The vagus nerve has 80% of its nerve fibers reporting to the brain. What is up with that?
Catherine Lienemann ‘UA 27 and James Stellar
In a previous blog, we wrote about Takotsubo syndrome where stress can actually alter the structure of the heart. That showed something already known, but perhaps not represented enough here – the interaction between behavior and the body’s physiology. Now we want to focus on what the body, and particularly the vagus nerve input from the heart, tells the brain about to alter its behavioral/psychological function. We will focus on the higher cortical brain regions and decisions, given that 80% of the vagus nerve fibers are sensory, as stated in the title. We will not focus on the classical parasympathetic outputs from the brain to the organs like the heart (also a vagus nerve function), but instead on how these inputs affect planning, conscious awareness, and decision making.
First, the Gut-Brain Axis:
The vagus nerve may be 80% sensory because it connects just about ‘everything’ in the body to the brain. By this input, the vagus essentially creates the gut-brain axis, monitoring the airway or esophagus for obstructions, examining the lungs for constant gas exchange and harmful particulates, monitoring blood, and pressure, etc. We can conceptualize the vagus nerve as the “nanny camera” of the body and it reports to the brain what it “sees.” That influences our subconscious feelings and gut reactions in the limbic system and the frontal cortex brain areas that seem to re-represent that limbic function (see an earlier blog).
The figure below depicts the extent of the vagus nervous connections to the body and therefore the relation of this information to and from the brain.
Information Sent:
What is the information being traded up and down the wandering vagus nerve? Well starting from the bottom of the pathway with the intestine and the stomach. The brain receives information on food absorption and changes in energy usage, influencing metabolic hormones, glucose levels, enzymes, contractions, and more. Then up to the next major system, the respiratory system, where the vagus nerve retrieves information on the condition of the airways and ensures the airway is clear, if not, it invokes coughing and other protective airway reflexes. From our perspective, this is not the input we’d believe would play a part in our subconscious feelings. The cardiovascular system is the last major system on which we would like to touch. Its nerves in the heart monitor blood pressure, rate, chemical composition, and volume; and it influences a large part of the limbic system
We are concerned here with the entire gut-brain axis and its role in mood control such as butterflies in the stomach theorized to occur through what some have called an “inverted reflex arc involving brain-to-gut-to-brain transmission.” Others call this general phenomenon interoception. Still others, like Antonio Damasio, refer to it as the somatic marker hypothesis which plays a role in decision-making. We think of it as the little voice from the gut that tells you you are on the right (or the wrong) track, perhaps what Daniel Kahneman called “System 1” in his book “Thinking fast and Slow,” or what Malcolm Gladwell referred to as thin-slicing in his book “Blink.”
The insular cortex (IC):
The vagus nerve then delivers these little voices from the ‘gut’ to the brain; in particular the insular cortex. The figure below from a comprehensive study depicts the pathway of inputs and outputs from the vagus nerve to the insular cortex’s anterior (pink) and posterior (brown) regions. Each distinctive region of the cortex is responsible for interpreting input (the little voices) into behavior/emotion motivation and body regulation. For example, another blog by Pauline describes the insula cortex output as a “vital structure in the organization of movement” of actions from emotions.
The complex nucleus of the solitary tract (NTS):
The NTS in the brainstem is the collection point of all sensory information in the brain. Looking at figure below, from the same study cited above, you can see from there that the input goes to other specialized regions of the brain, the most important of the couple being the Thalamus and the Hypothalamus being the “?rst level of control of hormonal, immune, and autonomic outputs.” The figure below also depicts a pathway of sensory input to distinctive regions of the insula cortex depending on the input. For example information to the hypothalamus then influences the hormonal output, the information to the ventrolateral medulla influences immune outputs, and the parabrachial nucleus influences the autonomic outputs.
As a key article on health and disease says,
However, inconsistency within the physiological and psychological literature regarding the definition of interoception, and use of terms such as interoceptive awareness, led to proposed dimensional frameworks for understanding and studying this set of senses. Within such a framework, interoception can be described from the physical responses in body and brain representation up to (and beyond) interoceptive metacognitive (i.e., available for explicit awareness and reflection) insight and conscious awareness.
While there is some ongoing discussion in the research cited in this blog about what exactly is interoception and how to define it, interoception refers to the sense of what’s going on inside your body, like feeling your heartbeat or your stomach growling. Because different studies use different terms, researchers have suggested ways to think about and study interoception in different stages. These stages range from how your body physically reacts (like a heartbeat) to how aware you are of those sensations and how much you can think about or reflect on them. In those reflections we make “gut feeling” based decisions and passively create opinions about the world around us without cognitive thinking.
What seems clear is that the sensory input from the vagus nerve is carried into the NTS where it is organized into specialized higher regions like the insula cortex that process the sensory input. What particular sensory information from the gut fuels interoception and what regions and what thoughts are altered will be the subject of our next blog.
