What is the difference between physical and chemical senses? And how do they form?

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Meaning senses like hearing, smelling and seeing and those

In: Biology

Chemoception (sensing a chemical) is when a specific chemical touches a specific nerve in your nose/tongue, in our case. That chemical neatly interacts with something in the cell membrane (or potentially inside the cell if it is membrane permeable), and eventually causes a cascade of reactions in the cell leading to the nerve firing. With smell, it’s usually more specific chemicals that have more specific interactions with the sensory nerves, which is why smell distinguishes so many different kinds of chemicals, but taste is pretty general – anything with low pH is “sour”, most ions are “salty”. Though sweet and savory (sugars and glutamates) are also just groups of chemicals, but much larger groups than most smell receptors.

Physical senses are responses to the actual physical environment – our balance system for example, in the ear, has several partially fluid-filled loops at different angles, and special cells that have hairs sticking into the loop, that send a signal when the fluid sloshes by them. Thermoception (temperature) we don’t really understand how it works in humans yet, but it is a response not to heat, but to heat flux (how fast heat is coming in/going out). Touch is very complex, with different sets of nerves for different kinds of touch, but generally it is pressure on a cell that changes it’s shape that triggers a response.

Sight is maybe the easiest to understand and explain. You have 4 different light sensing cells in your eye, 3 cones and rods. Rods sense light intensity and the 3 types of cones sense color. Each cone has a special pigment (a chemical) that absorbs one wavelength of light (rods are less specific). When it absorbs enough, the pigment has enough energy to start a chemical reaction in the cell, that leads to the neuron firing, telling your brain “I saw red here!” Your brain processes all the information in a super complicated way to generate shapes and objects and motion and stuff.

Really though, there’s not much of a difference between “chemical” and “physical” senses. They’re all just specialized neurons to read information from the environment.