How many senses

Image credit: G. Senses that rely on motion detection mechanoreception include hearing, balance, proprioception, and touch.

It also includes more obscure things like osmotic pressure and bowel and bladder control. To detect osmotic pressure, cells in the brain use stretch receptors to sense our level of hydration. When we're dehydrated, the cells shrink, deactivating stretch receptors, and triggering thirst. Similarly, stretch receptors on the bladder sense when it needs to be emptied.

Some animals use mechanoreceptors in other ways. Fish have lateral-line organs along their sides that function much like the organs in our inner ears to sense water movement and vibrations from their prey and other fish. Insects can "hear" sound vibrations, but instead of ears they have specialized organs on their antennae bees and flies , legs crickets and grasshoppers , or abdomen. Fish scales showing openings to lateral line organs. Inside the holes are motion-sensing structures much like the ones that sit inside our ears' balance organs, the semicircular canals.

Taste and smell are chemicals: they rely on receptors that detect chemicals in food and in the environment. And though we associate pain and itch with touch, they too are chemical senses that are activated by the contents of broken-apart cells pain or irritants itch.

We also have internal sensors that respond to pH, carbon dioxide, and other chemicals that are important for homeostasis. In other animals, pheromone detection is important for finding a mate. Some moths can use their antennae to detect extremely low concentrations pheromones released by a potential mate from miles away. The male cecropia moth uses chemical-sensing cells in its antennae to detect pheromones released by a female up to a mile away.

We have temperature-sensing receptors not only in our skin, but also inside our bodies. Internal temperature sensors are key for maintaining body temperature. Rattlesnakes and other pit vipers use heat-sensing pit organs to detect the body heat of their prey. Hearing in insects. Annual Review of Entomology, 61, Principles of Neural Science, fifth edition.

McGraw Hill Medical. Home Sensory Systems The Twenty senses. The Twenty senses. Sensory Receptors: A Basic Toolkit. In addition to the famous five, there are a slew of other senses constantly updating us about the world inside and outside of our bodies. They detect hunger and carbon dioxide and tell us where our arms and legs are.

They may not help us see the sunset, smell the roses or tune in to our favorite songs. This handy sense allows us to walk with our heads up, throw a ball while gazing at a target and control the steering wheel while looking at the road. They detect how much tension, strain and load are on our limbs, and are constantly sending that information to our brains. Based on these stats, our brains can discern where our limbs are in relation to our surroundings and the rest of our body.

Another key player is our sense of balance, or equilibrioception. It allows us to stand, walk and move around without toppling over. Our sense of balance relies on the vestibular inner ear system. Our inner ears have streams of fluid that flow between three winding canals. When we nod our head up and down or turn it left, right or sideways, this fluid will flow to one of the three canals, each detecting a specific sense of direction.

Along with input from the visual and proprioceptive systems, our brain uses this information to send messages to our muscles telling them how to stay upright and evenly distribute our weight. While proprioception and balance help us navigate the world outside, we have internal senses reporting on the world inside, too.

Perhaps most immediately obvious is our sense of hunger. This hormone travels to a region of the brain called the hypothalamus, where it activates neurons that stimulate hunger. A good example of this is our inner carbon dioxide detector. We get rid of CO2 by breathing it out, so things like excessive breathing during exercise can cause our levels to drop too low.

The idea of five classical senses dates back at least to Aristotle, himself a rather classy guy. In De Anima Of the Soul he argues that, for every sense, there is a sense organ. Because when you start counting sense organs, you get to six right away: the eyes, ears, nose, tongue, skin, and the vestibular system.

We now know that the vestibular system, located in the inner ear, is an integral part of how we balance ourselves, but it also plays a critical role in vision, allowing us to keep our two eyes focused on things even while our heads are moving about.

Instead of a sense organ, each separate sense really only requires a different kind of sensory receptor. In the skin alone, there are at least four different kinds of sensory receptors: those for touch, temperature, pain, and proprioception or body awareness. A sensory receptor is a specialized cell that sends electrical signals to the brain in response to the type of stimuli the cell is optimized for.

The rods and cones in the retina are sensory receptors. They send signals when stimulated by light of various wavelengths and intensities. The skin is brimming over with sensory receptors optimized not only for touch, but for other things as well, such as hot and cold. There are at least six different kinds of temperature receptors, each optimized for a different temperature range.