Smelling: Its More Than Meets the Olfactory Epithelium :: Biology Essays Research Papers

Smelling: It's More Than Meets the Olfactory Epithelium The ability to take a chemical sample of the environment and interpret that sample has long been a skill of earth dwelling life forms. We don't tend to think of the sense of smell as a mechanism that analyzes physical specimens. It is sort of a repulsive notion, considering some of the undesirable substances we are forced to smell every day. But, just as we cannot feel a book without touching it, we cannot smell an orange without guiding some orange molecules up our noses. The capacity that humans divide into smell and taste has a single evolutionary precursor. This was a common chemical sense that enabled single-celled organisms to identify food and alert themselves to the presence of harmful substances. While it is among the oldest and most universal senses employed by living creatures, science has been slow to understand it. One roadblock to knowledge progression has been the inherent difficulty in experimenting with the chemical senses. Delivering precisely timed and quantitative ly accurate amounts of chemical stimuli to receptors is a technological challenge, but often necessary in the study of olfaction (1). While in many animals, the chemical senses play the most important role in perception and survival, in humans they are less involved in behavior than sight or hearing. This relative insignificance is another reason why olfaction has received scientific short shrift. It has been comparatively neglected by our culture as well--the English language does not include a sufficient vocabulary for describing odors. It is very difficult to verbally depict an odor to an individual who has never encountered it. Our understanding of smell is ever increasing, and while some big questions remain, others are continually being answered. The olfactory epithelium of each of the two nasal passages in humans is a 2.5 square centimeter patch containing about 50 million sensory receptor cells (3). The reception of the odorant and the beginning of sensory signal transduction occurs in the olfactory cilia, which are hair-like extensions of the receptor neurons (10-20 cilia per neuron). The neurons have a turn-over rate of about 40 days (3). On the opposite side of the cilia, within the epithelium, the neurons form axons which penetrate the cribiform bone in bundles and synapse with neurons in the olfactory bulb (2). Via the olfactory tract, (cranial nerve I), olfactory information travels to the primary olfactory cortex without first passing through the thalamus.