Vomeronasal Organ

The vomeronasal organ (VNO), or Jacobson's organ, is an auxiliary olfactory sense organ that is found in many animals. It was discovered by Frederik Ruysch and later by Ludwig Jacobson in 1813. This organ is the sense organ involved in the flehmen response in mammals.

The VNO is the first stage of the accessory olfactory system, and contains sensory neurons that detect chemical stimuli. The axons from these neurons project to the accessory olfactory bulb, which targets the amygdala and bed nucleus of the stria terminalis, which in turn project to the hypothalamus.

The vomeronasal organ is mainly used to detect pheromones, chemical messengers that carry information between individuals of the same species. As with other olfactory systems, chemical messages are detected by their binding to G protein-coupled receptors. The neurons in the VNO express receptors from 3 families, called V1R, V2R, and FPR. The receptors are distinct from each other and from the large family of receptors in the main olfactory system. Stimuli reach the VNO in liquid phase via a pumping mechanism; the primary cues for the VNO are therefore non-volatile and require direct physical contact.

Its presence in many animals has been widely studied and the importance of the vomeronasal system to the role of reproduction and social behavior (through influence on anterior hypothalamus) has been shown in many studies. Its presence and functionality in humans was controversial, though most studies agree the organ regresses during fetal development. Many genes essential for VNO function in animals (such as TRPC2) are non-functional in humans. Chemical communication does appear to occur among humans, but this does not necessarily imply that the human vomeronasal organ is functional.