The activity level of the vagus nerve (cranial nerve X), the primary parasympathetic nerve innervating thoracic and abdominal organs. Vagal tone is commonly assessed via heart rate variability (HRV), reflecting the balance between sympathetic and parasympathetic control of heart rate.
The vagus nerve originates from dorsal motor nucleus (DMV) and nucleus ambiguus in the medulla. It carries both afferent (80% of fibers) and efferent signals. Efferent vagal activity releases acetylcholine at target organs, slowing heart rate, enhancing digestion, and activating the cholinergic anti-inflammatory pathway. Afferent vagal fibers transmit interoceptive information from organs to nucleus tractus solitarius (NTS). High vagal tone correlates with high HRV (beat-to-beat variability), indicating flexible autonomic regulation and parasympathetic dominance.
Low vagal tone (low HRV) predicts cardiovascular disease, inflammation, poor stress resilience, and mortality. In cPNI, vagal tone assessment via HRV guides autonomic interventions. Low vagal tone reflects chronic stress, poor sleep, inflammation, and metabolic dysfunction. Interventions to increase vagal tone include slow breathing (5-6 breaths/min), cold exposure, meditation, singing, and addressing underlying inflammation.