The olfactory nerve, via scent molecules and their receptors, is our direct connection to information about the “outside” world to the “inside” world in our brain.
The olfactory nerve is actually a collection of sensory nerve rootlets that extend down from the olfactory bulb and pass through the many openings of the cribriform plate in the ethmoid bone. These specialized sensory receptive parts of the olfactory nerve are then located in the olfactory mucosa of the upper parts of the nasal cavity. During breathing, aromatic molecules attach to the olfactory mucosa and stimulate the olfactory receptors of the nerve, which then send information along the nerve tract to the bulb, and finally into the brain.
From the olfactory bulb, mitral cells send their axons through the olfactory tract toward two separate sets of targets. One target is for perception/awareness of smell, and that is to the thalamus and then to the cortex. The other target is more primitive — to input to our autonomic nervous system and limbic system (emotions) so that certain smells lead to particular responses (like to get out if you smell fire). And remember, olfactory information doesn’t even have to go to the thalamus before heading on to its targets of the limbic system (some frontal lobe, temporal lobe, and other limbic targets).
For a long time, because of the location of the olfactory bulb and tract, many hours of mine have been spent trying to determine if in fact there is a relationship between essential oil circulation and cerebral spinal fluid circulation.
If nasally administered medication contacts the olfactory mucosa, there is good evidence that suggests molecule transport can occur directly across this tissue and into the cerebral spinal fluid. The olfactory mucosa, as previously stated, is located in the upper nasal cavity, just below the cribriform plate of the skull. It contains olfactory cells which traverse the cribriform plate and extend up into the cranial cavity. When medication molecules come in contact with this specialized mucosa they are rapidly transported directly into the brain, skipping the blood-brain barrier, and achieving very rapid cerebrospinal fluid levels (often faster than if the drug is given intravenously). and extend up into the cranial cavity. This concept of transfer of molecules from the nose to the brain is referred to as the nose-brain pathway and has implications when centrally acting medications such as sedatives, anti-seizure drugs and opiates are delivered nasally. Multiple authors have demonstrated that the nose-brain pathway leads to nearly immediate delivery of some nasal medications to the cerebral spinal fluid, by-passing the blood brain barrier.
Literature related to Nose brain pathway:
Nasal mucosa consists of a highly vascularized surface that easily absorbs many medications directly into the venous circulation. This medication is then transported to the heart and pumped out to the body where it can have its therapeutic effect. Because the absorptive surface is not the intestinal mucosa, the drug never enters the portal circulation and is not subjected to hepatic metabolism – thereby leading to far higher drug levels than oral or rectal medications. In addition, the nose brain pathway across the olfactory mucosal transports some of the nasally delivered medication directly into the CSF and brain – leading to early effects of centrally acting medications.
Unfortunately, there is no research that I can find related to essential oil inhalation absorption along the olfactory mucosal transport routes and cerebrospinal fluid circulation/absorption, but based on some of the most recent nasal drug administration research I have read, I would say as aromatherapists we can make the “leap of faith” here and apply these findings to essential oils. Which means not only do essential oils have a direct route to the brain via the limbic system, but there is a direct fluid transport of them via the CNS and the cerebrospinal fluid system.
How amazing to think that we humans are made up of somewhere between 70-85% water (based on age and other factors) and that cerebrospinal fluid, a blood by product and also made up of a high percentage of water and essential oils – another fluid albeit a hydrophobic one – but which requires water, in the form of steam to bring about its creation — that all these fluids transport astounding amounts of information almost within a blink of an eye. That our innate intelligence and our incredible bodies inherently “know what to do”, is exactly why I love anatomy, and aromatherapy and craniosacral therapy!