Olfaction is not one of the topics most people look forward to with bated breath when going into a neuroscience course; at least, that hasn’t been my experience. Most people (often myself included) are looking out for the trendier stuff — consciousness, phantom limbs, schizophrenia, hallucinations. But it should come as no surprise that the neuroscience of olfaction, or the sense of smell, is both a hotbed of current research and a fascinating area of study. The 2004 Nobel Prize in Physiology or Medicine was awarded to Linda Buck and Richard Axel for their discovery of the family of genes (about 1,000 genes total) which code for olfactory receptors in humans.
The human olfactory system is thought to have around 400 different receptor types for odorant molecules. These receptor types each react to a small variety of molecules, making the different combinations of molecules, and thus the different smells perceived, much greater than the 400 receptor types. Each receptor type is expressed on tissue inside the nose called the olfactory epithelium, which is covered in a layer of mucus through which cilia, or hair-like cells, protrude. The olfactory receptors are expressed on these cilia; this is where the molecules from the odorant (the source of the smell) first interact with the nervous system.
When a receptor (on a cilia) is activated by its respective odorant molecule, a signal is sent upward to a glomerulus (see above picture). Each glomerulus receives inputs from one single type of odorant receptor (the color-coding above should explain that better than words can). The cells pass through tiny holes in a bone known as the cribiform plate (the sponge-like stuff in the middle of the picture).
From the glomeruli, the signals are passed on to various areas of the brain, including the amygdala, which explains the highly emotional character that olfactory memories can have. Since this is a pathway with a few different steps, there are many ways in which a person’s sense of smell can be disrupted or disturbed.
In one case, the cribiform plate (which is very thin in reality, unlike its representation in the picture above) can shear the pathway between the sensory neurons on the glomeruli, for instance when the nose is broken. Luckily, these pathways can be regrown, and sense of smell can be restored, although it may be permanently altered.
In other cases, sense of smell can be lost or disrupted genetically, and tends to decrease even in normal, healthy aging. Recent research on mice has shown a promising genetic pathway to fix the loss of smell for genetic reasons — for instance, in the case of people who were born with anosmia, or lack of a sense of smell. Gene therapy is a promising avenue for disorders of the olfactory system; however, it probably won’t take its place in human research and treatment for a number of years.