Aristotle told us in the 16th century we have five senses or as he called them, wits.
You know, Sight, Smell, Taste, Hearing, Touch.
The idea of the “big five” from Aristotelian medieval thought still lingers so strongly in modern thinking that a sense beyond the natural ones is still called a “sixth sense”.
Well, no, dear Aristotle. It turns out, you’ve been wrong. There are at least 9 senses and most researchers think there are more like 21 or even 30. And no, the 6th sense is not a supernatural ability to see dead people.
Even the core five senses have something extra in them. For example, sight. Technically, it is two senses given the two distinct types of receptors present, one for color (cones) and one for brightness (rods).
Taste is yet another complex story. It is very often argued in the scientific community that it should be five senses due to the differing types of taste receptors — sweet, salty, sour, bitter, and umami. We can live with it being just one sense, though. . The taste sense, unlike sight, is a sense based on a chemical reaction.
Let’s begin with the “new” ones.
Here comes the thermoception, our ability to sense heat and cold. Thermoception is also thought of as more than one sense.
This is not just because of the two hot/cold receptors, but also because there is a completely different type of thermometer, in terms of the mechanism for detection, in the brain. These brain thermometers are used for monitoring internal body temperature.
Proprioception in this sense gives you the ability to tell where your body parts are, relative to other body parts. The word is kinda self-explanatory, it comes from Latin “proprius”, meaning “one’s own”. For instance, proprioception is the sense police officers check when they pull over for a drunk test. The “close your eyes and touch your nose”. Proprioception is used all the time in little ways, such as when you scratch an itch on your foot, but most of the time you don’t have to look to see where your hand is relative to your foot.
Nociception, or should we say, pain sense. This was once thought to simply be the result of overloading other senses, such as “touch”, but this has been found not to be the case and instead, it is its own unique sensory system. There are three distinct types of pain receptors: cutaneous (skin), somatic (bones and joints), and visceral body organs.
The sense that allows you to keep your balance and sense body movement in terms of acceleration and directional changes is named equilibrioception.
This sense also allows for perceiving gravity. The sensory system for this is found in your inner ears and is called the vestibular labyrinthine system. Anyone who’s ever had this sense go out on them on occasion knows how important this is. When your sense of balance is not working or is malfunctioning, you literally can’t distinct up from down. Moving from one location to another without additional help is nearly impossible. The vestibular labyrinthine system works by sensing the motion of fluid in three canals in your inner ear, as well as sensing the weight of small crystals of calcium carbonate on tiny hair-like sensory receptors.
Chemoreceptors trigger an area of the medulla in the brain that is involved in detecting blood-born hormones and drugs. It is involved in the vomiting reflex, too.
Stretch Receptors, on the other hand, are found in such places as the lungs, bladder, stomach, and gastrointestinal tract. A type of stretch receptor, that senses dilation of blood vessels, is also often involved in headaches.
Thirst is a system that more or less allows your body to monitor its hydration level and so your body knows when it should tell you to drink just like the sense of hunger allows your body to detect when you need to eat something.
Magnetoception is the ability to detect magnetic fields, which is principally useful in providing a sense of direction when detecting the Earth’s magnetic field. Unlike most birds, humans do not have strong magnetoreception, however, experiments have demonstrated that we do tend to have some sense of magnetic fields. The mechanism for this is not completely understood; it is theorized that this has something to do with deposits of ferric iron in our noses. This would make sense if that is correct as humans who are given magnetic implants have been shown to have much stronger magnetoception than humans without. One such method for testing whether humans have magnetoception is by placing a strong magnetic field near a person and then disorienting them. Results have shown that people in this scenario perform significantly worse at being able to re-orient themselves in terms of the cardinal points than people who are not near a strong magnetic field.
More conclusive evidence has been demonstrated by examining a subject’s brains when magnetic fields are produced near a person. It has been shown that these magnetic fields will evoke a response in the brain’s activity. There exists a type of bacteria, called magnetotactic bacteria, that build magnets inside themselves in order to orient themselves with the Earth’s magnetic field. They also migrate and form chains of themselves along magnetic field lines. Many avian life forms possess a region of their bodies that contain biological magnetite, generally in their beaks.
It is believed this gives them strong magnetoception and thus allows them to sense direction accurately. More recently, it has been shown that certain birds have the ability to see magnetic fields.
How this works is the Earth’s magnetic field affects how long a certain molecule, cryptochrome, in their photoreceptor cells stays in the active state. This then affects the light sensitivity of the bird’s retinal neurons.
The net effect is the birds can perceive magnetic fields with their eyes.
The biological magnate and ability to perceive magnetic fields with their eyes are thought to combine to form a very accurate mapping and directional system in the birds. Sharks, stingrays, and chimera all possess an electroreceptive organ called an ampullae of Lorenzini.
This organ gives them the ability to detect even small variations in electric potential.
They can use this to detect magnetic fields, among other things. Cattle tend to align themselves north-south, which leads some researchers to believe they have a strong magnetoception sense.
This one is debated as no singular mechanism has been found that allows people to perceive time. Chronoception.
However, experimental data has conclusively shown humans have a startling accurate sense of time, particularly when younger. The mechanism we use for this seems to be a distributed system involving the cerebral cortex, cerebellum, and basal ganglia. Long-term timekeeping seems to be monitored by the suprachiasmatic nuclei (responsible for the circadian rhythm).
Some people experience synesthesia where they may perceive some senses intertwined like thinking of a sound as a color. So a dog barking may be “red” those having the synesthetic ability. This condition does not generally occur naturally, though it can; it usually manifests itself when people are under the influence of hallucinogens.
We don’t only need to know where our body is in space, we need to keep it upright. For that, we have equilibrioception which is our sense of balance. This is accomplished with help from the vestibular system in the inner ear. As anyone who as a kid spun around and around on the front lawn knows, once your equilibrioception is thrown off you’ll fall down and it’ll take you a minute to get back up again.
Ever thought about how you can tell whether it’s hot or cold outside? This sense is known as thermoception. It turns out we detect outside temperature through thermoreceptors in our skin. This allows us to avoid things like getting burned or frostbitten and to avoid environments that are too hot or cold.
If you think these are interesting there are some even more fascinating senses found outside of our species. For instance, there’s electroception, which is the ability to sense electrical fields. Sharks use this to detect prey.
There are some senses that are only found in the animal world.
Electroception – the ability to feel electrical fields around us. Sharks can detect electrical fields in their environment, including those emitted by prey it cannot necessarily see.
Magnetoreception – several mammals, such as bats, can sense the Earth’s magnetic field and use it to navigate.
Polarized light – many animals, including insects and birds, are able to see and use polarized light to decide which direction to go in.
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