I've heard of the vagus nerve. What is the trigeminal nerve?

There’s been a lot of talk about the vagus nerve lately: where it is, what it does and how to manipulate it to increase healing of a variety of conditions. However, during a recent conference on mold, a toxicologist brought up the trigeminal nerve, which was news for our staff.  It’s a pretty large nerve with important functions related to our environment, so we think it’s important to know more about it.

First of all, the prefix “tri” does indeed mean three.  The nerve anatomy begins in the brain and extends all over the head and face, carrying sensations of touch, pain, temperature and proprioception (awareness of position, of the jaw in this case).  A quick 2-minute video on the trigeminal nerve shows it in profile.  Here are the three branches of this nerve (Trigeminal Nerve: What to Know):

• The ophthalmic nerve covers the scalp and upper part of the face, including the eye, eyelids, and forehead. It is a purely sensory nerve and carries sensations to the brain.

• The maxillary nerve covers the middle part of the face and is also a purely sensory nerve. It carries sensations from your nose, cheeks, lower eyelids, upper lip, and gums. This nerve also carries sensations from the inside of the mouth, the palate, the upper jaw teeth, and the sinuses.

• The mandibular nerve has both sensory and motor parts. It carries sensations from the lower part of your face, including the jaw, lower lip, and gums. It carries sensations other than taste from the front part of your tongue. The motor part of this nerve controls the muscles involved in chewing, biting, and swallowing.  

These sensations of heat, pain, touch and jaw position don’t seem to have much to do with mold.  However, there are also “chemoreceptors” in the trigeminal system.  This is where your “smell”, “taste”, and “feel” sensors may overlap.   In the nose, the capacity to detect and react to volatile chemicals is mediated by two separate but interrelated sensory pathways, the olfactory and trigeminal systems. Most chemosensory stimulants, at sufficient concentration, produce both olfactory and trigeminal sensations (i.e., stinging, burning or pungent). (Influence of nasal trigeminal stimuli on olfactory sensitivity).  The response threshold of trigeminal fibers to chemicals is generally much higher than olfactory thresholds for the same compounds, meaning that it takes much less of an odorant to produce a response in the olfactory system than the trigeminal.  (The exception is for the corneal endings in the eye that can be extremely sensitive to chemical irritants.)  (Chemosensory Properties of the Trigeminal System) However, activation of the trigeminal system ”sensitizes” individuals’ sense of smell which may account for the sensitivities experienced by sufferers of Multiple Chemical Sensitivity, or MCS.   In a 2004 study, 20 subjects had their trigeminal nerve exposed to allyl isothiocyanate (AIC, it is responsible for the pungent taste of mustard, horseradish, and wasabi), then they smelled either phenyl ethyl alcohol (PEA, a rose-like odor) or butanol (BUT, a strong alcohol smell with a hint of banana).  The study showed that whatever the odorant (PEA or BUT), AIC trigeminal activation produced a decrease in the olfactory thresholds, corresponding to an increase in olfactory sensitivity.  (Influence of nasal trigeminal stimuli on olfactory sensitivity).  What does this mean?  If you get hit with a pungent VOC (trigeminal and olfactory), it will also affect your smell and taste (olfactory).  This can be a pleasant reaction when tasting a complex food like aged cheese, or deeply insulting/irritating when it’s a non-food mold. (The Hidden Sense with Huge Power: “Trigeminal” flavours)  Conversely, according to Dr. Krause, when the olfactory nerve is being exposed to sulfur-containing compounds, it can lower the threshold of trigeminal nerve irritation by 50%. 

The reaction of the body to these VOCs may be either an allergic response, or non-allergic response, which don’t seem very different at first glance!  According to Dr. Krause, an allergy requires prior exposure to develop antibodies, and subsequent exposures can trigger severe allergic responses.  However, irritation of the trigeminal nerve can cause a cascade response of inflammation, even without a detectable odor.   It doesn’t need a pre-exposure to an allergen, it can happen the first time you experience it.  Odor and irritation are two separate nerve endings and processes, even though the same substances can activate both.  This is a “safety feature” of our bodies, telling us to “get out” of a space with certain poisonous gases, whether or not we’ve experienced them before!   These kinds of “irritant” responses outcomes not generally associated with an allergic response—including nervous-system effects, suppression of the immune response, hemorrhage in the mucous membranes of the intestinal and respiratory tracts, rheumatoid disease, and loss of appetite—have been reported in people who work or live in buildings that have microbial growth.  (Toxic Effects of Fungi and Bacteria) For example, Jason Earle of GotMold?, who used to perform mold inspections, reported a severe nosebleed (a type of respiratory hemorrhage) within 30 minutes of entering a severely mold-infested building (which had a high level of S. Chartarum), and has had at least one client who reacted the same way.

So, what kinds of things are going on when we breathe in mold VOCs?  These chemicals, which can be produced in a lab with or without fungi, in the short term can cause pain and inflammation.  Activation of the trigeminal nerve leads to blood vessel “leakiness” and can result in symptoms like nosebleeds.  (This is differentiated from pulmonary hemosiderosis, which is from long-term exposure to irritants).  Different irritants and toxins, when combined, can also have synergistic effects.  For example, sulfur-containing compounds can enhance effects on sensory nerves.  Depending on the chemical, a variety of physiological responses mediated by the trigeminal chemosensory system are triggered by exposure to irritants. These include increased salivation, vasodilation, tearing, nasal secretion (runny nose and post-nasal drip), sweating, decreased respiratory rate, and bronchoconstriction. (Trigeminal Chemoreception)  If you think about it, although bronchoconstriction (narrowing of the airways in the lungs) is scary-feeling, that and reduced respiratory rate are actually defensive mechanisms to avoid breathing more of the irritant!

Therefore, you can thank the trigeminal nerve for good and bad sensations.  It's good to feel warm sunshine on your face, and a little later a cue to get out of the sun.  It's good for heightening the taste of aged cheese, and even cues that there's unauthorized mold growing in your fridge or under your sink.   The human body is a wonder of complexity!

Photo by Robina Weermeijer on Unsplash