The Limbic System

We have all had the experience where a particular smell has brought us back to a time and place of memory. It could be a freshly baked apple pie transporting you back to childhood and the first time you smelled apple pie baking. Or maybe a cologne wafts by and wham, you’re thinking back to your high school sweetheart! There’s no telling when a “limbic moment” will hit you, but thanks to that ancient part of the brain, you have direct access to your past through the sense of smell.

The limbic system is part of the reptilian brain, the older embryological part of brain that is one of the first to form as a fetus. It is the interface between the brain and the outside world. The limbic system is the seat of the emotional center and is partly responsible for our fight or flight response, our emotional reaction to something, our hormonal secretions, motivation, pain reflex and our mood fluctuations. There are three main components to the limbic system: the hypothalamus, the hippocampus and the amygdalae. There are also the pituitary, the cingulate gyrus, the fornicate gyrus, the thalamus, the mammilary body, the nucleus accumbens that help form the system, plus the olfactory bulb that plays such an important part in aromatherapy.

To understand the limbic system we need to break it down into its main constituents. The first part that we will briefly look at is the hypothalamus, which strives to bring the body into homeostasis. If the body is feeling imbalanced, the hypothalamus will try to return it to “set point” much like a thermostat adjusting hot and cold. The hypothalamus regulates hunger, thirst and response to pleasure and pain. It also affects, through the parasympathetic and sympathetic nervous system, one’s blood pressure, heart rate, the sleep/awake response and sexual arousal. Its relationship to the endocrine system allows for hormone production and release.

The hippocampus converts short-term memory into long-term memory and deals with spatial navigation. Interestingly enough, in Alzheimer’s patients, this is one of the first parts of the brain to be affected and if there is damage to this part of the brain, memories prior to the damage are hard to retrieve. In a faulty functioning hippocampus, people somehow forget where they are and how they got there. The hippocampus allows us a sense of direction and a knowing of how to navigate through life both physically and internally.

The amygdalae are primeval arousal centers that process the memory of emotional reactions. They store all information concerning emotional events, which include fear conditioning and conditioning of emotional response. The amygdalae register non-verbal signals of anger, fear, defensiveness and aggression. They also contribute to the sexual response. When input from the olfactory bulb and olfactory cortex arrives at the amygdalae, impulses are sent out to other the parts of the limbic system and the fight or flight response takes over. There is a demand for increased sympathetic nervous response, asking for faster reflexes, an appropriate facial expression and increased hormonal production.

How does all this work? Let’s say that an aroma fills the room and the smell is inhaled. Odorant molecules infiltrate the nasal passages and travel through to the olfactory epithelium where neurons with cilia bind onto the odorant. Inside the neurons there are protein called odorant receptors. There are thousands of neurons in the epithelial tissue, but in humans we can only pick up smell with about 300 different odorant receptors (dogs have many more). So, the odorant receptors in the neurons bind to an odorant molecule. The receptor cells then convert the proteins into electrical impulses, which travel along the olfactory nerve to the cribiform plate and then to the olfactory bulb. From the olfactory bulbs, odor impulses travel directly to the olfactory cortex in the temporal lobe or to the limbic system (especially the hypothalamus and pituitary).

Now we need to see how aromatherapy plays into all of this. Each of the essential oils has therapeutic properties, in that they are stimulating, calming, sedative, balancing, etc. When we inhale an essential oil molecule, it travels through the nasal passage to a receptor neuron that transports it up to the limbic brain, especially the hypothalamus. Remember that some of the functions of the hypothalamus are to regulate blood pressure, control heart rate and adjust hunger and thirst? Well, if you smell an essential oil that has a vibrationally calming effect on the body, and the hypothalamus receives the input to relax, then it creates neurochemicals that are sent through the body to relax and calm. Likewise if an essential oil has a frequency known to stimulate the body, then the limbic system will conform by sending the message to energize and become more active. Through this manner of transport, one can understand how aromatherapy oils can increase immunity, balance hormonal secretions, dampen or enliven hunger and thirst, and create sexual desire. Aphrodisiacs are so potent because they affect not only the hypothalamus and the pituitary, but also other brain parts associated with sexual arousal, increased libido and desire for procreation. Most aphrodisiac oils send messages to relax peripheral nervous energy and instead concentrate that energy for stimulation of the sexual centers. Many parts of the primal (reptilian) brain in humans convert proteins into sexual impulses. All essential oils have aroma molecules that set up chemical chain reactions to affect all parts of the body, by directly interfacing with the limbic system.

Along with neurochemical changes, there are also emotional responses that get set up. If a situation is sensual, pleasing, fun, exciting or amusing, the odors present at that time help to cement the neuro-pathways of the brain. Each time the same aroma is present with a similar pleasure, the pathways deepens, until the memory transfers from short term to long term memory and is indelibly planted in the brain. Then years after the event, even if the smell hasn’t been present all that time, one whiff and the brain falls right back into the pleasure pathways and the memories kick in. The same applies for ‘bad’ memories. The unpleasant external events associated with the aroma become internalized and are remembered by the stimulus of the odor. An arousal or fight/flight state is triggered in the limbic system by the direct access of the aroma molecule.

Aromatherapy can play a big part in stress reduction, balanced heart rate, hunger control and sexual desire, all because of its inter-relationship with the limbic system. Whether the oils are rubbed on in massage oil or inhaled through an aroma lamp, the odor molecules will travel to their limbic destination and create the appropriate neurochemicals to affect a physical response. So the next time you smell something that transports you back in time, you can hum “thanks for the memory” to your limbic system.