Sensory Systems/Olfactory System/Pheromones and Vomeronasal System

Pheromones
Pheromones are a distinct class of species and gender specific chemical cues that provide information about sexual and social status. These airborne chemical signals are released by individuals into the environment. Pheromones can influence the physiology and behavior of other members of the same species, and play a crucial role in various biological processes, including communication, reproduction, territorial marking, and social organization. Indeed, there are alarm pheromones, food trail pheromones and sex pheromones.

For example, in adult male silkmoth (Bombyx mori), the antennae act as a sensing organ both for odors and sex pheromones. When the latter bind, it evokes courtship behaviour. .

It is important to note that while pheromones can have significant effects on individuals within a species, they generally do not cross species boundaries. Each species has its own unique set of pheromones that are specific to their reproductive and behavioral needs.

Vomeronasal System
The system allowing the perception of pheromones in mammals is called the vomeronasal system or Jacobson-organ. The sensory neurons found in the vomeronasal organ (VNO) contain cell bodies that house receptors capable of detecting pheromones from the surrounding environment. Although very close and similar, this system is independent from the olfactory system. In fact, it projects to a separate bulb, called the accessory olfactory bulb, and from there to the hypothalamus via the vomeronasal amygdala. It also has a distinct class of genes, the vomeronasal receptors (V1R and V2R), along with Trp receptors. Those genes sustain the production and maintenance of the vomeronasal system receptors and proteins, allowing its functioning as a whole system. For example, it has been shown that Trp2 knockout mice males do not present the territorial behaviour (urine marking), a normal behavior provoked by pheromones contained in the urine of other males: by inactivating the vomeronasal organ, the mice changed behavior. In addition, this shows that Trp2 channel is required in the vomeronasal organ to detect male specific pheromones and elicit an aggressive, territorial behavior in mice.

In humans it is yet still unclear if the behaviour is affected by pheromones. There is an extensive ongoing debate between experts. There are VR genes in human genome, but they seem to be non-functional. We even have an embryonic structure that resembles the vomeronasal organ and a foetal accessory olfactory bulb (AOB) but it regresses with growth. In addition, primates have responses that can be attributed to pheromone or pheromone-like-hormones. As of today, these issues are still unresolved.