The sense of smell of animals, which works in conjunction with that of taste, has very relevant functions: it detects the presence of food and gives information about the possible consequences of its consumption, contributes to physiological changes in 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. We will review the process of perception of olfactory stimuli, from their reception in the sensory neurons of the nasal cavity to the cognitive processing in the orbitofrontal cortex.

The olfactory receptors

The nose, structured from ethmoid bone and cartilage tissue, is the external part of the human olfactory system. The openings of the nostrils allow odorous molecules (also known as “odorants”) to reach the olfactory epithelium, which is located at the top or roof of the nasal cavity, when air is inhaled.

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 to the lungs. It is here, in the mucus layer of the epithelial tissue, that the receptor cells for odorous molecules are found.

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, synapses with the olfactory bulb through the bone known as the sieve lamina, which is located at the base of the brain.

The olfactory bulbs are located in the brain itself , at the bottom 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 all other sensory stimuli, to reach the primary cortex.

More than a thousand different types of neurons have been found to receive odorous molecules since the receptors are highly specialized, so that each one transmits information on a single class of odorant.

Transduction of information

Sensory transduction is initiated when airborne odor molecules are inhaled and dissolved in the mucus of the nasal cavity. Once this has happened, receptors located at the apical poles of the olfactory neurons detect the odorants.

When the receptor cilia, of the metabotropic type, capture and retain an odorous molecule, a system of second messengers 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 bulb neurons: mitral and spiral cells , which project olfactory signals to higher regions of the brain, 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 olfactory system and the accessory , also known as vomeronasal. As the name suggests, the main olfactory system is more relevant to the perception of odours than the vomeronasal, although the latter plays characteristic roles.

The main system starts in mitral cells and in the olfactory bulb’s ball that send projections to the rhinencephalon, a term used to name 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 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, which is responsible for decision-making and emotional processing, the perception and discrimination of odours takes place.

The orbitofrontal cortex is also stimulated by taste ; together with smell, this allows the perception of flavours. Sometimes we speak of a “chemosensory system” to refer jointly to the senses of smell and taste, which are very close from a neurofunctional point of view.

The accessory or vomeronasal olfactory system

Unlike the main olfactory system, vomeronasal contains only mitral cells. These are located in a differentiated region of the olfactory bulb: the vomeronasal organ, which is also called “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 the learning of emotions, especially negative ones, while the hypothalamus is the key structure in the release of hormones, so 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 because of interaction with members of the same species. It plays a fundamental role in the reproduction, aggressiveness and social behaviour of many animals, but it is not clear that it is still functional in humans .

When talking about the accessory olfactory system, 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, should be highlighted.