Sixth Sense: The Vomeronasal Organ
"We are all more influenced by smell than we know." (Hercule Poirot)
....Murder in Retrospect, Agatha Christie
Biologists have long realized that the noses of most vertebrates actually contain two sensory channels. The first is the familiar olfactory system, which humans possess. The second channel is the vomeronasal complex, a system that has its own separate organs, nerves, and connecting structures in the brain. The function of the vomeronasal system is the detection of pheromones, chemical messengers that carry information between individuals of the same species. It was widely believed (as I found in some of the older texts I examined) that humans had long ago discarded this sensory system somewhere along evolution's trail. But convincing behavioral and anatomical evidence has since brought the notion of a human vomeronasal organ (VNO) into the realm of scientific fact. Some thirty years ago, when anatomist David Berliner was studying human skin composition using scraped skin cells from the insides of discarded casts, he found that when he left vials containing skin extracts open, his lab assistants would become more friendly and warm than usual (1). When, months later, he decided to cover the vials, the warm and relaxed behavior was noticeably reduced. These findings led him to investigate the possible existence of odorless human pheromones and a "sixth sense" organ to detect their presence, a VNO.
While this early evidence was not empirical, anatomists have since found that all humans display two tiny pits, with duct openings, on both sides of the septum just behind the opening of the nose (3). The duct leads into a tubular lumen lacking a thick, distinct sensory epithelium. However, there are cells in the lining of the lumen that may be VNO receptor neurons. They appear to be bipolar neurons and respond to neuron specific stains (2). In a recent experiment, human VNO was reported to respond positively (by emitting electrical signals) to puffs of air laden with substances claimed to be human pheromones (2). If the experiment is valid, it presents strong evidence supporting the hypothesis that the human VNO is functioning, not vestigial. In some respects, however, the proof is lacking. The human VNO lacks the characteristic capsule and large blood vessels of other mammals' VNOs (2). The sensory epithelium, as mentioned earlier, is not well developed. In addition, connections between the presumed VNO receptor neurons and the brain have not yet been confirmed in humans. In other mammals, nerve impulses from the sensory cells of the vomeronasal organ enter brain structures known as the accessory olfactory bulbs and also project to brain structures that regulate sexual behavior and the secretion of gonadotropin, a pituitary hormone regulating the function of the testes (4). The accessory olfactory bulb, the normal termination of vomeronasal receptor-neuron axons (i.e. the doorway to the brain for...