In the back of the nasal cavity is a patch of tissue called the olfactory epithelium. The olfactory epithelium is populated with neurons called olfactory receptors. Each receptor has cilia, small eye-lash-like structures extending from the neurons, that are in direct contact with the air passing through the nasal cavity. Extending from the cilia are proteins that bind to molecules in the air, which acts as a chemical stimulus. This chemical stimulus generates an electric signal. There are many different proteins extending from the cilia, and each protein will only bind with certain chemicals.

When the electric signal from the proteins reaches a threshold level, the neuron fires sending a signal along the olfactory nerve to the olfactory bulb, a part of the brain where interpretations of the smell begins. This is when things get complicated. The olfactory bulb acts as a relay station passing signals to the olfactory cortex in the brain, but before this happens, the olfactory bulb has already started relating the signal to past experiences with the substance inhaled. Additionally, which neurons fire and how many sets a pattern the olfactory bulb can use to identify particular odors. From this point on, everyone’s sense of smell differs due to past experiences, emotions, and memories of emotions. The olfactory cortex further processes the input from the olfactory bulb before sending the signal on to the central nervous system, which controls emotions and behavior, as well as basic thought processes.

From the olfactory cortex, signals are then sent to the amygdala, which is involved in the expression and experience of emotion, the hippocampus, which is involved with associative memory, and the hypothalamus, which is involved with the formation of memories. The olfactory cortex has a shorter synaptic path to these areas than any other sense, which is probably why associations between odors and emotions are so readily formed. So, it would seem that whether an odor is pleasant or unpleasant would be greatly influenced by associated memories and experiences tied to that particular odor.

Another important aspect to an odor that is often overlooked is that most odors have a ‘feel’ to them as well. For example, menthol feels cool, ammonia burns, etc. These feelings come from the trigeminal nerve, a nerve that runs through the face and nose that is used to convey sensations from the face to the brain. The trigeminal nerve is responsible for your eyes tearing up when cutting onions and sneezing when you smell pepper. Almost all odors have a trigeminal component, varying from mild to intense. For example, sweet rose is a mild stimulation, while acetone is an intense stimulation. Intense stimulations of the trigeminal nerve can be uncomfortable to irritating and even painful. It is often difficult to distinguish between the olfactory and trigeminal senses, so they are usually associated as one.

This would seem to explain why some people actually like the smell of ammonia and some don’t. In short, ammonia smells the way it does because of which neurons the ammonia molecules bind with and the memories and emotions you associate with that particular pattern, and it burns because of the intense trigeminal stimulation.

AMMONIA STINKS, SAFETY DOESN‘T

Today Jeremy Williams of GCAP brings another R-717 Video Tip of the Day.  Question is: Where Does Ammonia Go When the System is Off?  This videocast is sponsored by P1 Group!