HOW DOES THE SENSE OF SMELL WORK
The sense of smell, known as olfaction, uses a sensory organ called the nose to pass scent information on to the olfactory cortex in the brain. Diffuse suspensions of relevant molecules, called odors, are analyzed by the nose using a molecular lock-and-key scheme whereby odors are identified by their unique chemical signatures. The sense evolved as a means of detecting survival-relevant information about the external world, especially appraising food. Smell is the oldest of the senses, with analogues dating all the way back to the first animals 600 million years ago. One of the five primary senses, smell is most intimately associated with the formation of memories. Olfactory receptor neurons, the cells responsible for smell, are located on a 1-by-2 inch strip of tissue called the olfactory epithelium, located about 3 inches above and behind the nostrils. The human olfactory epithelium is about 16 cm², contrast with some dogs which have 150 cm². With advanced color vision, sight can be
Smell receptors in the innermost parts of the nose bond to gas molecules from the air. The receptors then send electrical signals to the olfactory bulb, which signals the orbitofrontal cortex, where the firing pattern reveals to the rest of the brain what smells so nice (or bad) out there. Most receptors (though not all, according to preliminary research in Don Leopold’s lab) are arrayed in two dime-sized patches, one per nostril. The genetics and biochemistry of the system are a hot field of study, and much of what’s known is new. The first gene for a smell receptor was discovered as recently as 1991, by a team from Columbia. That triumph broke the subject wide open. One major surprise, discovered in 1992, is how very many receptor types take part in smell–500 to 1,000, researchers infer from genetic evidence. By contrast, color vision uses three receptor types, taste four or five, and hearing only two. Several hundred receptors sounds like a lot, except that the world has thousands
