MadSci Network: Anatomy |
Rachael: Wow, that is a big question. I’m going to answer it pretty generally and give you some references so you can carry on at your own pace. Olfaction, the sense of smell, involves the detection and perception of tiny odor molecules (odorants). The odorant gets inhaled through the nose (or via the oral cavity when eating) and moistened on its way up the nasal cavity. Once there, it hits the olfactory epithelium, which is a layer of sensory nerve cells (neurons) specifically designed to detect odorants. Olfactory neurons have cilia, little hair-like structures that have specific receptors for odorants. The upshot of this is that one neuron may detect one type of odorant (e.g. a floral smell), while another may detect a completely different one (e.g. vinegar). Think of it being similar to taste, in that different parts of your tongue detect sweet, salty, sour and bitter. The olfactory epithelium neurons then send signals to the olfactory bulb, which lies above and behind the nasal cavities. The olfactory bulb is organized into groups of neurons called glomeruli (think about your tongue again). From there, smell signals are sent to the rest of your brain, so you can act on the smell. Humans don’t compare very well with many other animals when it comes to olfaction, but the average person can perceive about 1000 different odorants. Remember that olfactory acuity varies a lot between people, so there is a large range of what is considered normal. Some people, like perfumers, who develop scents, have trained themselves to perceive about 5000 different odorants. As for disorders of olfaction in humans, the most common is specific anosmia (anosmia meaning have no sense of smell), whereby the person has a lowered ability to perceive a specific odorant, but normal ability to perceive others. This is probably due to a lack of the specific receptor on the cilia. It’s estimated that up to 20 % of people may have some sort of specific anosmia. General anosmia is a complete loss of smell and may result from infections or head trauma, or may be congenital. Parkinson’s disease sufferers may experience alterations in their olfactory capability. Olfactory hallucinations (particularly of bad smells) can also occur after epileptic seizures. Olfaction in gender and age. It’s generally accepted that women are better at identifying odors, at all stages of life. There hasn’t been a whole bunch of studies on this though. Have a look at http://zebra.biol.sc. edu/smell/ann/myth1.html which addresses sex differences and http://zebra.biol.sc. edu/smell/ann/myth2.html which addresses whether olfactory ability declines with age (the general feeling is we don’t know very well yet). Check out http://www.cf.ac.uk/biosi/staff/jacob/teaching/sensory/olfact1.html for a tutorial on the sense of smell. If you want books, any standard textbook on physiology (e.g. Physiology by Berne and others) or neuroscience (e.g. Principles of Neural Science, by Kandel and others) will have a section on olfaction (and it won’t be nearly as big as the section on vision!!)
Try the links in the MadSci Library for more information on Anatomy.