Thursday, May 12, 2005

Five Wild Horses

The spicy component of peppers is called capsaicin. It's an interesting molecule, because it doesn't actually bind to taste receptors; it binds to thermoreceptors. This means that capsaicin actually "tastes" like heat. This fascinates me. Normally we can't taste much heat because if something gets that hot in temperature, the tongue gets burn damage. But capsaicin literally tastes like burning, without actually burning.

It's curiously synesthetic.

Synesthesia, which refers the crossing of sensory modalities (seeing sounds, hearing tastes), raises the question of just what a sense is, anyway. By convention we think of the five human senses as sight, hearing, taste, smell, touch. To be more precise/pedantic, these are sensory modalities and the terms are olfaction, gustation, vision, audition, tactition.

Actually, it is not so straightforward. We also have, as previously mentioned, a sense for thermoreception, as well as senses for balance (vestibular), pain (nociception), and locating ourselves in space (proprioception).

When it comes right down to it, senses aren't nearly as clearly delineated as we were taught in elementary school. What makes a distinct sensory modality? Is it a distinct type of receptor? Then we're already a bit confused with tasting capsaicin, and it gets worse--hearing, balance, and tactition are all mediated by mechanoreceptors, receptors that sense physical pressure such holding a pencil, or having a sound wave push against your ear.

The senses of smell and taste both use all kinds of chemoreceptors that are sensitive to all kinds of chemicals. So what's the difference? Standard reply is that smell detects airborne chemicals, while taste detects water-soluble chemicals. It must be noted, then, that aquatic animals have no sense of smell--although it may be somewhat less disturbing to think of sharks smelling blood from miles away rather than tasting it. But even on land there are some hazy cases; mice have a vomeronasal organ which detects water-soluble chemical cues carried by the nasal mucus (=snot). This information is described in the scientific literature as mapping* to an anatomically distinct set of olfactory neurons and brain centers. So is this water-soluble olfaction?

Incidentally, the VMO is not unique to mice. It's present in a wide variety of terrestrial vertebrates, including humans.

However, there are a number of intriguing sensory modalities that remain entirely unavailable to humans. One of the most outlandish is electroreception, developed in a number of fish and, uniquely among mammals, in the platypus. These organisms can detect changes in nearby electric fields, and some even generate their own. While discussing electroreception one must also mention magnetoreception, famous for allowing birds to orient themselves to the compass directions for migratory flights. If you remember your E&M from high school physics, you'll recognize that electric fields generate magnetic fields and vice versa, so what's the difference in sensory modalities? The main distinction between electroreception and magnetoreception as I understand it is that sharks and platypuses are sensing disturbances in small local electric fields, while birds are paying attention to the Earth's magnetic field as a whole.

So we could also categorize sensory modalities based on the type of energy they recieve and transduce. All cues come in some form of energy, be it light or vibration or gross matter. I should like to classify energy cues thusly:


We already know chemicals (matter) are energy, and light and heat and pressure are just different kinds of waves or vibrations, and if the physicists ever come up with their Grand Unified Theory we'll know that all forms of energy are the same anyway. I feel this rather esoteric thought ties in nicely with the title of this entry, which refers to the yogic tradition of referring to the five senses as five horses drawing the chariot of the body, where the mind is the charioteer. Yoga means union, and if all the senses are just energy receptors, and the mind is made up of electrical impulses, and matter is energy, then perhaps horses-charioteer-chariot are all one.

But that's not a really useful perspective for studying squid pheromones, which may be the next topic of rambling...

* The astute reader will notice I've been skirting around the issue of classifying sensory modalities by mapping them to different regions of the brain. That's because I know relatively little about neuroscience and it's all much more complicated that I want to get into right now.

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