Vagal tone
（重定向自Sinus arrhythmia）
Vagal tone is an internal biological process referring to the activity of the vagus nerve, the tenth cranial nerve located in the medulla oblongata of the brainstem. The vagus nerve serves as the key component of the parasympathetic branch of the autonomic nervous system, regulating the homeostasis (or “resting state”) of the majority of the body’s internal organ systems that operate on a largely subconscious level, such as the heart, lungs, eyes, glands and digestive tract. Due to the regulatory nature of the parasympathetic nervous system (PSNS), vagal activity is continuous, chronic, and passive (“tone” in this usage is analogous to “tension”).