<P> The vagus nerve also has a sympathetic function via the peripheral chemoreceptors . Peripheral chemoreceptors are activated primarily due to hypoxemia . When stimulated, these chemoreceptors relay impulses throughout the vagus nerves in order to enable a vasoconstrictor response and increase blood pressure . The vagus nerve is the longest nerve in the human body . </P> <P> Parasympathetic innervation of the heart is partially controlled by the vagus nerve and is shared by the thoracic ganglia . Vagal and spinal ganglionic nerves mediate the lowering of the heart rate . The right vagus branch innervates the sinoatrial node . In healthy people, parasympathetic tone from these sources are well - matched to sympathetic tone . Hyperstimulation of parasympathetic influence promotes bradyarrhythmias . When hyperstimulated, the left vagal branch predisposes the heart to conduction block at the atrioventricular node . </P> <P> At this location, neuroscientist Otto Loewi first demonstrated that nerves secrete substances called neurotransmitters, which have effects on receptors in target tissues . In his experiment, Loewi electrically stimulated the vagus nerve of a frog heart, which slowed the heart . Then he took the fluid from the heart and transferred it to a second frog heart without a vagus nerve . The second heart slowed down without an electrical stimulation . Loewi described the substance released by the vagus nerve as vagusstoff, which was later found to be acetylcholine . Drugs that inhibit the muscarinic receptors (anticholinergics) such as atropine and scopolamine, are called vagolytic because they inhibit the action of the vagus nerve on the heart, gastrointestinal tract, and other organs . Anticholinergic drugs increase heart rate and are used to treat bradycardia . </P> <P> Excessive activation of the vagal nerve during emotional stress, which is a parasympathetic overcompensation of a strong sympathetic nervous system response associated with stress, can also cause vasovagal syncope due to a sudden drop in cardiac output, causing cerebral hypoperfusion . Vasovagal syncope affects young children and women more than other groups . It can also lead to temporary loss of bladder control under moments of extreme fear . </P>

Where is the vagus nerve and what does it do