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At some point, you’ve probably learned about the five senses: sight, sound, smell, taste, and touch. However, the classic list doesn't account for all the sensations we experience and use to navigate the world around us!

Hosted by: Hank Green
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Sources:
Thermoception:
http://web.pdx.edu/~zelickr/sensory-physiology/articles/2014-articles/05-21-wednesday/thermo-only.pdf
http://www.nature.com/nrn/journal/v15/n9/full/nrn3784.html
http://www.ncbi.nlm.nih.gov/gene/7442
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3775668/
http://www.ncbi.nlm.nih.gov/books/NBK5260/#
http://www.ncbi.nlm.nih.gov/books/NBK5238/

Proprioception:
http://cirrie.buffalo.edu/encyclopedia/en/article/337/
http://neuroscience.uth.tmc.edu/s3/chapter05.html
http://www.wired.com/2014/09/24-woman-discovers-born-without-key-brain-region-cerebellum/
http://www.ncbi.nlm.nih.gov/books/NBK10812/
http://www.centropiaggio.unipi.it/sites/default/files/course/material/Cutaneous_mechanoreceptors.pdf
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3988398/

Equilibrioception:
http://www.ncbi.nlm.nih.gov/books/NBK279394/
http://neuroscience.uth.tmc.edu/s2/chapter10.html
https://www.nidcd.nih.gov/health/balance-disorders#1
http://www.ncbi.nlm.nih.gov/books/NBK10792/
http://www.ncbi.nlm.nih.gov/books/NBK10863/
[SciShow intro plays]

Hank: At some point you've probably learned about the five senses: sight, sound, smell, taste, and touch. These five don't explain all of our sensations. How can we tell how hot or cold we are, or keep ourselves balanced? Now, scientists are beginning to add more senses that classic list. Here are three of them.

It's probably no surprise that sensing temperature is pretty important, which we call thermoception. It helps us keep our body temperature constant and lets us know when our environment is too hot or too cold so we can avoid tissue damage, like from burns or frostbite.

So how do we do it? Scientists have found a couple of potential mechanisms connected with the Transient Receptor Protein Channel or TRP [trip] family. There are lots of these channels and they react to lots of different stimuli. We're still trying to figure out what they all do. One thing's for sure: a lot of them help us respond to changes in temperature.

Scientists aren't exactly sure how these channels work, but with physical stimuli of the environment getting warmer or colder, depending on the channel they're more likely to open. One of these channels, TRPV1, plays a role in the sensation of painful heat. The receptor is activated when temperatures get uncomfortably warm around, 40 degrees Celsius. TRPM8 on the other hand responds to cold stimuli, below 20 degrees Celsius, so pretty much anything below room temperature.

These channels, and others, can be found throughout our bodies, but when they're on nociceptors or pain sensing nerves, activation of the channel triggers a rush of calcium into the cell and sends a signal to the brain about painful temperature. All that information goes to the primary somatosensory cortex, a thick fold of tissue on the top of the brain where most of the mechanical sensations like touch, pain, and vibration are processed. Then, you can consciously process the temperature and yank your hand away from that campfire, or decide whether you want to put on a jacket.

Now, have you ever thought about how you just know where your body is in space? Well that's proprioception. The word comes from the Latin "for one's own grasp." It's how you can type without looking at a keyboard, and walk without looking at your feet. And there are a bunch of specialized receptors in our skin, joints, and muscles that help us do it.

For example, muscle spindles respond to changes in muscle length and the speed of muscle movement, while Golgi tendon organs send signals about muscle tension and exertion. And then cutaneous mechanoreceptors respond to stretch and pressure in the skin and joints. All these receptors work together to provide the brain, especially the cerebellum, with information about your movement and the positions of your limbs.

The cerebellum is responsible for coordinating things like balance, posture, and voluntary movement. Weirdly though, scientists recently discovered a case of a woman born without a cerebellum who has some balance and movement issues but seems to be doing relatively fine. So there is still a lot to understand about how our brains process proprioceptive information.

Separately, we have equilibriaception, our sense of balance, and we need balance whenever we move like walking and running. Ears are important for our sense of hearing but they're also a key part of equilibriaception, especially the inner ear. It contains the vestibular system which includes three fluid-filled semi-circular canals lined with tiny hair cells. When your head moves, these hair cells are sloshed around by the fluid and send signals to the brain, specifically to the vestibular nuclei in the brainstem. Each canal is responsible for a different kind of movement: one for up and down, one for left, and right and one for side-to-side.

The otolith organs located just below the semicircular canals are similar, but in addition to liquid they have tiny crystals made of calcium carbonate. As the head moves, these crystals rub against the hair cells attached to the membrane, which send information to the brain stem. The brain then sends information out to your eyes, joints, and muscles, so they can respond accordingly and help you navigate the world.

Now problems with this system can lead to issues with balance. Vertigo, for example, can be caused by loose stones in the otolith organs. They can also fall into the semi-circular canals, disrupt the normal fluid movement and put unexpected pressure on hair cells. That pressure conflicts with what your eyes are seeing which can make you feel dizzy when you move your head. Together, these three senses are really important in helping us navigate our environment successfully and safely. So even though they don't make the list of our traditional senses, I think we do ourselves a disservice by forgetting about them.   Thanks for watching this episode of SciShow which was brought to you by our president of space SR Foxley. Thanks SR! If you want to help support this show you can go to Patreon.com/SciShow, and if you just want to keep getting smarter with us you can go to YouTube.com/SciShow and subscribe.