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The anatomy of polyvagal structures

Updated: Dec 10, 2019


It was last year that I dove deep into studying about Dr. Steven Porges' Polyvagal Theory. As a manual therapist and bodyworker I found many aspects of the theory relevant to me. Perhaps I will elaborate on what the Theory is another time, or you may have already heard about it. Today I want to share with you my sketches that helped me understand the anatomy of the vagus nerves and also how the poly-vagal theory translates into anatomy.

A quick run down of the Autonomic Nervous System (ANS) first. The ANS is that part of our nervous system that is automatic and autonomic. It regulates digestion, breathing, heart rate, sleep, all of those things we rely on 24/7 but never need to give it a thought. It does so many things and this blog is not intending to go down that lane. However, according to the Polyvagal Theory (PVT) we now understand that our social engagement is also regulated by the Autonomic Nervous System, of which vagus nerve is a big part.


The Autonomic Nervous System has two branches: the Parasympathetic and the Sympathetic branch. But you know that already, so I'll jump to the Vagus nerve. Although we read about the Vagus nerve as if there is one singular nerve, it is a paired cranial nerve (no. 10, or CN X). We have a left and a right Vagus nerve branches. FYI Some fish (like goldfish) don't have vagal nerves, but possess a vagal lobe instead, which among other crucial functions helps them sort food from stones :)


As 'polyvagal' suggests that there isn't one vagal nerve, but many - poly. Left and right, as well as Dorsal and Ventral vagal complexes, which really translate into myelinated and unmyelinated Vagus nerves. We can also divide the Vagus nerves into supra diaphragmatic and sub diaphragmatic.


As per PVT, the differentiation between myelinated and unmyelinated fibers of the vagus nerve goes beyond the myelin division. The differentiation and consequent naming of each implies that unmyelinated fibers are phylogenetically older and associated with survival in unsafe environment, whereas myelinated fibers evolved much later and serve mammalian species exclusively and are associated with function within safe environment. Unmyelinated fibers belong to the Dorsal Vagal Complex, and myelinated fibers to the Ventral Vagal Complex.


Keep on reading, here comes a picture of that...



THE VAGUS NERVE DISSECTION


Let's take a single Vagus nerve and look inside.



Anatomists largely agree that between 75%-85% of fibers within a single Vagus nerve are sensory or afferent fibers. Please don't let my drawing confuse you, they are certainly not arranged like I circled them with blue color. Proportion is my point. These sensory fibers originate in the viscera and end in the Nucleus Solitarius in the brainstem. These fibers are both myelinated and unmyelinated (my sketch says only myelinated, but that's not a fact).

The rest of the fibers within any single Vagus nerve (roughly 20% of them), are the motor or efferent fibers. Most of these originate in the Dorsal (or posterior) Motor Nucleus within the brainstem. These are unmyelinated fibers and they comprise what Dr. Porges named the Dorsal Vagal Complex. His research also point to the fact that this complex of fibers is the most prominent in the sub-diaphragmatic region.

Now, within the motor/efferent fibers we find a subgroup, which is proportionately small - only 15% of the 20% of motor neurons that are actually myelinated! This small portion of the motor fibers originates in a different area of the brainstem - Nucleus Ambiguus - and Dr. Porges names it the Ventral Vagal Complex. This group of neurons primarily innervates the supra diaphragmatic structures.


So easy, right?

Let's take a look at the origin of the Vagus in the brainstem.



THE ORIGIN OF THE VAGUS NERVE(s)