Following our introduction to the 7 senses and sensory receptors with the sense of touch, we are now going to explore the types of receptors that enable us to see and to smell the world around us.

Vision

The whole neural network making vision possible is absolutely remarkable but for this article we will only mention the cells that can actually perceive “light.”

Receptors involved in vision are called photoreceptors. This word is easy to remember as “photoreceptor” stands for “Receptor of Photons.” (Photons travel with light rays).

Different species have different types of photoreceptors. We humans have only two types: Rods and Cones. They are both located in the retina of our eyes.

Rods

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• Rods are very sensitive to variations of low light intensity. (Imagine that, a rod cell is so good at perceiving light that it can discriminate even a single photon!!!  Cone cells on the contrary can only perceive light when there is at least tens or even thousands of photon hitting them…)
• While they can’t discriminate between different wave lengths (they can’t discriminate colors very well – except blue-green) they are the ones that enable us to see in the dark.
• Location: Rods tend to be evenly distributed in the retina, with the exception of the fovea – the fovea contains mostly cones.
• In our eyes rods outnumber cones by a factor greater than 10 to 1.

Cones

As we have just seen, cone cells are mostly located in the fovea, are less sensitive to low light intensity (i.e: they work best in bright light), they are less numerous than rod cells and they can discriminate colors.

Actually, one rod cell cannot discriminate every color of the visible spectrum. There are 3 sub-categories for rod cells:

(Attention the nomenclature is… very uncreative!)

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• L cones (yes, “L” is the actual name of the cone cell… It stands for “Long”) Are sensitive to long wavelengths of light (red color).
• M cones (M = medium) are sensitive to…. medium wavelengths of light (green color).
• S cones (S = Short) are sensitive to short wavelength of light (Blue color).

All other colors are due to different rod types being activated at the same time by intermediate wavelengths.

Olfaction

Olfaction is a VERY important sense that is highly undervalued… until we loose it. More on that later.

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Olfactory receptor neurons contain lots of tiny cilia that are responsible for sensing olfactory molecules. They can be located in the roof of the nasal cavity.

I won’t say much more about the neural network that is involved in the full perception of smells, however I do want to bring your attention to the sense of smell!

You never knew the sense of smell was so important!

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• The sense of smell is what gives taste to most of the food we eat. If you thought that taste came through the mouth and the tongue you’d be… mostly wrong. (The tongue can only discriminate between 5 basic types of taste… everything else that your brain registers as “the taste” of the food you are eating comes from all the tiny olfactory molecules that reach your nasal cavity while you are chewing).
• Most people who loose the sense of smell become depressed… It is apparently very difficult to be happy without a working nose.
• Among all our senses, the sense of smell is the one most strongly connected to memories and emotions.
• The sense of smell is involved in “dating” and procreation. While we are not sure exactly how this works in humans, we know that pheromones (sex-related olfactory molecules), while totally odorless (ie: we can’t become aware of them consciously), influence our behavior and who we feel attracted to (the perfume industry is a multi-billion dollar industry for a good reason….)

Minimum to Remember:

Vision

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• Vision is possible because of two types of cells: Rods and Cones. We can see in the dark because of rods, and we can see colors during daytime because of cones.
• There are 3 sub-categories of cones: L, M, and S cells that are responsible for perceiving Red, Green and Blue wavelengths

Olfaction

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• Olfaction is possible because of Olfactory receptor neurons with lots of cilia in the cavity of the nose.
• If anything, remember that smell is SO much more important than you ever thought it could be!