THE CONTROL DEVICE OF THE CARDIAC RHYTHM

The cardiovascular system is controlled by several antagonistic systems. It was first believed that the heart was stimulated by the (ortho)sympathetic system and moderated by the parasympathetic system (represented by the vagus nerve also called X). The recent research carried out by PORGES (ref. P1) shows that there are three distinct centers of command: two allotted to the parasympathetic system and one to the (ortho)sympathetic system.

During the evolution of the species, the mammals developed a nervous vagal system which is not present in the other animals. In the mammals, the vagus (or nerve X) divides in two types of nervous fibers associated with two different driving nuclei located in the medulla. The oldest driving center (which is found in the reptiles) is located in the posterior part of the bulb and is called dorsal motor nucleus of vagus (DMNX). This center commands slow nervous fibers (C fibers) because of the absence of myelination. In the mammals, there is a second center called Nucleus Ambiguus (NA), located in front of the dorsal command center. This center orders fast nervous fibers (B fibers) because they have a myelin sheath.

These two centers of command of the vagus will both moderate the heart, but in a completely different way.

Because the size of their brain and their great mobility, the mammals need a great amount of oxygen. According to PORGES (Ref. P1), this need of oxygen induced the development of the autonomous nervous system specific to the mammals. The polyvagal theory (Ref. P2, P3) developed by PORGES highlights the respective roles of the two parts of the vagus nerve for the conservation of energy and the survival of the species:

• the most vegetative part of the vagus (dorsal motor nucleus) is linked to reptilian brain and in case of stress induces an answer adapted to this kind of animals: initiates immobilization and death feigning response. This attitude is completely inappropriate and very dangerous for the mammals.
• The most recent part (NA) and specific to the mammals, innervates and controls the heart, but also the facial somatic muscles related to the expression and food capture. This center is thus connected to emotions and communication. It controls the heart and the bronchi to promote calm and soothing states.

Considering their various origins, the two vagus systems of the mammals are programmed to deal with different strategies. According to PORGES, the competition between these two systems can cause unsuited answers (in particular in autistics and asthmatic patients).

According to the polyvagal theory, our autonomous nervous system presents three hierarchical levels corresponding to the phylogenic evolution. :

• the system related to the communication (most recent), corresponding to the part innervated by the nucleus ambiguous. This system allows a fast regulation of the heart according to the environment. It is associated with the cranial nerves which control the sociability through the expression of the face and the speech.
• The defense system (escape/fight) which depends on the (ortho)sympathetic nervous system: increases the metabolic production and moderates digestion in order to metabolize all the resources related to mobility.
• the primitive system corresponding to the dorsal motor nucleus of X which supports digestion and strongly depresses the metabolic activities.

According to the polyvagal theory, the most recent systems inhibit oldest. When our three systems are quite functional and in good correlation, we should be in a calm state, and opened to communication. If our NA system is defective or is repressed, we automatically pass through the (ortho)sympathetic nervous system. If the latter is repressed or defective, the ancestral DMNX system will prevail.