Olfactory system: reception, transduction and brain pathways

The sense of smell of animals, which works in conjunction with that of taste, fulfills very important functions: it detects the presence of food and gives 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 the 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.

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Olfactory receptors

The nose, structured from the ethmoid bone and cartilaginous tissue, is the external part of the human olfactory system. The openings of the nostrils allow that when we inhale air the odorous molecules (also known as “odorants”) reach the olfactory epithelium, which is located in the upper part or roof of the Nasal Cavity.

The olfactory epithelium is composed of layers of tissue covered with mucus, which is found throughout the cavity nasal and has the function of dissolving odorous molecules and retaining potentially dangerous particles for lungs. It is here, in the mucus layer of the epithelial tissue, where they are found receptor cells for odorous molecules.

These cells are bipolar neurons specialized in the reception of chemical compounds. This function takes place at the apical pole of the neuron, while the opposite end, the basal pole, synapta with the olfactory bulb crossing the bone known as the lamina cribrosa, 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" of the rest of sensory stimuli, to reach the primary cortex.

More than a thousand different types of receptor neurons for odorous molecules have been found since the receivers are highly specialized, so that each of them transmits information of a single kind of odorant.

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Information transduction

Sensory transduction begins when airborne odorous 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 metabotropic receptor cilia capture and retain an odorous molecule, a second messenger system is activated that depolarizes the neuron. This causes action potentials to be fired from the cell body that will be transmitted through the axon.

As we have said, the axons of olfactory neurons synapt with dendrites of neurons located in the olfactory bulb. This allows the indirect connection between the olfactory epithelium and the cerebral cortex.

Odorant receptor neurons make connections with three different types of neurons in the bulb: mitral and ball cells, which project olfactory signals to higher regions of the brain, and inhibitory periglomerular interneurons, which modulate the function of the other two types.

The main olfactory system

There is an anatomical and functional division between the main olfactory system and accessory, also known as vomeronasal. As its name indicates, the main olfactory system is more relevant for the perception of odors than the vomeronasal, although it plays characteristic roles.

The main system begins in mitral cells and in the ball of the olfactory bulb that send projections to the rhinencephalon, a term used to describe the brain regions related to the sense of smell. The piriformis cortex, which is located in the medial part of the temporal lobe, is especially important in this regard.

From these areas the olfactory information is transmitted to the dorsomedial nucleus of the thalamus, from where it will reach the orbitofrontal prefrontal cortex. In this region, in charge of decision-making and emotional processing, the perception and discrimination of odors takes place.

The orbitofrontal cortex also receives taste-like stimulation; Along with smell, this allows for the perception of flavors. Sometimes we speak of the "chemosensory system" to refer together to the senses of smell and taste, which are very close from a neurofunctional point of view.

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The accessory olfactory or vomeronasal system

Unlike the main olfactory system, the vomeronasal 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 the regulation of temperature.

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.