Neuro-Regulation Theory 2

Neuro-regulation 2

Now, let’s go back to the heart. With the understanding that the sympathetic and parasympathetic nervous systems were interrelated in cardiac regulation, interest began to focus on heart rate variability (HRV). HRV is the moment by moment, beat by beat, small variations in the heart beat. HRV was found to be associated as a marker of health, with higher HRV associated with overall well-being, both physical and emotional, particularly with cardiac health, better immune function, and resiliency. Lower HRV was associated with overall decreased well-being. HRV was studied in a wide variety of scientific disciplines, and these findings are quite robust. The question is, what are the mechanisms that connect HRV to overall better health?

In the mid 1990’s two of the researchers who studied this in detail, particularly how HRV was related to mental health, were Stephen Porges and Julian Thayer. Porges developed the Polyvagal Theory (PVT) and Thayer and others the Neuro-viscera; integration model. These theories have many similarities but a few important differences. This section will focus on the polyvagal theory, the next section on the neuro-visceral regulation model.

Porges was doing research on infants. He wondered why mild bradycardia in adults is generally associated with good cardiac health while in the fetus and infant bradycardia is a sign of serious trouble and high risk for death. He postulated there were two autonomic systems that caused bradycardia both associated with the vagus nerves. The parasympathetic system comes almost entirely from the vagus nerves (one on the right and one on the left) and their branches which innervate the heart, lungs, and almost all of the abdominal organs. From here on because of convention, I will refer to the vagus nerves in the singular.

The vagus nerve is the tenth cranial nerve. The cranial nerves innervate structures in the head and neck and are described by roman numerals. The vagus or X cranial nerve arises in two locations in the brainstem, one from the dorsal nucleus of the vagus (DMN) and the other from the nucleus ambiguous which we will refer to as the ventral vagal complex (VVC). This is where we get the polyvagal theory. Although not completely anatomically correct, the DMN sits posterior to the nucleus ambiguous. The fibers from the DMN are older evolutionary, generally are unmyelinated (without a protein-fat insulation) and conduct relatively slower than the fibers of the VVC. The DMN has some innervation to the heart but mostly innervates the abdominal structures. Porges postulated these fibers were associated with the “shutdown” response or what we often refer to as the freeze reaction. In infants this shutdown is associated with immobility and sometimes complete collapse or death.

The DMN of the vagus is found in fish, reptiles, and mammals. I know nothing about bird brains. The VVC is only found in mammals and in a rudimentary fashion in some fish and reptiles. Unlike mammals, fish and reptiles have limited responses to threat. They have the fight/flight response or they can freeze with very little gradation in between. Why lizards do pushups I do not know. The fibers of the VVC are myelinated, thus very fast conduction to the heart, which allows moment by moment flexibility to respond to threat and stress.

Porges talked about the concept of the “vagal brake”. The vagus exerts a continual varying degree of baseline tone to the heart. For example, without the vagal brake heart rates at rest would be about 120 beats per minute instead of the usual 70-80 brats per minute. Porges wanted to measure the strength of the tone the vagus exerts on each individual. He used a form of heart rate variability called the respiratory sinus arrythmia (RSA) to approximate the strength of the vagal brake. The RSA is the variation of heart rate associated with the breath. When we breathe in the heart rate increases a little, and when we breath out, it slows a little. In studying infants, he found that those with a lower RSA had a higher risk than those with a higher RSA of unexplained infant death. In addition, he found that lower RSA in infants was associated with and a predictive factor of behavioral problems at age 3! There are other interesting studies by a variety of researchers (thus a robust finding) that lower RSA is associated with greater emotional reactivity in patients diagnosed with borderline personality disorder.

So, the first main point of the polyvagal theory is we can use the RSA to measure the strength of the vagal tone thus a marker of increased or decreased resiliency.

Another main point Porges makes that has enormous clinical implications, is that 80% of the vagal fibers are afferent coming from the organs to the brain) and only 20% efferent (from the brain to the organs). So, the heart talks to the brain four more times than the brain talks to the heart. The gut talks to the brain four more times than the brain talks to the gut. This means the heart and gut regulate the brain more than the brain regulates the heart. We get “heartbroken” and “gut feelings”.

Another central point of the PVT is how the RSA, a physiological marker, is associated with complex human behavior. This has to do with the relationship of the vagus, the X cranial nerve, to our facial and neck musculature which Porges labels the muscles of social engagement. The nucleus ambiguous, which is the origin of the ventral branch of the vagus, also contains the origins of cranial nerves IX and XI. Cranial nerves V and VII are right next door. Cranial nerve V provides sensory input from the face. Cranial nerve VII provides the motor input to the facial muscles. Cranial nerve X is the vagus. Cranial nerve IX along with branches of the vagus innervate the structures that affect or middle ear, pharynx, and larynx. Cranial nerve XI is responsible for the rotation of the head and neck and coordination of the shoulder muscles.

So, our cranial nerves are responsible for facial sensation, facial movement, chewing, sucking, swallowing, vocalization and prosody of the voice, and tuning of the ear. Again, these are referred to as the muscles of social engagement, a huge way we communicate with others including our judgement of safety or threat from other people. They are tightly connected to the vagus which results in a significant conclusion by Dr. Porges.

The neuro-regulation of the heart and lungs is directly, anatomically and functionally, to the muscles of social engagement.

We will review the enormous clinical implications of that in Neuro-regulation 4.

Many physiologists, anatomists, and physicians have a big problem with the polyvagal theory. I have heard it sometimes as “debunked” or not scientifically sound. Dr. Paul Grossman of the University of Bessel Hospital in Belgium, also a physiologist who has studied heart rate variability intensely, has had a big problem with the PVT for many years. He helped discover something very wrong with the PVT. That is that the PVT posits that the bradycardia associated with the shutdown, immobility, collapse, and freeze response is mediated through the dorsal

motor nucleus of the vagus. The issue is that it has been shown that the DMN has no fibers connected to the heart that affect heart rate, just the strength of the heart’s contraction. So, if the DMN does not affect the heart rate it cannot be associated with the bradycardia that comes with the immobility/shutdown response. So, many believe the polyvagal theory collapses. So, is the PVT true or false? That is the wrong question to ask. The question to ask is, “What parts of the PVT are likely correct and what parts incorrect?” The only part of the PVT which is incorrect is the DMN of the vagus is not the origin of the bradycardia associated with the shutdown response. For us as clinicians that does not matter at all. When I am working with a patient in dissociation or freeze, I do not care one bit where that response comes from in the brain. I don’t think any of us would be.

So, when you hear that the polyvagal theory has been debunked or is not scientifically sound, take a deep breath. Everything that matters clinically about the theory is not only correct, but has clinical implications for how best to work with our patients.