By Chris Jung
To continue with my “senses” series on the blogs, today we’ll be talking about our olfactory and gustatory senses. In my opinion, these senses are a bit less exciting than the visual or auditory systems, but they are nonetheless extremely important—imagine living in a world where the food we ate had no smell or taste (kind of like what happens when we get sick)! Both these senses are often referred to as the chemosensory system. Unlike sight and hearing, where mechanical waves are transduced into neural signals, the transduction behind taste and smell occur as a result of classic ligand-binding interactions at receptor sites (chemosensory = quite literally, sensing chemicals).
Your sense of smell (or olfaction) is the only part of the sensory system that does not pass through the thalamus. There are millions of sensory neurons lining your mucous membrane. Olfactory cells are lined with microscopic cilia that are receptive to molecules associated with certain odors. The olfactory receptors on these hair cells are actually G-protein coupled, and stimulation of these receptors will result in an enzyme cascade commonly involving second messengers such as cAMP. These cells are widely variable, with over 1,000 types of olfactory receptors, which can create over 20,000 types of smells.
Axons of these olfactory nerves (the olfactory receptor cells) pass through the olfactory foramina of the cribriform plate of your ethmoid bone, and end at the olfactory bulbs (one for each nostril). They transmit their signals to the dendrites of olfactory bulb neurons in glomeruli, with each glomerulus made of thousands of receptor neurons converging on ~50 olfactory bulb neurons. Each glomerulus receives input from one type of odorant receptor, which makes it susceptible to multiple odors because each receptor cell is receptive to multiple odorants. However, there still seems to be organization in the olfactory bulb: odorants with similar structures stimulate glomeruli close to each other.
There are four main neuron types in the olfactory bulb: mitral cells and tufted relay neurons form connections with olfactory receptor neurons. These are the cells with dendrites that receive signals from the olfactory receptor cells. These cells also carry the neuronal information from the olfactory bulb to the olfactory cortex in the superior temporal lobe. The olfactory cortex is the general term for several areas: the piriform cortex, entorhinal cortex, the periamygdaloid cortex, and the olfactory tubercle and anterior olfactory nucleus. Granule cells and periglomerular neurons are the two other cell types that are interneurons regulating scent processing.
The olfactory bulbs are a huge area of interest in neuroscience: along with the dentate gyrus, it is the only place where neurogenesis occurs (new neurons are made over a lifespan). Although the extent of neurogenesis in humans is not as well studied as it is in rodents, it is thought that new neurons are produced in the ventricular zone lateral to the ventricles and then migrate to the olfactory bulbs to differentiate into certain cell types.
Although humans seem to have relatively smaller olfactory bulbs than other animals like dogs, the human sense of smell is not immune to the process of long-term potentiation. Humans who have a greater need to use their sense of smell, such as wine tasters, have highly refined olfactory senses, and some humans can even “track” scents like bloodhounds (Never underestimate the power of repetition!)
Taste buds are organs! Humans have around 5,000 - 10,000 taste buds, which we lose as we age (start to decay after 50 years of age). Taste buds are located in some of the papillae of the tongue. The filiform papillae provide friction on the tongue and do not contain any taste buds, whereas fungiform papillae have around ten taste buds. Circumvallate papillae contain around 100 taste buds. Each taste bud is made of 50 to 100 gustatory cells that are receptive to one of the five known basic “tastes”: sweet, sour, salty, bitter, and umami. Each receptor on taste buds has a specific stimulus. For example, the “salty” taste results from activation of a simple ionic channel that lets in sodium ions from NaCl. The “sour” taste is thought to be caused by acids or proteins that increase H+ concentrations, causing depolarization. The other tastes (sweet, bitter, and umami) are caused by stimulation of more complex G-protein receptors.
I’m sure you’ve seen one of the diagrams of the tongue with different “taste regions”—unfortunately, this is a common misconception. Taste isn’t region-specific—rather, cells in each taste bud have different cells that are “activated” by different compounds corresponding to certain tastes. The axons of these nerves converge into three different cranial nerves: the facial (CN VII), glossopharyngeal (CN IX), and the vagus nerve (CN X).
These nerves pass through the thalamus to the gustatory cortex and the insula where tastes are further specified (ex: a certain pattern of “activation” of sweet neurons in your taste buds might be processed into the taste of a “strawberry” in your higher brain regions).
So why do our chemosensory systems appear to be “dulled” when we are sick? It’s actually only your sense of smell that is blocked because of inflamed or stuffy mucous membranes in your nostrils. The way we perceive taste is greatly heightened by our sense of smell. For example, when “sweet” chemicals such as sucrose are detected in a certain pattern, they often “taste” a lot sweeter when combined with the odorant molecules associated with strawberries (can you tell I really like strawberries?) Neurologists believe that neuronal pathways of gustation and olfaction often converge, and this crossing of information contributes to specific smell and taste combinations.
So that’s a basic overview of our chemosensory system—not too bad, right! As with most neuroscience and sciences, you can go as deep as you want into this subject and still come up short for answers, but I hope this “senses” series on the blog inspires at least one person to consider the millions of processes going each second of their daily lives! What we perceive as taste, or similarly, what we perceive as reality or fact, may be more complex or simple than originally thought.
Citations (for further information/background):
Bende M, Nordin S. Perceptual learning in olfaction: professional wine tasters versus controls. Physiol Behav. 1997;62(5):1065-1070. doi:10.1016/s0031-9384(97)00251-5
BrainFacts Book: https://www.brainfacts.org/the-brain-facts-book
Handwerk, Brian. “In Some Ways, Your Sense of Smell Is Actually Better Than a Dog's.” Smithsonian.com, Smithsonian Institution, 22 May 2017, www.smithsonianmag.com/science-nature/you-actually-smell-better-dog-180963391/.
I really find Neuroscientifically Challenged helpful (Dr. Dingman’s YouTube channel and blog posts).
I also like NinjaNerd lectures on YouTube for neuroanatomy. Any anatomy/neurology textbook probably covers olfaction and gustation. And as always, living life and personal observation is the best resource to learn!
No changes were made to the following image, https://commons.wikimedia.org/wiki/File:Head_olfactory_nerve_-_olfactory_bulb_en.png, License: https://creativecommons.org/licenses/by/2.5/legalcode