongue detect salty, sweet, sour, savory, and bitter substances that, along with fat and spicy, are some of the ingredients of the taste constellations of our food. The olfactory information blends with and sharpens the taste information, and these two systems, smell and taste, combine in the brain to produce the perception of flavors. These flavors do not exist in the foods. They are a product of an interaction of the food’s tastes and our brain. As we’ve discussed before, the brain doesn’t just process information, it creates information. Flavor is an emergent property of our nervous system. It is a part of our phenomenology, our subjective experience. We are conscious of this flavor, and we can focus our awareness to detect more subtle aspects of tastes and flavor in the food.

Flavor, this flow of neural information influencing our enjoyment and memory is not just a product of smell and taste. Our experience of flavor can be altered by visual, auditory, and tactical information. This has been verified experimentally and can be found in scientific literature, and in your everyday experiences. Food that is the wrong color hits differently. The flavor is off. Eating seafood by the coast is a different experience than eating inland. Italian food is better on a rustic, vine covered patio, with soft Italian music playing, and the smell of garlic and fresh baked bread infusing the air.

Thinking about food can make you hungry. What is Hunger? It’s four o’clock in the afternoon and you missed lunch, now you’re hungry. Not only are you starving, but you might also desire a specific food. You need food, but you desire a certain food. You want pizza. You’re hungry for pizza. You might know a good place close by, they are quick, and it’s cheap as well. Several factors play into your decisions to eat, including location, a food choice, time, and costs. How do you know you are hungry? How does your body tell you?

We’ve talked about some cranial nerves. Cranial nerves are pairs of nerves, one on each side of the body, and they are numbered with Roman numerals I – XII. The Vagus Nerves (cranial nerve X), come off the medulla of the brainstem. The medulla oblongata. It is a huge nerve that innervates most of the body’s organs and digestive tract. The Vagus nerves wind down the esophagus and branches to the vocal cords, lungs, and heart. This pair of nerves carries parasympathetic innervation (rest and digest) to most of the body, from the head to almost two-thirds of the gastrointestinal tract. Branches of the Vagus nerves line the stomach, small intestines, and most of the large intestine. The Vagus nerves control resting heart rate, can lower the heart rate, and much of the processes involved in digestion. They let the brainstem and brain know when food is in the stomach and increase peristalsis, the waves of contractions that move food through the digestive system.

The hypothalamus monitors the parasympathetic nerves, and initiates a drive, like seeking food, or shutting down hunger to stop the drive. The hypothalamus directly controls the pituitary glands causing the release of hormones that affect our hunger and satiety (feeling full and satisfied). Weight loss drugs can mimic these hormones and convince the body that it is full. Damage to some of these pools of neurons in the hypothalamus can result in anorexia or obsessive overeating.

From the olfactory input into the amygdala and the hippocampus, to the input from the other senses in the production of flavor, to the parasympathetic and hypothalamic influences, we can see that eating behavior and experience involves most of the human nervous system. Like sex and violence, eating behaviors are not just reflexive, they involve emotion, memory, expectations, and attention. To understand these behaviors, we need to look at how they developed over time, deep time.

The cognitive neuroscience of eating is the story of more than a million years of the evolution of the human nervous system. Like other living things, our survival depends on the continual attainment and consumption of food. The food preferences of each species are shaped over millions of years of behavior and changes in their environment. Animals spend time and energy seeking, detecting, and consuming a specific range of environmental items. In the same way, each animal’s nervous system samples a selective subset of environmental energy. This tuning of the nervous system is directly related to eating. Every living thing on the planet is eaten. Some plants and animals have evolved traits making them inedible to some species, but even those animals are eaten, after death, by the trillion microbes around us. All species get their slice of the pie.

The cranial nerves that are intimately involved in our eating behavior function like those of other vertebrates but can differ in importance of function. I’m going to use the term “animal” to refer to vertebrates, as only vertebrates have cranial nerves. It’s easier than typing and reading “vertebrates”. I mean no offence to invertebrates such as worms, insects, spiders, and crabs. I’m not advocating speciesism. I’m just being lazy.

Different species of animals utilize their senses in different ways. Some animals rely more on olfaction, and others on vision, for example. The cortical areas devoted to those senses differ among these animals. For example, our occipital lobes, and the amount of neural circuitry in them devoted to vision, are larger than in rats, for example. The number of pairs of cranial nerves, the specific anatomic routes, and exact functions of the nerves vary amongst animals, but their general purposes of sensory, motor, and visceral functions are the same. We can see that animals’ nervous systems are adapted to their niche of the environment. Their eating behaviors vary from foraging to predation, and combinations of these.

Let’s consider the cognitive neuroscience of our eating behaviors, and especially the nuances such as flavor and enjoyment. Flavor is a quality that is processed downstream from taste and smell and emerges (is produced) as an experience from the integration of touch, smell, taste, vision, hearing, emotion, memory, and current physiological state. The experience, the phenomenology of enjoyment, involves the dopamine circuit for reward, the circuit of Papez, and several small pools of neurons where all these pathways cross called the septal nuclei. These deep brain nuclei connect emotion (the cingulate cortex) and memories and feelings of social reward. Why is there so much neural circuitry involved in what could be a reflexive behavior?

One to three million years ago, Homo Erectus was a widespread, social creature that learned to control fire. Archaeological evidence suggests they huddled around the fire, sharing cooked communal food. Homo erectus competed with other hominid species, and survived long after the others became extinct. One reason is likely the use of fire, and the cooking of meats and vegetables. We love to sit by the fire, poke it, cook on it, warm ourselves, and huddle and cuddle together.

The consumption of cooked food slowly shaped our anatomy. The modern human face is flatter, with a less pronounced snout than our ancestors of four million years ago (and of other mammals). The skull has changed dramatically from homo habilis, about two million years ago, to homo erectus, a million years ago, and neanderthals one-half million years ago. The middle of the face, the maxilla, is larger and more prominent in modern skulls. The back, the occipital bone, is much less pronounced, giving us a more compact and symmetrical skull, rounder than long. Jaw musculature changed as well, as less power is needed to consume cooked good. The routes of the cranial nerves compensated for these gross changes, and the functions of these nerves began to change as well.

The human mouth, compared to the rest of the face, is much smaller than a Neanderthal’s, or a chimp’s, or a monkey’s mouth. Cooked food provides more calories per weight than raw food, and less food is necessary to sustain energy levels, meaning less time is needed to find and consume food. That found time, for humans, is social time. This additional social time can be comforting and can cause stress, forever tangling our eating behaviors with the neurobiology of anxiety. Our eating behaviors are profoundly motivated by social cues.

The cranial nerves of all animals move muscles that produce signals that are understood by other animals. Flashing of teeth, positioning of the ears, and raising the hair or fur, are all meaningful social signals produced by the anatomy and function of the cranial nerves. Without words, we can speak volumes with our tiny mouths, puckering our lips or flashing that debonair smile. These are social signals, developing alongside our eating habits. The Facial nerve (cranial nerve VII) is a motor and a sensory nerve. It moves the muscles of facial expressions, for example smiling, frowning, and winking, that express specific social emotions. It also sends taste bud information from the front two-thirds of the tongue to the brainstem, then the thalamus, then to the insular cortex of the brain. The insular lobe lies deep inside the longitudinal fissure separating the temporal lobes from the frontal and parietal lobes above. It processes visceral information from the organs and guts.

The anterior part of the insular cortex combines input from taste with somatosensory input from texture and temperature receptors. Olfactory and visual input are added, and this newly created information is sent to the emotional cortex and then around the circuit of Papez for memory. Our phenomenology, our subjective experience of enjoyment is created by this neural chain, providing further reward for the experience.

Our eating behaviors are an example of appetitive behaviors. These behaviors lead to the fulfillment of a need or a desire. We need food to survive. We desire certain foods for pleasure. There is an added neural component, a cognitive component to our desires. Sex is an appetitive behavior. Exploration, exercise, and addictions are other examples. These appetites vary from individual to individual, and from situation to situation. These appetites share the same neural mechanisms, and their social expressions define aspects of our personalities, and of our consciousness. Again, we see that the ghost is not in the machine. The ghost is a product of the machine.