the vagus nerve and vagal tone

see also:

• Anatomy of the autonomic nervous system
• parasympathetics
• autonomic dysfunction and autonomic neuropathy

• the vagus nerve is one of the main parasympathetic nerves

• vagus nerve itself (cranial nerve X):
  • leaves the medulla oblongata between the pyramid and the inferior cerebellar peduncle
  • passes through the jugular foramen, then into the carotid sheath between the internal carotid artery and the internal jugular vein down to the neck
  • branches in neck:
    • auricular nerve
    • pharyngeal nerve
    • superior laryngeal nerve
    • superior cervical cardiac branches of vagus nerve (right side only, as left arise from rec. laryngeal N)
    • inferior cervical cardiac branch
  • right vagus nerve:
    • gives rise to the right recurrent laryngeal nerve, which hooks around the right subclavian artery and ascends into the neck between the trachea and oesophagus
    • crosses anterior to the right subclavian artery, runs posterior to the superior vena cava, descends posterior to the right main bronchus
    • branches to cardiac (innervates the sinoatrial node and thus may cause sinus bradycardia), pulmonary, and oesophageal plexuses
    • forms the posterior vagal trunk at the lower part of the oesophagus and enters the diaphragm through the oesophageal hiatus
  • left vagus nerve:
    • enters the thorax between left common carotid artery and left subclavian artery and descends on the aortic arch
    • gives rise to the left recurrent laryngeal nerve, which hooks around the aortic arch to the left of the ligamentum arteriosum and ascends between the trachea and oesophagus
    • gives off thoracic cardiac branches (innervates A-V node and may cause conduction block), breaks up into pulmonary plexus, continues into the oesophageal plexus
    • enters the abdomen as the anterior vagal trunk in the oesophageal hiatus of the diaphragm
• efferent fibres:
  • pre-ganglionic neuronal nuclei for the nerve are in 2 areas of the medulla:
    • dorsal motor nucleus of the vagus - mainly GIT
    • nucleus ambiguus - mainly cardiac
  • efferent fibres terminate at terminal ganglia in end organs such as heart, lungs, gut as far as 2nd segment of transverse colon and the neurotransmitter is acetylcholine acting on muscarinic receptors
  • controls some skeletal muscles including:
    • cricothyroid muscle
    • levator veli palatini muscle
    • aslpingopharyngeus muscle
    • palatoglossus muscle
    • palatopharyngeus muscle
    • superior, middle and inferior pharyngeal constrictors
    • muscles of the larynx (speech)
  • other actions:
    • sinus bradycardia
    • slowing of AV-nodal conduction and potential for AV block
    • induction of AVNRT with a premature atrial complex in those with dual AV nodal pathways
    • may trigger an episode of lone atrial fibrillation - onset during sleep or after meals (large meal causing gastric distension, cold bolus, spicy food)
    • vaso-vagal syncope / dizziness
    • bronchial secretion
    • bronchospasm
    • increased gastric acid and intrinsic factor secretion
    • increased hunger
    • decreased hepatic gluconeogenesis following insulin actions on the arcuate nucleus
    • increased GIT motility

• afferent fibres:
  • arise from cell bodies in the inferior ganglia of the vagus nerve
  • account for 80-90% of nerve fibres in the vagus nerve
  • pass to the following medullary nuclei:
    • solitary nucleus - taste, visceral organ afferents
    • spinal trigeminal nucleus - deep/crude touch, pain, and temperature of the outer ear, the dura of the posterior cranial fossa and the mucosa of the larynx
  • fibres apparently supply as distal as the uterus as they apparently mediate orgasms in spinal cord injured women

• improve seizure control
• reduce tonal tinnitus
• reduce depression refractory to drugs
• appears to reduce chronic inflammation in rheumatoid arthritis (perhaps via actions at the spleen to suppress TNF and other factors)
• via Valsalva manouvre:
  • revert supraventricular tachycardias (SVT)

• the absolute level of cardiac vagal activity or “vagal tone” appears to result from the excitatory drive from peripheral baroreceptors
• vagal tone can be measured by assessing the heart beat variability with respiration as high levels of vagal tone result in respiratory sinus arrhythmia (RSA) as is detected by changes in the R-R interval on an ECG with respiration
  • RSA is pronounced in children but decreases with age unless one adopts endurance sports
• chronic low vagal tone is linked to:
  • impaired ability to cope with stressful events
  • negative moods
  • loneliness
  • cardiovascular morbidity and mortality
  • increased chronic inflammation
  • higher cortisol and TNF levels
• vagal tone can be increased by:
  • endurance sports
  • meditation, yoga
  • deep diaphragmatic breathing with long, slow exhalation
  • Vagal Nerve Stimulator (VNS)