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In most cases, vagal tone is not measured directly. Instead the processes affected by the vagus nerve – specifically heart rate and heart rate variability – are measured and used as a surrogate for vagal tone. Increased vagal tone (and thus vagal action) is generally associated with a lower heart rate and increased heart rate variability.
Vagusstoff (literally translated from German as "Vagus Substance") refers to the substance released by stimulation of the vagus nerve which causes a reduction in the heart rate. Discovered in 1921 by physiologist Otto Loewi , vagusstoff was the first confirmation of chemical synaptic transmission and the first neurotransmitter ever discovered.
The vagus nerve plays a crucial role in heart rate regulation by modulating the response of sinoatrial node; vagal tone can be quantified by investigating heart rate modulation induced by vagal tone changes. As a general consideration, increased vagal tone (and thus vagal action) is associated with a diminished and more variable heart rate.
Vagus nerve stimulation (VNS) is a medical treatment that involves delivering electrical impulses to the vagus nerve. It is used as an add-on treatment for certain types of intractable epilepsy , cluster headaches, treatment-resistant depression and stroke rehabilitation.
The vagus nerve is also responsible for regulating inflammation in the body, via the inflammatory reflex. [7] Efferent vagus nerve fibers innervating the pharynx and back of the throat are responsible for the gag reflex. In addition, 5-HT 3 receptor-mediated afferent vagus stimulation in the gut due to gastroenteritis is a cause of vomiting. [8]
Methyldopa acts on the vasomotor center, leading to selective stimulation of α 2-adrenergic receptor. [8] Guanfacine also causes the same stimulation. [9] This reduces sympathetic tone to vascular smooth muscle. [9] This reduces heart rate and vascular resistance. [9] Digoxin increases vagal tone from the vasomotor centre, which decreases ...
A vagal maneuver is an action used to stimulate the parasympathetic nervous system by activating the vagus nerve. The vagus nerve is the longest nerve of the autonomic nervous system and helps regulate many critical aspects of human physiology, including heart rate, blood pressure, sweating, and digestion through the release of acetylcholine .
The reflex was originally demonstrated by Bainbridge in 1915 who observed an increase in heart rate following infusion of blood or saline into the jugular vein of anaesthetized dogs. [6] The response was reduced by cutting the cardiac sympathetic nerves and abolished by cutting the vagus nerve and he therefore concluded that it was a neural reflex.