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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”).〔(【引用サイトリンク】author=Heathers, J. )〕 In the context of psychophysiological research, vagal tone (and specifically its influence on heart rate) represents an index for the functional state of the entire parasympathetic nervous system. Heart rate is normally controlled by multiple centers in the brainstem; one of these centers, the nucleus ambiguus, increases parasympathetic nervous system input to the heart via the vagus nerve. Vagal tone decreases heart rate by inhibiting the firing rate of the sinoatrial node (S-A node, the "pace-maker" tissue of the heart). The absolute level of cardiac vagal activity or vagal tone appears to result from the excitatory drive from peripheral baroreceptors. In animals, cardiac vagal activity disappears at very low pressures or if the afferent nerves are cut. Thus, a major determinant of resting heart rate is the beat to beat activation of the arterial baroreflex with each cardiac cycle. Cardiac vagal tone has been treated as a physiologic substrate of regulation of emotion and arousal. ==Relation to respiratory sinus arrhythmia== Respiratory sinus arrhythmia (RSA) is a naturally occurring variation in heart rate that occurs during a breathing cycle. RSA is also a measure of parasympathetic nervous system activity - which denotes "rest and digest" behaviors. Vagal tone cannot be directly measured. Instead, other biological processes are measured that represent the functionality of vagal tone. An increase in vagal tone both slows the heart and makes heart rate more variable (i.e. there is more beat-to-beat change between heart beats). During the process of RSA inhalation temporarily suppresses vagal activity, causing an immediate increase in heart rate. Exhalation then decreases heart rate as it causes vagal activity to resume. Thus, while vagal tone is not explicitly measured, the resultant changes in heart rate are. This is done by measuring periodic changes in the heart rate during a resting state of cardiovascular activity, a process known as heart rate variability (HRV). There are about 40 published methods of quantifying HRV, but the vast majority of studies use only a few of the available measurements. Most common is high-frequency HRV, a measurement of the amount of heart rate variability there is between typical breathing cycles (approximately 2.5 to 6.7 seconds). On an electrocardiogram ECG, RSA is seen as subtle changes in the R-R interval (time between two of the distinctive, large, upward "R" spikes on an electrocardiogram) synchronized with respiration. The R-R interval on an ECG is shortened during inhalation and prolonged during expiration. Meditation and relaxed breathing techniques can temporarily alter RSA. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Vagal tone」の詳細全文を読む スポンサード リンク
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