Olfaction
Just like taste, olfaction [5, 133, 141] is a sense whose stimuli are chemical in nature. It is through this sense that we can detect and analyze volatile chemical substances present in the air, these molecules are what we perceive as odors.
While this sense is of vital importance in other species, in humans, its significance is secondary. Indeed, in humans, the sense of smell is less developed compared to other animals [5] and is subject to great subjectivity, making it difficult to study.
Reception :
The receptor organ for olfaction is located in the upper part of the nasal cavities. This is the olfactory mucosa [94, 130], it contains olfactory sensory cells - 10 million in humans compared to 200 million in dogs for reference [141]. These cells are bipolar neurons equipped with olfactory cilia; they have the capacity to detect odors at the tips of their dendrites.
Olfactory neurons are unique: they continue to proliferate in adults (neurogenesis) [4, 116]. The lifespan of a primary olfactory neuron is approximately 30 to 60 days [99].
We are able to smell between 4,000 and 10,000 different odors [5] ; each odor preferentially activates a specific group of receptor neurons [5]. The mucus of the olfactory mucosa captures molecules that bind to receptor proteins on the ciliary membrane of the olfactory receptors. This binding triggers a cascade of biochemical reactions that ultimately depolarize the membrane and lead to the generation of an action potential [38, 41].
Transmission :
The axons of the receptor neurons cross the cribriform plate of the ethmoid bone, forming the olfactory nerve [41, 50, 116].
The olfactory nerve is a very peculiar nerve. First, it is the shortest nerve, as its length barely exceeds the thickness of the cribriform plate; second, it does not have an anatomically solid and firm structure like the rest of the body's nerves, but is instead a collection of nerve fiber bundles that traverse the cribriform plate at various points.
Some authors confuse the olfactory nerve with the olfactory tract [83] between the bulb and the olfactory striae, whereas the majority of authors agree that the olfactory nerve is simply the set of nerve fibers between the olfactory mucosa and the olfactory bulb [116].
The olfactory bulb [4, 38, 41] is characterized by the presence of glomeruli [38, 96] - spherical structures containing the synaptic junctions of receptor neurons-, mitral cells (the main relay neurons of the olfactory bulb) [5], and local interneurons. There are approximately 1,000 glomeruli in the olfactory bulb [5], each averaging 25,000 synaptic junctions.
Olfactory neurons that possess the same affinity for a specific odor group together in the same glomerulus, where they synapse with the relay mitral cells. These pass through the olfactory tract [116] and then the lateral olfactory stria [45], terminating directly in the piriform and prepiriform cortex (primary olfactory cortex) [50] without first relaying through the thalamus.
Perception :
From the primary olfactory cortex, several fibers project to the hypothalamus, thalamus, amygdala, hippocampus, and orbitofrontal cortex [38].
Fibers projecting to the limbic system (the hippocampus and amygdala in particular) trigger emotional reactions and induce the formation of memories.
Certain odors, such as those of smoke, gas, or skunk, stimulate the sympathetic nervous system.
Appetizing smells stimulate salivation, while unpleasant odors trigger defense reflexes such as sneezing, choking, or vomiting.