Table of contents
By Torsten Liem & Winfried Neuhuber
Comparative anatomical and functional studies argue against the postulated phylogenetic rooting of the polyvagal theory (PVT). Also, the term "polyvagal" in the neuroanatomical construct of PVT and in the functional construct of the social engagement system is misleading, as the term implies that a "new" ventral vagal complex would exert coordinating function alongside an "old" dorsal one. The vagus nerve is certainly an important factor in the system of social engagement (in which the hypoglossal nerve should be included), but it is not the coordinator. Rather, the mesencephalic periaqueductal grey, together with the limbic system and neural brainstem networks, coordinates behavioural states such as fight and flight and freezing - with associated motor, autonomic and endocrine effects. Ultimately, numerous other brain areas, if not the entire brain, act as a social engagement system. Consequently, a reformulation, or at least a clarifying new term, would be indicated.
1.1 The Polyvagal Theory
Since its first description by Stephen Porges in 1995 [1][2], polyvagal theory (PVT) has received much attention among mind-body therapists including osteopathy worldwide, especially with regard to the treatment of trauma patients. PVT is an attempt to explain the relationship between parasympathetic activity and behaviour from an evolutionary perspective [3]. It aims to provide an understanding of the connections between the processes of the brain and the body [1][2].
The term "polyvagal" refers to 2 vagal circuits. One is the phylogenetically older unmyelinated system represented by the motor nucleus dorsalis n. vagi, which mainly innervates subdiaphragmatic organs (especially the gastrointestinal tract), but also the heart, and is associated with immobilisation and dissociation. Evolutionarily younger, according to Porges, a 2nd younger vagal pathway is thought to have developed, which has only been observed in mammals, but not in reptiles, and has the ability to down-regulate immobilisation and fight and flight behaviour. According to Porges, the anatomical structures of this component of the vagus interact in the brainstem with structures that innervate the striated muscles of the face and head to create an integrated system of social engagement [4]. This younger system is represented in particular by the nucleus ambiguus. In PVT, it is combined with the other branchiomotor (specifically visceroefferent) nuclei of the Vth, VIIth, IXth and XIth cranial nerves as the ventral nucleus ambiguus. Cranial nerve as the ventral vagal complex [5]. This system regulates the heart and lungs via myelinated nerve fibres to enable resting states and is thought to be associated with safety and social behaviour [6].
The focus of PVT is on the phylogenetic shift between reptiles and mammals that resulted in specific changes in the vagal pathways regulating the heart. Accordingly, primary vagal efferent pathways regulating the heart shifted from the dorsal nucleus of the vagus in reptiles to the nucleus ambiguus in mammals, establishing a face-heart connection with properties of a social engagement system that allows social interactions to influence visceral state and visceral dysfunction manifested in neural regulation of the heart [7].
1.2 Comparative anatomical and functional studies related to PVT
These studies argue against the proposed phylogenetic basis of PVT. It is undisputed that in mammals myelinated cardioinhibitory axons arise from the nucleus ambiguus. However, already in cartilaginous fish (elasmobranchs, e.g. sharks), which have existed for 400 million years, the cardioinhibitory vagus neurons are myelinated and conduct at speeds between 7 and 35 m/s (which corresponds to the B-fibres of mammals). Moreover, their cell bodies are located at 2 different sites in the brainstem (dorsal vagus nucleus and primordium of the nucleus ambiguus) [8][9]. Thus, cartilaginous fish are already "polyvagal".
Lungfish, which are evolutionary precursors of air-breathing animals, also have a myelinated cardiac vagus nerve that originates in dorsal and ventrolateral brainstem nuclei [10]. These myelinated, fast-conducting axons enable beat-to-beat slowing of the heart rate, which is mandatory for the cardiorespiratory interactions observed in these ancient vertebrates, similar to mammalian respiratory sinus arrhythmia [10][11].
The unmyelinated vagal cardiac neurons of the nucleus dorsalis n. vagi most likely have no significant influence on heart rate and therefore cannot be held responsible for bradycardia observed in the freezing state. They seem to influence ventricular inotropy and might protect cardiomyocytes from ischaemia [12].
1.3 Response patterns in PVT
PVT assigns reactions to perceived risks to 3 categories: feeling safe, being in danger or perceiving a threat to life. These categories follow one another in phylogenesis. They are related to the adaptive behaviours of social communication (facial expressions, speaking, listening), which are thought to be controlled by the nucleus ambiguus, and to defence in terms of mobilisation (fight, flight) and immobilisation response (vasovagal syncope, dissociation, immobilisation or freezing state), which are controlled by the nucleus dorsalis n. vagi [1][6][12][13][14].
Again, the proposed connection of these behavioural phenomena with the old unmyelinated or the new myelinated vagus nerve is misleading. The mammalian nucleus ambiguus contains, in addition to the cardioinhibitory neurons, mainly the branchiomotor (especially visceroefferent) neurons for laryngeal, pharyngeal and striated oesophageal muscles [15], but controls neither facial expression (mimic muscles are innervated by the facial nerve) nor hearing via the median nerve. facialis), nor hearing via the middle ear muscles (tensor tympani muscle, innervated by the motor branch of the trigeminal nerve, and stapedius muscle, innervated by the facial nerve), nor other head and neck muscles as suggested by PVT. Also, conversely, the facial nucleus does not influence the ambiguous nucleus.
All these motor nuclei, including the hypoglossal nucleus, are coordinated by premotor networks in the lateral parvocellular and intermediate reticular formation [16][17][18][19][20]. The intermediate reticular formation, located between the medial magnocellular and lateral parvocellular areas, also houses the neuronal networks for cardiovascular regulation (circulatory centre) and the central generators for respiratory rhythm (pre-Bötzinger complex, respiratory centre) and for swallowing and vomiting. Nucleus dorsalis n. vagi and nucleus ambiguus are anatomically and functionally embedded in these networks, but not as coordinators but as output elements. Vagal afferents are connected via the solitarius nucleus (nucleus tractus solitarii) not only to the motor vagus nuclei (nucleus dorsalis n. vagi and nucleus ambiguus), but also to the premotor networks of the formatio reticularis and the circulatory and respiratory centres [20]. Equally important for the coordination of the entire head and neck motor system, however, are trigeminal and upper cervical spinal afferents, which are also fed to the premotor reticular networks.
1.4 Role of the mesencephalic periaqueductal grey (PAG)
A bilateral periventricular nucleus in the ventral mesencephalon, showing a similar location to the mammalian PAG, has already been described in the lamprey, which belongs to the oldest group of vertebrates living today [21]. Behavioural states such as fight and flight, immobilisation or freezing state and risk assessment - with associated motor, autonomic and endocrine effects - are coordinated by the mesencephalic periaqueductal grey (PAG) [22][23][24][25]. The PAG is connected to the hypothalamus and the limbic system (especially the amygdala and prefrontal cortex) [23][24] as well as to various premotor and autonomic brainstem nuclei that coordinate respiration and the emotional motor system [25]. The PAG receives afferents from almost all sensory systems - not least the nociceptive system - and modulates their processing [24].
Undoubtedly, the vagus nerve has a significant influence on emotions and various behavioural states due to its large afferent component. Vagal afferents, accounting for about 80% of its axons, are transmitted through the nucleus tractus solitarii to the PAG, hypothalamus, amygdala, as well as to the insular, cingulate and prefrontal cortex, where they are integrated into emotional and cognitive processes [26][27][28][29]. Recent studies suggest that subdiaphragmatic vagal afferents influence innate fear, learned fear and other behaviours [30][31]. Furthermore, vagal afferents modulate spinal nociceptive processes in various experimental models [32][33].
It is true that Porges [34] mentions the representation of neuroanatomically already long known relations of the limbic system and PAG with the bidirectional connections to the vagus complex. However, since it is not the ventral vagus complex but the PAG in association with limbic and other brainstem networks that is responsible as a coordinator for these behavioural states and, moreover, numerous brain areas, if not the entire brain, function as a system of social engagement, the term "polyvagal" to characterise it appears to be a misleading misnomer.
1.5 Conclusion
As Grossman and Taylor [11] already showed, phylogenetic references are questionable as a basis for PVT. Facts of cranial nerve anatomy are also sometimes incorrectly presented in PVT. Instead of extending the concept of the ventral vagal complex to all branchiomotor nuclei, it would make more sense to leave them their independence and emphasise their coordination by a network of brainstem neurons.
The concept of the social engagement system is plausible and seems relevant to practice. However, it is misleading and should be avoided to link it with the "old unmyelinated or new myelinated vagus" and the term "polyvagal". In addition, the hypoglossal nerve, which is not a branchiomotor nerve but innervates the socially important tongue muscles, should also be included in the concept of the social engagement system.
The mesencephalic trigeminal nucleus and other sensory trigeminal nuclei also play a major role in the coordination of orofacial motor function. The vagus nerve is certainly an important factor in the social engagement system, both efferent and afferent. However, since the "new" vagus nerve in the form of the nucleus ambiguus does not exert a coordinating function on the other branchiomotor nuclei (V, VII, IX, XI), even though vagal afferents are fed into these coordination networks via the nucleus tractus solitarii, PVT turns the causal connections on their head. Consequently, the term "polyvagal" is a misleading misnomer. The functional construct of the social engagement system should not be associated with the term "polyvagal". Perhaps a clarifying new designation would be indicated.
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