vagus nerves

vagus nerves ‘Vagus’ means ‘wanderer’ — and that is indeed what these nerves are. Attached to the brain stem, and emerging through the base of the skull into the neck, the right and left vagus nerves innervate through their branches a widespread range of body parts, from the head down to the abdominal organs.

These nerves contain fibres that are both incoming to the central nervous system (the majority) and outgoing from it. Sensory information comes from the external ear and its canal, and from the back of the throat (pharynx) and upper part of the larynx. Longer fibres travel in the branches of the vagi from the organs in the chest and in the abdomen: from the lungs and the heart, and from the alimentary tract, including the oesophagus and right down to half way along the colon. The incoming signals lead to many reflex responses, mediated at cell stations in the brain stem, and entailing either autonomic or somatic motor responses. For example: irritants in the airways stimulate vagal sensory nerve endings and lead to a cough reflex; information on the state of inflation of the lungs causes modification of the breathing pattern; distension of the stomach leads to reflex relaxation of its wall.

The outgoing, motor fibres in the vagus nerves represent most of the cranial component of the parasympathetic division of the autonomic nervous system. Vagal stimulation slows the heart beat, and excessive stimulation can stop it entirely. When Otto Loewi first showed, in 1921, that stimulation of the vagus nerve to a frog heart caused something to be released that could slow down another heart that was linked to the first only by fluid perfusion, he called the unknown factor Vagusstoff. We know now that vagal nerve endings act on the heart's pacemaker by the release of the transmitter acetylcholine; this modulation of the heart rate is continuous, counterbalancing the action of the sympathetic nerves at the same site. The vagus nerves also provide a pathway for reflex reduction of the cardiac output if the blood pressure tends to rise. In the lungs, they stimulate the smooth muscle in the wall of the bronchial tree, tending to increase the resistance to airflow (by causing bronchoconstriction), again counterbalancing the sympathetic effect which tends towards relaxation. In the alimentary tract they stimulate smooth muscle in the walls of the stomach and of the intestines, acting through the nerve networks between the layers of smooth muscle, but they have the opposite action on the smooth muscle sphincter that tends to prevent the stomach contents from moving on. They stimulate glandular secretions of stomach acid and of the digestive enzymes that are released into the stomach and intestine, and the ejection of bile from the gall bladder. They also influence the release from the pancreas of the hormones that promote the storage of absorbed nutrients. All these effects add up to support of activity in the alimentary system during and after eating, when the parasympathetic effects predominate over the opposite quietening effects of the sympathetic nerve supply.

The term ‘vaso-vagal’ attack refers to fainting, when — from a variety of causes ranging from emotional shock to the pain of injury — there is a strong parasympathetic outflow in the vagus nerves, causing slowing of the heart that leads to a fall in blood pressure sufficient to cause unconsciousness.

Sheila Jennett

See also alimentary system; autonomic nervous system; cranial nerves; visceral sensation.