Greetings to all and sundry,
It is a beautiful day today and already the weekend has ended, the lot is dreading the start of another, the rat race begins again but do not fret, everything is going to ok, let's just continue to do our best always and things will continue to fall in place for all.
As usual i am about ocular health and as an optometrist my mission in the blockchain is to educate members about the eye for the benefit of their ocular health as well as make the job of my colleague ODs out there easier when my readers visit their facilities for eyecare. I am certain that my readers are gaining something from what i share everyday and thus i would continue to do more.
Today we would try to understand how the eye works and how come each and everyone of us end up seeing, how are you able to appreciate that what is in front of you is red or black or that is it a human being or an animal, that is it a girl or a dog. How come you are able to keep following an object in space and everything else we can look at regarding vision.
Let me hasten to add that vision is not an easy topic which can be completed comprehensively in a single writeup in a day and thus i would admonish readers not to be alarmed should we end up needing a part 2 or part 3 of this to enable us touch more on other divisions such as color vision, dynamic visual acuity, visual acuity generally, contrast sensitivity and the likes.
Now the eye is considered a part of the brain or an extension of the brain if you want to look at it from another perspective and this is probably due to the fact that the optic nerve begins directly from the back of the eye and the optic nerve is the first cranial or brain nerve, thus a part of the brain. The eye ball as a whole does some good amount of work when it comes to seeing however a lot of the heavy lifting is also done by the brain thus their joint force ensures that we appreciate what we call vision.
The Visual Pathway
So how does vision start when what goes on into its processing? Vision starts from the object of regard, when we look at something or someone, light impulses from the person or something which we would be referring to as the object of regard would travel through space with information into your eye. It is this impulse or light energy that would be telling us the height, color, direction etc. about the object.
Once this light energy moves from the object of regard, the first point of contact is the cornea (the transparent layer covering the black part of the eye when looking directly into someone's eyes). The cornea is one of the most important refractive component of the eye, and so what it does here is to bend the light rays with the help of its refractive index and radius of curvature for it to be directed into the light.
The light then passes through the pupil (which is the hole in the eye which is seen as black when looking into someone's eye) and then is further refracted by the lens into the eye. The lens doesn't do much of the refraction as compared to the cornea however it does refine it and help the impulses to be positioned right. The lens can change its shape in a phenomenon known as accommodation, this would help the rays to be refracted a bit more or less to help place it on the retina cells for what is needed to be done.
The light rays then moves through the vitreous which is a jelly-like substance that fills the space in the eye ball. The vitreous adds some amount of refraction to the light but it is not considered much same as the aqueous humor in front of the lens. Before i forget let me add that the pupil size is controlled by the muscles around it which may cause the pupil to either increase in size or decrease in size depending on the intensity of light in the atmosphere so as to protect the inner structures or enable vision in very dim light.
After the vitreous the light gets to the retina and that is where the phenomenon or process known as photo-transduction occurs. This process involves the conversion of the light impulses from the object of regard into electrical impulses for interpretation by the brain. This is mostly done by the retinal cells rods and cones before it goes through the other 9 layers of the retinal for further fine tuning and separation of information to the right places in the brain. The photo-transduction process is a biochemical process which would not be delved into however i would leave links at the end of this writeup for those who would want to know the details of the biochemical processes.
Once photo-transduction is done and the various information is sorted out according to size, heigh, orientation, color, contrast etc as it goes through the 9 layers the finally come together through the optic nerve head to form the optic nerve itself which leaves the eye socket through the optic canal into the brain. Once it leaves the socket it decussates or cross over from the two eyes at a joint known as the chiasma to form the optic tract. What this means is that whiles each eye undergoes photo-transduction separately their information cross over and join each other at the optic chiasma before forming the optic tract.
This cross over ensures that information from the right eye regarding the upper field of view joins the one from the left regarding the same upper field of view, thus summing up and enhancing vision together. This is one of the reasons why vision with both eyes or binocular vision as it is technically known is always more powerful than with both eyes. It is also the reason why one may still enjoy good vision if one eye has a poor vision. Binocular vision is crucial to what we call stereopsis which would be another interesting topic to discuss later.
From the optic tract things get quite interesting however i am certain that my readers would like to take a pause and imbibe the information for now and so would be leaving it here and continue tomorrow with the part two. I hope you stay tune.
But basically vision is done with the brain with the help of the eye as an organ system, it is a very beautiful thing to understand, its complexity is one to be much appreciated and it also help you appreciate the beauty of creation and the wonders of the eye. I do hope you will continue to stay with me for the next chapter.
I really do appreciate the time you have spent to read what i had to share with you today and i certainly look forward to more time with you. Thank you and i wish you a wonderful and a fruitful week. Be safe.
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Detwiler P. B. (2018). Phototransduction in Retinal Ganglion Cells. The Yale journal of biology and medicine, 91(1), 49–52.
Contreras, E., Nobleman, A. P., Robinson, P. R., & Schmidt, T. M. (2021). Melanopsin phototransduction: beyond canonical cascades. The Journal of experimental biology, 224(23), jeb226522. https://doi.org/10.1242/jeb.226522
Masland R. H. (2001). Neuronal diversity in the retina. Current opinion in neurobiology, 11(4), 431–436. https://doi.org/10.1016/s0959-4388(00)00230-0
Wässle, H., Yamashita, M., Greferath, U., Grünert, U., & Müller, F. (1991). The rod bipolar cell of the mammalian retina. Visual neuroscience, 7(1-2), 99–112. https://doi.org/10.1017/s095252380001097x