Vomeronasal System

Some animals have sensory systems that humans lack completely, making it difficult for us to even imagine what their perceptual worlds are like. For example, many fish can detect electric fields in the water around them, including, in some cases, electric fields produced by other members of their own species. Other animals have sense organs that detect stimuli outside the ranges that humans can detect, such as insects that can see ultraviolet or polarized light or bats that can hear extremely high frequencies that we call “ultrasound.” The vomeronasal system, or accessory olfactory system, is a sensory system lacking in humans, but [Page 1135]present in most tetrapods (amphibians, reptiles, and mammals). Although our understanding of the sense of smell has undergone a revolution in the past two decades, the vomeronasal system is still so poorly understood that we don't even know if it functions like an extra sense for animals that have it, or simply extends the range of odorants that can be detected by their noses.

The vomeronasal system is physically separate from the rest of the olfactory system: The two systems have discrete sense organs, the sensory epi-thelia and receptor neurons inside the organs are different, and the axons of the receptor neurons project to distinct regions of the forebrain. In mammals, the vomeronasal organs are a pair of cigar-shaped organs located between the floor of the nasal cavity and the roof of the mouth. In reptiles, they are often called Jacobson's organs and vary considerably in size, shape, and location; for example, in most snakes the vomeronasal organs are roughly spherical, large, and are located above the roof of the mouth. In contrast, in some turtles the vomeronasal organs are little more than grooves in the floor of the nasal cavity. In salamanders, the vomeronasal organs are generally pouches that extend from the outer edge of the nasal cavity; and in frogs, the organs are usually separate from and below the nasal cavities.

Overall, vomeronasal receptor neurons look like other olfactory receptor neurons, except that the tips of the dendrites, where the cells contact odorants, are covered in tiny hairs called microvilli. The receptor neurons in the olfactory epithelium are more diverse, and their dendrites can be covered in larger hairlike cilia, or microvilli, or both. Both the receptor molecules and the ion channels that open when an odorant is detected differ completely between vomeronasal and other olfactory receptor neurons. These disparities in appearance and biochemistry suggest that vomeronasal receptor neurons detect different odorants, or respond differently to odorants, than do other olfactory receptor neurons.

The receptor neurons in the vomeronasal organ have long axons that project to the accessory olfactory bulb in the forebrain. The accessory olfactory bulb is generally located behind the main olfactory bulb, which receives input from the receptor neurons in the olfactory epithelium. The types of neurons and circuits they form differ between the accessory and main olfactory bulbs, again suggesting that the vomeronasal system performs a different function than the rest of the olfactory system.

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