These posts make more sense when read in order.
Please click here for the first article in this series to enter the rabbit hole.
We’re commonly told we have five senses, but we actually have many more. How many depends on how we define a sense and how we group them, so the lists vary from 9 to 33. Some senses use several types of receptors, while some receptors are used for several types of senses.
Sight, sound, smell, and touch are more focused on the external world. Taste seems a bit more internal, although we are sensing something external that’s entering the body. Most of our additional senses are internal, such as our sense of hunger, thirst, stomach fullness, whether our bowels and bladders are full, and proprioception—the sense of the locations of our bodies and their parts in space. It’s related to, but separate from, our sense of balance, motion, and acceleration.
Proprioception receptors are in your muscles and tendons, while the others are in our ears. Drinking alcohol interferes with this sense, which is why the police ask those they suspect of being drunk to touch their nose with their forefinger, balance on one foot, and walk a line.
Other senses are a bit more difficult to place, such as our sense of temperature and time. We also have a number of senses related to maintaining our daily rhythms. Some suggest we should even include our immune systems and our sense of math. In addition, there are subconscious senses, like those that detect our blood pressure and the pH level of our blood and cerebrospinal fluid. The so-called sixth sense—intuition or gut feeling—would probably fall under the subconscious, although it’s hard to define as a sense and is heavily influenced by past experiences, knowledge, and biases.
We also have a chemosensory sense that uses a separate pathway from those of smell or taste, which are also chemical senses. This is what detects the heat of peppers, the coolness of mint, the burning of ammonia, and the tingle of carbonation. You’d think the last one is sensed by touch of the bubbles, but it’s not. What you think you feel actually comes from the formation of carbonic acid, which is not felt, but detected as a mild chemogenic pain.[1] Incidentally, carbonic acid reduces sweetness, which is why sodas contain so much sugar or sweeteners.
We probably have more senses that we’ve yet to discover. Opsins are the light-sensitive proteins in photoreceptors that enable us to see. Nature tends to repurpose basic elements and opsins are an example of this. So calling them taste receptors is much too limiting since they’re also used by a wide array of animals for sensing temperature, hearing, and in regulating circadian rhythms. And they’ve been found in some unusual places where we don’t yet know what they’re doing.
Both us and cephalopods—such as octopuses and squids—have them in their brains, so they’ve probably been there for millions of years and must be doing something important. They’re also found in our lungs, liver, bladder, fat, and in men’s testicles. When the sweet or savory sensors in our heart detect certain nutrients, they make it beat faster, perhaps to aid digestion. We’re still discovering what they’re doing elsewhere.
Fruit flies use opsins—light receptors—for temperature detection, so they can keep themselves from getting too hot or too cold. Sperm cells use them as one of their several methods of navigation, as they can detect a difference in temperature of just a thousandth of a degree Fahrenheit (500th of a degree C), allowing them to move towards warmth.
We have olfactory receptors in our hearts, some of which detect fatty acids, reducing our heart rate. We also have scent receptors in our skin that react to smells. And they’re found in our lungs, livers, kidneys, stomachs, intestines, and bones where they’re involved in system regulation, immunity, and healing. They might even be monitoring our nutritional status.
Many animals crave certain foods when their diets lack vital nutrients. Some evidence suggests scent receptors were repurposed in all of our organs, but perhaps the receptors were repurposed to detect scents, depending on which came first. Those in the lungs can cause your bronchial tubes to expand or contract, depending on what they sense, while cilia—tiny hairs that sweep out unwanted particles—can sense bitterness, such as nicotine, making them sweep more vigorously.
So, without realizing it, we have sight, smell, and taste receptors throughout our bodies sensing things that we’re completely unaware of.
When the Coast is Clear
On a clear, quiet night the average person can just barely make out a candle that’s 30 miles away and they can hear a ticking watch that’s 20 feet away. They can smell a single drop of perfume that is evenly dispersed throughout a three-room apartment. Dissolve a teaspoon of sugar in two gallons of water (7.5 l) and they can taste it. And if you drop a bee’s wing on their cheek from a height of half an inch (1 cm), they will feel it. These are the minimum thresholds for our primary senses that half the population is able to detect.
The tips of our fingers are sensitive enough to feel a bump that’s only 1/25,000th of an inch high (6 micrometers)—that’s about the diameter of a cell of bacteria. But it’s not just mechanical differences that we can feel, we can also sometimes sense chemical differences, such as when scientists swapped a carbon atom for a nitrogen atom in a completely smooth single-layer coating of atoms.[2]
Touch isn’t a single sensation, but arises from a collection of distinct receptors. Mechanoreceptors sense pressure and vibrations, with different ones sensing light touch, pressure, skin stretch, and the movement of hairs. There are receptors that make us feel itchy, while pain receptors respond to damage, low cold, and high heat. We also have at least six different temperature receptors—two for cold, two for warm, and two for hot. We also appear to have separate receptors for vibration and flutter—flutter being slightly slower. These senses are actually not much different from sight and sound. They all work using receptors that send signals to the brain, which converts, interprets, and enhances them, before the final processed sensations enter our awareness.
Despite seeming straightforward, once the information gets to our brain, things can go wrong. Healthy people sometimes wrongly think they were touched on one side of their body, when they were touched on the other side, or even feel a touch on their hand that was actually on their foot, and vice versa, and they’ll never realize anything is amiss.[iii] There is also ciguatera poisoning, where, after eating neurotoxic seafood, some people’s temperature sensations reverse so that they sense hot things as being cold, and cold things as hot. This disconcerting reversal can last for several weeks.
Something Stinky This Way Comes
We like to think humans are exceptional among animals, but our self-professed superiority falls flat when it comes to our senses. Elephants can hear lower sounds than us, while bats can hear higher. Eagles can see farther, birds in general see more colors, cats see better in the dark, although dolphins probably beat them. Cockroaches’ sense of touch is a hundred thousand times more sensitive than ours. Octopuses are a hundred times more sensitive to tastes. And, of course, some dogs have a phenomenal sense of smell.
For most of us, our sense of smell is poor. Some of this is probably our own fault. We work hard at getting rid of odors. We buy products that mask them (I don’t think deodorizers can actually eliminate them). And we avoid smells as much as possible. Compared to past centuries where people were constantly in the midst of many smells—both good and bad—our lives tend to be sanitized olfactory wasteland. And since nature leans toward conservation, if you don’t use it, you begin to lose it. Among animals, our sense of smell is now at the lower end of the spectrum.
Dog nose. Salem Eames, CC BY-SA 2.0 (adjusted). |
It doesn’t have to be this way. Some people cultivate their sense of smell—perfumers, wine connoisseurs, and the like. So we could increase our ability to detect scents, but it might take some dedication. It’s unfortunate a dog can’t tell us the wondrous aspects of the world we’re oblivious to. Not just with aromas, but other facets our senses can’t detect.
I’m sure most of us have memories of favorite scents. They are probably associated with childhood memories or the holidays. Perhaps it’s the smoke from a burning pile of leaves or a brush fire; the smell of rain or a damp forest; the scent of coffee brewing, freshly baked bread, or freshly mowed grass. Maybe it’s the smell a jack-o-lantern gives off from the candle burning inside, fireplace smoke wafting from a chimney, or a misty ocean shoreline with the waves crashing on the beach.
Scents are much more important to us than we realize and our sense of smell is not nearly as bad as scientists used to think it was before they started testing it.
Smell and taste combine to give us a sense of flavor. Without one or the other, eating and drinking becomes very unpleasant. Loss of smell has even been linked to suicides. For those who completely lose their sense of smell, most things taste like sugar or salt, or they’re bitter or sour. It’s also dangerous, since you can no longer tell whether there’s a gas leak, the house is on fire, or whether you’re eating rotten food. Still, people do learn to live without it.
One woman who temporarily lost both taste and smell said everything tasted like ice cubes and cardboard. As her sense of taste returned she said chocolate tasted like sweet rubber. Her sense of smell gradually returned and everything smelled horrible, while everything tasted like it had chemicals in it.[4] The Stowers Institute for Medical Research reports that others have described roses as smelling like cigarettes, coffee like gasoline, and fresh bread like rancid meat.[5]
Flavor is related to taste and smell, but it’s a sense that’s constructed in your brain. We don’t have any flavor receptors. Your brain creates flavors using information from taste, smell, spiciness, and moisture (which is sensed by touch and temperature receptors). Mouth feel and vision also play a role. Even hearing can influence flavor, so some restaurants carefully select the background music they play.
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[1] Paul M. Wise, Madeline Wolf, Stephen R. Thom, and Bruce Bryant, “The Influence of Bubbles on the Perception Carbonation Bite”, PLoS ONE, 8(8): e71488, August 21, 2013, https://doi.org/10.1371/journal.pone.0071488.
[2] Leah Crane, “Your finger can feel the change of a single atom in a material”, New Scientist, no. 3333, May 8, 2021, https://www.newscientist.com/article/2276384-your-finger-can-feel-the-change-of-a-single-atom-in-a-material/, May 4, 2021, citing Soft Matter, https://doi.org/10.1039/D1SM00451D,
[3] Bielefeld University, "Phantom sensations: When the sense of touch deceives", ScienceDaily, June 14, 2019.
[4] Stephani Sutherland, “How Covid Scrambles the Senses”, Scientific American, vol. 324, no. 2, pp. 60-63, and as “Mysteries of COVID Smell Loss Finally Yield Some Answers”, https://www.scientificamerican.com/article/mysteries-of-covid-smell-loss-finally-yield-some-answers1/.
[5]
Stowers Institute for Medical Research, "Decoding smell: Neural code
determines instinctual responses to attractive or aversive odors", ScienceDaily,
March 29, 2021, https://www.sciencedaily.com/releases/2021/03/210329153350.htm.