People are generally familiar with the five basic senses: sight, smell, hearing, touch, and taste. However, it’s less well-known that humans have closer to 21! Aristotle is credited with the traditional “five senses” model, and science has advanced a little bit since that time. There can be some debate about what a “sense” actually is: the common definition is “any system that consists of a group of sensory cell types that respond to a specific physical phenomenon and that corresponds to a particular group of regions within the brain where the signals are received and interpreted.” Here are the senses humans have (or at least the ones discovered so far!)
- Sight – color: sight is actually two senses. Color is detected with the cone receptors in the eye.
- Sight – brightness: detected with rod receptors
- Smell: based off a chemical reaction, and separate from taste, though the two combine to produce flavor. This is why food tastes funny when you have a cold.
- Sound: detecting vibrations along a medium that is in contact with your ear drums, such as air or water. The density of the medium affects the vibrations reaching your ear, which is why things sound different underwater.
- Touch: This is an interesting one, because there is a unique touch sense, separate from your senses of pressure, temperature, pain, and itch. Your skin is the largest organ on your body and it gives you lots of data!
- Taste: This can be argued to be five senses, each based off chemical reactions with the taste receptors on your tongue. There are different receptors for sweet, sour, salty, bitter, and umami (which detects the amino acid glutamate, often found in meats).
And here already we can see the beginning of the debate. I only listed the “five” traditional senses, and depending on how those are counted, they could be as many as 14!
- Pressure: something is pushing on a part of you. This has to be differentiated from detecting atmospheric pressure changes, because that is still debated. There’s plenty of anecdotal evidence that humans can detect changes in atmospheric pressure – becoming sleepy or experiencing joint pain in low pressure periods – but there’s nothing conclusive.
- Itch: this is actually distinct from other touch-related systems! Evolution has provided us with some strange things.
- Thermoception: Ability to sense heat and cold. This can be counted as two senses, because the body has different types of thermoreceptors for detecting external and internal temperature.
- Nociception: sense of pain. This was originally thought of as overloading other senses, but in fact it is a separate system. There are three different types of pain receptors: cutaneous (skin), somatic (bones and joints), and visceral (body organs).
- Proprioception: my favorite sense, and one of my favorite words. The sense of where your body is without looking at it. This is how you know where your foot is even if you aren’t watching it move.
- Tension: sensors in your muscles monitor how much tension they are under
- Stretch: receptors found in the lungs, bladder, stomach, and intestinal tract, to tell when those organs are full. A type of stretch receptor that senses dilation of blood vessels is involved in headaches.
- Equilibrioception: this is what your inner ear is for. Detects balance, acceleration, and directional changes. Without this sense you can’t tell which way is up – it’s a little important!
- Hunger: self explanatory. Your body says to give it calories! The brain can actually sense the difference between different macronutrients, and the body has some learned appetites which explain a craving for, say, salty foods, when you are short on sodium.
- Thirst: my rule of thumb is that if I feel thirsty, I should have been drinking already…
- Chemoreceptors: involved in detecting blood-borne hormones and drugs. Can also detect carbon dioxide levels in the blood: the brain uses that information to control breathing rate.
- Magnetoception: the ability to detect magnetic fields. Not as strong as in many other animals like birds, but experiments have shown humans do have some sense of direction in this way. This can be tested by placing a person next to a strong magnetic field and disorienting them. People in this scenario are much worse at re-orienting themselves in terms of the earth’s magnetic field than people who are not near a strong magnetic field.
- Time: No singular mechanism has been found to explain how humans tell time, but there is conclusive proof that this sense is startlingly accurate. There seem to be different mechanisms for short term (minutes to hours) and long term (circadian rhythm) time keeping.
One interesting fact about humans’ time sense is that it is dependent on age: people within the age range of 19-24 years were able to tell within 3 seconds when 3 minutes was up, whereas those in the age group 60-80 thought 3 minutes had passed when the actual time was 3 minutes and 40 seconds. So as you get older, it’s not that the world is speeding up; it’s that you are slowing down!
It’s always a source of joy for me that there are so many things we still do not know about our own bodies. It’s easy to get jaded and think that science has answered every question, but the fact that we still have so many gaps in our knowledge is tremendously gratifying. We’ll never run out of new things to learn!