Sinus arrhythmia

Vagal tone is an internal biological process referring to the activity of the vagus nerve (the tenth cranial nerve), which originates in the medulla oblongata of the brainstem. The vagus nerve serves as the key component of the parasympathetic branch of the autonomic nervous system, which homeostatically regulates the 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", reflecting a sense of continuous baseline activity, much like muscle tone relies on nervous baseline activity).

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 usual natural pacemaker 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.

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).

Respiratory sinus arrhythmia (RSA) is a naturally occurring variation in heart rate that occurs during the breathing cycle, serving as a measure of parasympathetic nervous system activity that promotes "rest and digest" responses.

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