The Vomeronasal Organ (VNO), also known as Jacobson’s Organ, is a specialized chemosensory structure providing animals with sensory input distinct from the main system responsible for smell. While the primary olfactory system detects volatile, airborne chemical molecules, the VNO senses non-volatile chemical signals often carried in liquids or moisture. This organ is integral to the accessory olfactory system, which processes specific chemical cues that govern instinctual behaviors. Understanding the VNO’s structure and function reveals a unique biological mechanism for chemical communication.
Anatomical Identity and Location
The Vomeronasal Organ typically exists as a pair of small, blind-ended tubes or sacs located near the nasal septum. This structure is situated at the base of the nasal cavity, close to the roof of the mouth in many mammals. The name “vomeronasal” comes from its proximity to the vomer bone, which forms part of the nasal septum.
The organ’s lumen, or internal cavity, is lined with specialized sensory neurons distinct from standard olfactory neurons. These unique sensory cells possess microvilli that project into the fluid-filled lumen for chemical detection. In some species, a narrow duct connects the VNO to the nasal passage or the oral cavity, allowing chemical substances to enter. This distinct cellular makeup means the VNO processes sensory information differently and sends signals to a separate brain region called the accessory olfactory bulb.
The Mechanism of Detection
The fundamental difference between the VNO and the main olfactory system lies in the types of chemicals detected and the manner of signal delivery. The main system detects volatile, vaporized chemicals inhaled with air, which is commonly understood as smell. Conversely, the VNO specializes in detecting non-volatile, often larger chemical compounds dissolved in liquid, such as urine, saliva, or bodily secretions. Since these compounds cannot easily become airborne, animals must employ specialized behaviors to actively transport them into the VNO.
A common behavioral mechanism for delivering these liquid-borne chemicals is the “Flehmen response,” observed in mammals like horses, cats, and cattle. During this action, the animal curls back its upper lip, drawing the chemical stimulus through a duct and into the organ’s lumen. Inside the VNO, two families of G protein-coupled receptors, V1R and V2R, are responsible for binding these specific chemical cues. These receptors are distantly related to the main olfactory receptors, underscoring the VNO’s separate sensory pathway. The VNO’s sensory neurons utilize a distinct signaling cascade involving the TRPC2 ion channel, differentiating its signal transduction from the main olfactory system.
Role Across the Animal Kingdom
The VNO plays a central role in the social and reproductive biology of non-human species by detecting pheromones. Pheromones are chemical signals released by one individual that influence the behavior or physiology of another member of the same species. This specialized detection system allows for rapid, instinctual responses to chemical communication cues.
In reptiles, such as snakes and lizards, the VNO is well-developed and aids in foraging and identifying prey. Snakes flick their forked tongues to collect chemical particles, which are then delivered to the VNO openings located on the roof of their mouth. Among mammals, the VNO is involved in regulating reproductive behavior, signaling sexual readiness, and identifying potential mates. Rodents use VNO signaling to recognize social status, mark territory, and exhibit innate aggressive or mating behaviors. The VNO’s ability to detect these species-specific chemical messages ensures coordinated social responses necessary for survival and reproduction.
The Human VNO Debate
The question of whether humans possess a functional Jacobson’s Organ has been a subject of ongoing debate. Anatomical studies frequently identify small pits or depressions on the anterior portion of the human nasal septum, corresponding to the VNO location found in other mammals. These structures are histologically present in most adults, often as a small, tube-shaped canal opening into the nasal cavity.
Despite the presence of this anatomical structure, the scientific consensus suggests the organ is vestigial and non-functional in adult humans. Molecular and anatomical evidence indicates that the necessary components for VNO function are largely absent or mutated. Humans specifically lack the specialized sensory neurons that express the functional V1R and V2R receptor proteins required for pheromone detection. Furthermore, the accessory olfactory bulb, the distinct neural structure that processes VNO signals, typically regresses during human fetal development. While some studies suggest physiological responses to certain chemicals in the VNO region, current anatomical and genetic data suggest that any structure found in adults is non-operational for pheromone communication.