© Theresa Reed | The Tarot Lady 2011
Though there are always exceptions, most of us were born with the five basic senses of human perception: sight, hearing, taste, touch and smell. These aren’t the only means of interpreting sensory clues we have in our arsenals, though—the sixth, seventh, eighth and ninth senses are all equally important for our survival. I'm not talking about ESP or telekinesis; these are actual physiological tools we're utilizing right now.
THERMOCEPTION, or the ability to detect heat and cold.
The detection of heat (or lack thereof) is removed from the sense of touch. Mammals have two different methods of thermoception: one detects heat (temperatures above body temperature) and the other signals cold (or temperatures below body temperature). The most advanced thermoception in nature belongs to the pit viper and boa snakes, which use ultra-specialized thermoreceptors to “see” infrared radiation emanated by nearby warm-blooded animals.
Right:Master of thermoception: Pope's Pit Viper. (Image: Amod Zambre)
The fire-chaser beetle Melanophila acuminate uses a similar system to detect forest fires over long range distances; the fire-chaser lays its eggs inside of newly burned conifers. This sixth sense may not seem as awesome as reading minds or seeing dead people, but it’s pretty useful for keeping us from dying of exposure or getting burned. Aside from constantly catching your fingers on the iron, there are some drawbacks to losing your ability to sense heat and cold. Especially with injury-induced neuropathy and HSAN disorders (see below), the risk of heatstroke, hypothermia, and serious household injury are ever-present dangers.
NOCICEPTION, or how you know when something hurts.
Again, just as with temperature detection, the sense of physiological pain is removed from the sense of touch. Nociceptors (often called “pain receptors”) are an integral part of the human nervous system. When nociceptors detect potentially damaging stimuli--say, a bump on the elbow or a splinter in your finger—a signal fires from the source of pain through the spinal cord into the brain, giving you both an awareness of danger and the approximate location of injury. So what happens when your pain receptors go on vacation? It sounds really pleasant on the surface, the idea of not feeling pain, but the reality is a little bit horrifying.
Loss of nociception is a hallmark symptom of leprosy; damaged receptors leave sufferers unaware of injuries, which are susceptible to repeat injury, viral, fungal and bacterial infection, and excessive blood loss. Leprosy doesn't cause body parts to "fall off" as is commonly believed, but amputation, accidental removal and necrosis aren't uncommon in cases of untreated leprosy. If, say, you're one of the few people in a hundred-thousand who are born with any of the four types of Hereditary Sensory and Autonomic Neuropathy (HSAN), a set of congenital disorders which inhibit (or completely prevent) the nerves from transmitting sensations to the brain, then you're likely to have difficulty detecting the movement of your deep muscle tissue, have limited or absent sensation in the lower limbs, and will probably have issues with skin ulceration and reduced circulation from limited range of motion.
PROPRIOCEPTION: knowing where your parts are.
Proprioception tells us how our body is arranged; that is, it lets us know our arms are attached to our shoulders, and that our hands are at the ends of those arms. It also informs us of our body's muscular activities--whether they're functioning as intended and how well our muscular system responds to command.
If you've lost your sense of propioception after a highly experimental brain surgery or spinal cord injury, you may end up suffering from Alien Hand Syndrome just like Peter Sellers in Dr. Strangelove. Well, sort of like that. Your hand or other affected limb will seem to move independently of your command, with apparent agency and purpose. It probably won't be a Nazi, though.
If you've had a few too many drinks and your friendly neighborhood police officer asks you to take a field sobriety test, you're probably going to miss your nose when your fingertip gets lost in the space between your face and the end of your arm. Likewise, you'll have a hard time walking a straight line and judging the distance of objects from yourself. (Seriously, just don't drink and drive.)
EQUILIBRIOCEPTION, the sense of balance.
Image: Bikram Yoga Queens, NY
If you enjoy walking upright and not falling down when you turn your head, then you're a fan of equilibrioception. This is one of the more complicated senses in that, ideally, it requires the function of at least three others to work properly.
A sense of sight, a working vestibular (ear) system, and a working sense of proprioception work in conjunction to keep us upright. If sight or vestibular health are impaired--by sudden blindness or an inner ear infection, perhaps--equilibrioception is also impaired.
A fun result of having a multiple-system sensory network is that it can be knocked askew pretty easily. Most balance disorders are caused by changes in fluid level in the inner ear--as with Ménière's disease and perilymph fistula, both of which cause vertigo, dizziness and nausea--but aging, traveling at sea, and upper respiratory infections can all make you wobbly, clumsy, or unable to stand or walk. Source info: 9senses