Yesterday I went for an eye exam, only about three years overdue (I bow my head in shame…).
The eye is one of nature’s wonders. It does not matter the type of eye that we are talking about, from the relatively simple eyecup of many flatworms, to the rather complex vertebrate eye, this organ allows for the capture of photons of various wavelengths that in turn trigger a series of chemical and physiological reactions that influence animal behavior and survival. Eyes are also remarkable because they represent a beautiful example of convergent evolution, as the scientific consensus is that eyes have independently evolved at least 40 times in the history of life! There is a rather nice review of eye evolution here.
As you can see above, there is a lot of variability of eye anatomy, and this is a rather small sample! Even though it is not evident in the picture, there is also a lot of variation on the specific vision mechanisms depending on the type of organism. For example, here’s a simple diagram of the human eye:
One of the mechanisms that this type of eye uses to adapt to a variety of light intensities is the contraction and relaxation of certain muscles of the iris, which is the pigmented part of the eye (what determines whether you are a brown- or blue-eyed person for example). This contraction/relaxation controls the diameter of the pupil, an aperture that determines how much light enters the eye to interact with the photoreceptors of the retina, which eventually forms what you perceive as an image. Interestingly, a lot of psychological factors affect the diameter of the pupil, and in fact, in general people displaying dilated pupils are “rated” as more attractive for some obscure reason. This has been known since at least the middle ages, when women used eye drops based on a plant they called Belladonna (which means “beautiful woman”); this plant is also called Deadly Nightshade and it is a deadly plant indeed. This extract dilates the pupils, which made the women more attractive.
How does this happen? Briefly, the pupil has two set of muscles, circular and radial, as shown below. These two types of muscles control the contraction/relaxation of the pupil.
Which brings us to pharmacology. There is a series of chemical messengers (neurotransmitters) that control the contraction/relaxation of these muscles. We know quite a bit of the pharmacology involved, and we use this knowledge to our advantage. One of the standard procedures for a thorough eye examination is to dilate the pupil so that the doctor can take a good look at the retina. I was given eye drops containing a compound called Tropicamide, shown below:
Tropicamide is a synthetic compound that mimics the action of atropine, the active principle of the Belladona plant. Atropine (shown below) is largely responsible of Belladona’s pupil-dilating properties. One of the advantages of using the synthetic compound is that is has a shorter effect, useful in a clinical setting.
Credit: Wikimedia commons
Anyway, this is what happened when I was given the drops. As any self-respecting scientist, I documented it! I took a picture of my right eye (upper left, 0 minutes) right after the doctor applied the drops. I then took pictures of the same eye at the post-application times as indicated.
I took the first four pictures at the doctor’s office. After the fact, I thought about what the other people in the waiting room thought of the bald guy taking eye-selfies… (:-).
The atropine/tropicamide example is a great illustration of one of the most interesting connections between the fundamental and clinical sciences, the development of useful medications based on substances that in their natural context are toxic (remember the magnificent conotoxins?). This toxin-medication connection is something that i hope to keep exploring. Talk to you soon!
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