What is the vagus nerve and why is it so important?

The answer

The vagus nerve (cranial nerve X) is the longest and most complex cranial nerve, running from the brainstem to the abdomen and innervating the heart, lungs, gut, liver, spleen, and other organs. It serves as the main nerve of the parasympathetic 'rest and digest' system, slowing heart rate, promoting digestion, and reducing inflammation. The vagus nerve carries about 80% of its signals from the body to the brain (afferent), making it a crucial channel through which the body's state influences mental function. Vagal tone, measured by heart rate variability, is associated with emotional regulation, social engagement, and stress resilience. Vagus nerve stimulation (VNS) is an FDA-approved treatment for drug-resistant epilepsy and depression, and is being investigated for inflammatory conditions, PTSD, and obesity.