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The Vagus Nerve

'Trouble and bustle always finds its way into the bosom of my stomach.' James Joyce.

The vagus nerve provides sensory information to the brain from peripheral structures critical to food intake including the stomach, the intestines, and the liver.

The vagus nerve is the 10th cranial nerve and innervates the head, neck, thorax and abdomen. It is a mixed sensory and motor nerve and has 75% control of your parasympathetic system with some control over the heart and viscera. The parasympathetic part helps regulate the visceral system (digestive organs) and forms a link to your enteric (gut) nervous system.

When you inhale and exhale deeply through your nose, it activates the vagus nerve, which extends from your abdomen up to your brain stem. It, in turn, triggers the parasympathetic nervous system, which is the rest and relaxation system of your body.

Relaxation is vital to a healthy dining experience. Eating on the run or when busy at your desk – either physical or mental stress - is not conducive to healthy eating and we are more likely to eat quickly. Ideally one should take time to enjoy the aroma of food so that the Olfactory nerve stimulates the Vagus nerve.

The complexity of the vagus nerve in regulating hippocampal activity and the importance of the hippocampus in modulating eating behaviors is being extensively researched: http://www.pnas.org/cgi/content/full/103/42/15641

Major neurotransmitters found in the brain, including serotonin, dopamine, glutamate, norepinephrine and nitric oxide - occur plentifully in the gut as well. Enkephalins - described as the body’s natural opiates - also occur in the intestinal tract, as do benzodiazepines, psychoactive chemicals similar to mood-controlling drugs like Valium and Xanax.

Two Nervous Systems

Early in our embryogenesis, a collection of tissue called the ‘neural crest’ appears and divides during fetal development. One part turns into the central nervous system, the spinal cord and the brain and the other becomes the enteric nervous system composed of neurons and glial cells, organized as interconnected ganglia within the gut wall, which controls peristalsis of the gut wall and secretions from its glands.

Then the two nervous systems link through a neural cable called the "vagus nerve" the longest of all cranial nerves. When we eat, our stomach sends a signal to the brain via the vagus nerve. The ingredients of food touch the wall of the stomach and the signal goes through to the brain to say ‘eat more’ or ‘eat less’.

Controlling Appetite

The vagus nerve is involved in feelings of satiety after eating. As the gut digests fat, it releases CKK, which communicates via the vagus nerve to the brain, stimulating it to release oxytocin, which then communicates back down the vagus nerve to tell the body it's had enough to eat.

Latest research shows these gastric stimulators not only activate the ‘hunger centre of the brain, the hypothalamus, but also the seahorse - shaped parts of the brain, called the hippocampus. The hippocampus is the area related to memory and the reward system.

In fact, 95 percent of the body’s serotonin the neurotransmitter that plays a role in mood — is in the gut, and the gut uses it to send messages to the brain. Serotonin in the gut is thought to initiate peristalsis, the rhythmic movement of food through the digestive tract. Drugs like Prozac actually divert serotonin from the intestinal tract to the brain, leading to digestive problems including constipation in many patients.

Two of these chemicals whose job it is to help regulate hunger are called Leptin and Ghrelin and they communicate with the brain via the Vagus nerve. How these peptides work is a new area of research:

Ghrelin is a peptide secreted by the stomach that affects appetite by making people feel hungry. Levels of ghrelin increase before meals and then decrease after meals. Ghrelin increases hunger by acting on hypothalamic feeding centers in the brain. It also slows down the metabolism so that the body burns less fat.

Leptin has the opposite affect as ghrelin. Leptin works by decreasing appetite and increasing metabolism. Leptin binds to a portion of the hypothalamus, known as the ’satiety center’, signaling to the brain that the body has had enough to eat. Leptin is produced by fat cells and when fat cells decrease so does leptin. People who are extremely overweight have higher levels of leptin. However their leptin does not stop hunger as expected due to the development of leptin resistance, similar to insulin resistance in diabetics.

Cholecystakinin, CCK, is a peptide hormone released by the small intestine when fat or protein leaves the stomach. CCK stimulates the pancreas and gallbladder to secrete digestive enzymes and bile into the small intestine. It also tells the body to stop eating via the vagus nerve and turns off ghrelin.

Since CCK is released in response to fat, eating some healthy fats will turn off hunger. Legumes are rich in essential fatty acids but do not contain saturated fat and will cause the intestines to secrete CCK and stop hunger in about twenty minutes.

Oxytocin is a neuropeptide that helps regulate many of the body's functions, including hunger, thirst and blood pressure. Oxytocin may also be responsible for emotional eating as it activates the same neural pathways and is released in our brains when we are touched in loving relationships.