Olfactory System: Reception, Transduction And Brain Pathways
The sense of smell of animals, which works in conjunction with the sense of taste, fulfills very relevant functions: it detects the presence of food and provides information about the possible consequences of its consumption, contributes to the physiological changes of digestion and even provokes responses. towards members of the same species through pheromones.
In this article we will describe the main aspects of the olfactory system, both at a structural and functional level. To do this, we will review the process of perception of olfactory stimuli, from reception in the sensory neurons of the nasal cavity to cognitive processing in the orbitofrontal cortex.
The nose, structured from the ethmoid bone and cartilaginous tissue, is the external part of the human olfactory system. The openings of the nasal passages allow odor molecules (also known as “odorants”) to reach the olfactory epithelium, which is located at the top or roof of the nasal cavity, when we inhale air.
The olfactory epithelium is composed of layers of tissue covered with mucus, which is found throughout the nasal cavity and has the function of dissolving odor molecules and retaining particles potentially dangerous for the lungs. It is here, in the mucus layer of epithelial tissue, that they are found odorant receptor cells
These cells are bipolar neurons specialized in receiving chemical compounds. This function takes place at the apical pole of the neuron, while the opposite end, the basal pole, synapses with the olfactory bulb through the bone known as the cribriform plate, which is located at the base of the brain.
The olfactory bulbs are located in the brain itself, in the lower part of the frontal lobes. These structures are part of the central nervous system, so signals from the olfactory system do not need to pass through the thalamus, the “relay station” for the rest of the sensory stimuli, to reach the primary cortex.
More than a thousand different types of neurons that receive odor molecules have been found since the receptors are highly specialized, so that each of them transmits information about a single type of odorant.
Information Transduction
Sensory transduction begins when airborne odor molecules are inhaled and dissolve in the mucus of the nasal cavity. Once this has happened, the receptors located at the apical poles of the olfactory neurons detect the odorants.
When the cilia of the metabotropic receptor capture and retain an odorant molecule, a second messenger system is activated that depolarizes the neuron. This triggers action potentials from the cell body that will be transmitted through the axon.
As we have said, the axons of olfactory neurons synapse with the dendrites of neurons located in the olfactory bulb This allows the indirect connection between the olfactory epithelium and the cerebral cortex.
Odorant receptor neurons establish connections with three different types of neurons in the medulla: mitral and tangle cells which project olfactory signals to higher brain regions, and periglomerular inhibitory interneurons, which modulate the function of the other two types.
The main olfactory system
There is an anatomical and functional division between the main and accessory olfactory system, also known as vomeronasal. As its name indicates, the main olfactory system is more relevant to the perception of odors than the vomeronasal system, although it plays characteristic roles.
The main system begins in mitral and tuft cells of the olfactory bulb that send projections to the rhinencephalon, a term used to refer to the regions of the brain related to smell. The piriform cortex, which is located in the medial part of the temporal lobe is especially important in this sense.
From these areas, olfactory information is transmitted to the dorsomedial nucleus of the thalamus, from where it will reach the orbitofrontal prefrontal cortex. In this region, responsible for decision-making and emotional processing, the perception and discrimination of odors takes place.
The orbitofrontal cortex also receives gustatory stimulation ; Along with smell, this allows the perception of flavors. Sometimes we speak of the “chemosensory system” to refer jointly to the senses of smell and taste, which are very close from a neurofunctional point of view.
The accessory olfactory or vomeronasal system
Unlike the main olfactory system, the vomeronasal system contains only mitral cells. These are located in a differentiated region of the olfactory bulb: the vomeronasal organ, which is also called the “accessory olfactory bulb” and is located at the base of the ethmoid.
These neurons do not project signals to the neocortex, but to the amygdala and hypothalamus. The amygdala is related to learning emotions, especially negative ones, while the hypothalamus is the key structure in the release of hormones, which is why it intervenes in basic functions such as thirst, hunger, sexuality or temperature regulation. .
The vomeronasal system is related to behaviors and physiological responses that occur due to interaction with members of the same species. It has a fundamental role in the reproduction, aggressiveness and social behavior of many animals, but It is not clear that it is still functional in humans
When talking about the accessory olfactory system, it is worth highlighting the role of pheromones, chemical compounds secreted by living beings that are only captured by animals of the same species and are perceived through the vomeronasal organ.
