A frog's heart revealed that the vagus nerve talks in chemicals
Otto Loewi stimulated the vagus nerve of a frog's heart and watched it slow. He then moved fluid from that heart onto a second one with no nerve attached, and it slowed too. That simple experiment proved nerves release chemical messengers, and the vagus, the body's great wanderer, became famous for it.
The vagus is the tenth cranial nerve and the longest in the autonomic nervous system. It leaves the brainstem at the medulla, exits the skull through the jugular foramen and runs down inside the carotid sheath beside the carotid artery and jugular vein, reaching the heart, lungs and gut as far as the colon. There are two, a right and a left, and fibre counts vary sharply between sources, from about 105,000 in the right nerve to as few as 25,000. Its old name, the pneumogastric nerve, reflected its hold on lungs and stomach, though its control of the heart is just as central.
Most of its traffic runs upwards: between 80 and 90 percent of its fibres carry reports about the state of the organs back to the brain. Going the other way, it drives the rest-and-digest side of the nervous system, slowing the heartbeat, moving food through the intestines, and working muscles of the throat used in swallowing and speech. It triggers the gag reflex, helps signal fullness after a meal, and calms inflammation through what is called the inflammatory reflex. A branch even supplies part of the ear canal.
Loewi called the substance vagusstoff; it turned out to be acetylcholine. Drugs such as atropine block its effects and speed the heart. When the nerve overreacts during strong emotion, a sudden drop in blood flow to the brain can cause fainting, known as vasovagal syncope, which affects young children and women more than others.
Doctors exploit the nerve in several ways. Implanted stimulators have been used to control epileptic seizures and are approved for drug-resistant depression, and manoeuvres such as the Valsalva can halt certain fast heart rhythms. Cutting the nerve, once a treatment for peptic ulcers, is now obsolete for that purpose. Researchers at Aarhus found it roughly halved the risk of Parkinson's disease, hinting that the illness may begin in the gut and travel up the vagus to the brain.
Source: Vagus nerve
