accessory olfactory bulb

Meaning: The accessory olfactory bulb is a small structure next to the main olfactory bulb. It receives input from the vomeronasal organ, a chemosensory patch in the nose of many mammals. It processes pheromones, which are chemical signals released by one animal that influence the behavior or physiology of another of the same species, as in mating or territorial behavior. Its output goes mainly to the amygdala and hypothalamus, which are involved in emotion, reproduction and hormone control. In humans, most accounts treat the vomeronasal organ and accessory bulb as vestigial or nonfunctional in adults. Other researchers argue that humans still respond to pheromone-like cues, possibly through the main olfactory system.

On the MCAT: A passage on pheromones or animal behavior may ask which structure carries those signals, or why chemical cues can shift mood or reproductive behavior. You may also compare pathways. Ordinary odorants bind receptors in the olfactory epithelium and travel to the main olfactory bulb, then to the olfactory cortex and limbic areas. Unlike most other senses, this route does not first relay through the thalamus.

Common confusion: Students often treat the accessory bulb as part of how humans smell everyday odors, or as proof that humans detect pheromones. The main olfactory system handles ordinary smells. The accessory bulb is a separate pheromone pathway that is clearly functional in many other mammals, while its role in humans is debated.