The viscosity, that is, the, uh, stickiness (OK, I know this is a little gross, but bear with me) of the mucous layer also plays a role in how well odors are absorbed. A healthy mucous layer is a warm, wet, gushy place where odor molecules can be easily absorbed and identified. This is important because odor molecules are best identified when they are dissolved in a liquid. If the mucous layer is dry, it is difficult for odors to be absorbed. If the mucous layer is too thick, then there is a greater distance across which the odor molecules need to pass in order to reach the olfactory cilia, the primary receptors for odors. The olfactory membrane of the mucous layer contains pigment molecules, which are do not exist in other areas of mucous in the nasal cavity. These pigment molecules contain colored compounds which give the molecules a yellowish color. Although scientists aren't entirely sure what these pigments do, they recognize that they must play some role in smell, since albino animals don't possess these pigments, and also have no sense of smell. Within the mucous layer are odor-binding proteins which attach to the odor molecules. Once attached, an electrical discharge by the cilia sends them to the olfactory receptor cells. What is interesting about these neural cells is that they regenerate, unlike any other of our nerve cells. This makes sense when you think about the fact that our noses smell lots of funky stuff, including harmful odors, such as ammonia and gasoline. Once damaged, a new cell replaces the old one. Each cell lasts about four to five weeks. Anyway, these cells, directed by primary olfactory neurons, carry the odor message through the ethmoidal bone (a perforated bone in the nasal cavity) to an area of the brain the size of a matchstick head, called the olfactory bulb (of which they're are two—one on each side of the nasal cavity). Up in the rhinencephalon (from the Greek meaning "smell in the head") is where things get a wee bit complicated (as if things weren't complicated enough!). The rhinencepahlon collectively refers to the area of the head where the olfactory bulb connects to the brain and the limbic system. Imagining a telephone network may help. Here, all of the primary neurons (about ten million little buggers) meet at hubs called glomeruli (there are 2,000 glomeruli), which are like little switchboards with lots of lines coming in. From each of these switchboards are dispatched 24 secondary neurons which travel up to the brain in a series of on/off switches like a computer. The iterations of these on/off combinations are seemingly endless, though in fact there are about 16 million combinations, so theoretically, we are capable of distinguishing between 16 million different odors! Once an olfactory signal is sent to the brain, it is able to confirm that what looks like a lemon actually smells like one, so it must be a lemon. Speaking of food... |
