Summary and Conclusion
The following seven points summarize the Poly-Vagal Theory.
1. The vagal system does not represent a unitary dimension. The vagal system includes general visceral efferent fibers regulating smooth and cardiac muscle, as well special visceral efferent fibers regulating the somatic muscles of the larynx, pharynx, and esophagus. These somatic muscles control vocalization, sucking and swallowing and interface these processes with breathing. The vagal system also is linked neuroanatomically to the source nuclei that control facial expression, mastication, and head turning.
2. There are two vagal motor systems. One vagal system is the vegetative vagus, which originates in the dorsal motor nucleus and is associated with passive reflexive regulation of visceral functions. The other vagal system is the smart vagus, which originates in NA and is associated with the active processes of attention, motion, emotion, and communication. The two systems are neuroanatomically distinct, have different ontogenetic and phylogenetic origins, and employ different adaptive strategies.
3. In mammals the concept that vagal tone represents a single or summed system may have limited physiological or heuristic value. For example, in mammals high tone from the dorsal motor nucleus vagal system may be lethal, while, high tone from the NA-vagal system may be beneficial. Based upon the proposed Poly-Vagal Theory, an accurate measure of the NA-system is critical to the evaluation of psychophysiological relationships.
4. The functional output of the NA-vagus on the heart may be monitored via RSA. NA is part of a common neuronal network producing a cardiorespiratory rhythm. Thus, the output from the branch of the vagus originating in NA and terminating on the sino-atrial node of the heart conveys a frequency common to both respiratory and cardiac systems. In contrast, the output from the dorsal motor nucleus does not convey a respiratory rhythm.
5. The magnitude of neurogenic bradycardia is mediated by the dorsal motor nucleus. Rapid heart rate changes, such as conditioned anticipatory heart rate deceleration and decelerations associated with orienting, are neurogenic bradycardia. Additional neurogenic bradycardia are reflexes such as opto-vagal and chemo-vagal. In the absence of NA influences to the sino-atrial node, local conditions such as hypoxia may greatly potentiate the vagal effect.
6. There is a common cardiopulmonary oscillator. The common respiratory rhythm observed in heart rate and breathing is produced by a network of interneurons located in NTS and NA, which communicate with the motor neurons that control respiratory, laryngeal, and cardiac function.
7. Primary emotions are related to autonomic function. Since the primary emotions are often survival related, they must be integrated into cardiopulmonary regulation. Moreover, primary emotions have a right hemisphere bias, ipsilateral with the regulatory bias of the medullary structures controlling visceral function.
Based upon the Poly-Vagal Theory, additional hypotheses may now be tested evaluating the relationship between RSA (the measure of NA vagal tone) and processes and states dependent upon the coordination of cardiopulmonary processes with the special visceral efferents of the cranial nerves. This, of course, includes all processes associated with vocalizations, feeding, breathing, and facial expression.
In developing the Poly-Vagal Theory, the most striking insights come from the phylogenetic approach. Not only does a phylogenetic approach explain the vagal paradox in terms of the medullary source nuclei of the dorsal motor nucleus and NA, but it highlights the importance of oxygen needs in the evolving nervous system. As the nervous system gets more complex, there are greater demands for oxygen. Oxygen needs may have provided a major environmental pressure leading to the evolution of the adaptive and sophisticated autonomic nervous system found in mammals. Thus, constructs such as orienting, attention, emotion and stress are by-products of the evolutionary pressure to optimize oxygen resources.