What is the vagus nerve?
Out of all 12 nerves which arise from the brain, the longest is the 10th also known as the vagus nerve. It is the most complex nerve of the body’s autonomic nervous system, which is the system controlling non voluntary functions such as heart rate and digestion. These are actions you do not have to consciously think about and are a gift from evolution.
In the early days, as soon as single cells started to develop into multicellular organisms having an autonomic system became crucial in order to control various organs in the body simultaneously. In essence, backstage, your body is constantly multitasking in a heroic effort to maintain internal balance.
What is the function of the vagus nerve?
The vagus nerve is partially responsible for the control of the heart, lungs and the digestive tract. It is the main transporter of information from the gut to the central nervous system, also known as the gut-brain axis: from the gut the nerve projects to the brain, specifically the caudal brainstem.
Caudal brainstem- from latin cauda meaning tail, the brainstem is a structure which connects your brain to your spinal cord, the structure running down your back and neck.
The vagus nerve and tackling obesity
In terms of controlling food intake, vagal afferents transmit satiety signals from your gut to the brain. Removal of thia nerve is currently being researched as a less invasive alternative weight loss procedure to gastric bypass surgery. 
Afferent nerves-carry impulses from organs towards the central nervous system
Efferent nerves- carry impulses from the central nervous system to organs
Gastric bypass surgery- a surgical procedure where staples are used to create a pouch at the top of the stomach. The plumbing of your digestive tract is changed by connecting the small pouch to the small intestine meaning less food is required to achieve satiety.
During this surgery parts of the vagus nerve, such as the ventral and dorsal aspect, are removed whilst other parts remain (such as gastroduodenal branch). Therefore innervation to certain parts of the GI tract such as the pancreas are still provided.
Why is partial/ full removal of the vagus nerve associated with weight loss?
Studies have shown that when the vagus nerve is removed ghrelin levels decrease because of the loss of connection of the autonomic nervous system to ghrelin cells in the stomach.
Ghrelin- the hunger hormone stimulating an increase in appetite and food intake.
Therefore the intensity of messages from the stomach to the brain conveying information about hunger is decreased.
How can gastric bypass surgery and vagus nerve removal have beneficial effects on balancing out glucose levels?
After parts of the vagus nerve being removed in gastric bypass surgery, the secretion of hormones PYY and GLP1 is increased because of dumping of nutrients. This means fast movement of food down the digestive tract decreasing time for the absorption of nutrients. Both hormones affect blood glucose levels.
PYY- Peptide YY is an anorexigenic hormone, meaning it suppresses appetite
GLP1- glucagon like peptide 1 is a hormone promoting insulin secretion so encouraging the uptake and storage of glucose by muscle and liver cells.
All in all the vagus nerve is a key structure in communication between the gut and brain and future studies should be conducted into how altering its anatomy can help sufferers of metabolic diseases.
 Science Direct. (2012). Chapter 141 – Evolution of the Cardiovascular Autonomic Nervous System in Vertebrates. [online] Available at: https://www.sciencedirect.com/science/article/pii/B9780123865250001414 [Accessed 19 Jul. 2019].
 NHS. (2017). Weight loss surgery. [online] Available at: https://www.nhs.uk/conditions/weight-loss-surgery/types/ [Accessed 19 Jul. 2019].
 PMC. (2009). The vagus nerve, food intake and obesity. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2597723/ [Accessed 19 Jul. 2019].