With our every breath, the brain receives and interprets vast information about our environment. Our brain is capable of identifying at least 10,000 scents, and can detect the presence of vanilla at concentrations as low as .00000000762 grains per cubic inch.
When we inhale an aroma, a small portion of the odor molecules of inhaled air land on the olfactory epithelial membrane in the upper nasal cavity, melt and initiate an electrochemical cascade of responses leading into the limbic system. Others are exhaled back out into the air; still some enter the lungs and cross the capillary membrane, causing a subtle physiologic response dependent on the chemical constituents of the odor molecules themselves. Finally, a few molecules enter the stomach to join digestion.
Interestingly, the mucous covered olfactory epithelium is the size of a nickel with approximately 10 million olfactory neuron receptor cells that continue to regenerate throughout our lifespan. The dendrites of the receptor cells contain cilia that project into the mucous. As odor molecules dissolve, it is believed that they bind to specific receptor proteins on the cilia.
The thin unmyelinated axons of the receptor cells form slender bundles that pass through tiny openings in the cribiform plate of the ethmoid bone to reach the olfactory bulb. Physiologic studies indicate that a considerably large amount of informational processing occurs in the bulb. Mitral cells, olfactory bulb relay neurons, transmit not just stimulating messages, but reciprocal as well as inhibitory responses to locations in the brain.
The olfactory bulb is the only part of the brain exposed to the external environment. On the other side of the cribiform plate, the receptor cell bundles of the olfactory bulb are in proximity to the sheaths of the meninges and the lymphatic system of the nasal cavity, making a relatively easy pathway for nasal infections to spread to the meninges of the brain. Studies suggest that environmental toxins can invade the receptor cells and reach the brain through this route, the basis for the theory that aluminosilcates enter the brain through the olfactory pathway and contribute to the development of Alzheimer's disease.
In other words, the response to scent is a real, mechanical, electrochemical process in which odors are converted into a cascade of messages, amplified by the olfactory bulb, transmitted along the olfactory tract and sent to the center of the brain, the limbic system.(formerly called the rhiencephalon, or smell brain). It's not just in your mind.