Wednesday, June 25, 2008
As I wrote in the first post of this series about “Basic Ocular Anatomy”, the retina (10) is a layer of the eye that deserves one blog post just to itself.
It is the most important layer of the eye, because it contributes to the first part of the Visual Information Processing that we receive: the formation of the image, which will be processed later on.
I am going to show the retina from different points of view, in order for it to be easier to understand. Its importance is appropriate to its complexity.
In one hand, in order to understand how the eye works, you must think about it as an “old picture camera” (that is, with film ;-)); just as the image is expressed in the camera film, it is expressed in the eye retina.
In the other hand, you must also think that the eye behaves as a “dark chamber”; this consists for example, of a dark box which has a little hole in one of its walls (as the pupil in the eye). In opposite wall (as the retina in the eye) an inverted image is made from the external objects.
One eye without any problem of refraction (myopia, hyperopia or astigmatism), that looks at distance (more than fifteen feet), is at a state of rest. The iris (2) behaves as a camera diaphragm, whose contraction controls the quantity of light that gets into it through the pupil (3); in this situation, the lens (8), the other dynamic part of the eye, is at rest. Therefore, the human eye does not have to strain when it looks at distance.
When one person looks at near, if these dynamic parts of the eye do not modify their state, he will see blurred. It is the same thing that happens if the lens of a camera does not modify its focusing in order to clear a close object (after focusing something that was far): the picture will be blurred.
In the eye, in order to clear a close object, the thickness of the crystalline lens (8) varies. This mechanism is called accommodation, but I will write about this later.
When we look distant something as well as close something, this has to get to the film inside of a camera, or to the retina in the eye, in order to achieve a clear image.
Once you have understood how the eye works, now I am going to show you how the light gets into it, and it reaches the retina (10):
The light thrown by one object converges on the cornea (1), passes through the pupil (3), and gets to the crystalline lens (8). At this point, the image is inverted (according to the optics laws). If there is no reaction in the lens, the image that reaches the retina(10) can be more or less blurred, depending on the distance to the object (considering that this is not further than fifteen feet, as I explained before). That way if the image is blurred, this information gets to the brain and it answers by sending a command to the crystalline lens in order to modify its shape and therefore, allowing the image to get to the retina, thus creating a clear image. This action happens automatically and the process is very fast; it is similar to the autofocus process of a picture camera, which focuses automatically on what it detects in the central framing of the viewfinder.
So, after the light passes through the gelatinous vitreous humor (9), it will reach the end of the route in the eye: the retina (10); in this point the image is clear (I will explain later which are the cases where the image is not clear and why).
But unlike a picture camera, the image does not just stay in the retina. This eye’s innermost nervous layer is the one responsible for converting the photons of the light that it receives into nervous signals that can be relayed to the brain; there it interprets them and gives them a right meaning. Therefore, the brain is the part responsible for “developing the film”, that is, interpreting it (knowing what the object is, what it means to us, what emotion causes, how to answer before it, and so on).
In order to achieve this transformation, the retina (10) is composed of five kinds of nervous cells, which collect all luminous information. They are not just responsible for making the image clear, but they also extract the basic information of the object about its color, its shape, its orientation, its movement, transmitting it to the brain. The human eye transmits visual data to the brain at about the same speed as two computers can share data.
These cells are placed in the retina in different layers, but as I do not want to make it very complicated, I am just going to point out three of them:
The light passes through all retina nervous layers to reach the outermost one: the one composed by photoreceptors (called rods and cones) (A); there the incoming light is reflected and passes through the retina again in the opposite direction, transforming this light into nervous impulses and transmitting the neurological information through different layers of nervous cells to reach the last one: ganglion cells (C); in this layer their thin axons bundle together in order to get out the eye and to the Optic Nerve (11) and send the impulses to the brain, thus starting, the VISUAL PATH.
There are two subtype of photoreceptors: rods and cones (A) distributed by the whole retina, where each one has a certain function and location.
- Neither of them exists in the point of the ganglion cells exit (Blind Spot -12-).
- Cone cells are found mainly in the central area of the retina, because they are the cells responsible for the details (the clarity, the shape and the color) of the object. This central area of the retina is where the eye “machine” leads the image of an object, in order to see it clear and with 20/20 of vision. This central area is the macula (14) (http://rosavisionenglish.blogspot.com/2008/05/little-bit-of-basic-ocular-anatomy-eye.html) and its central point with higher vision in the retina is the fovea (13) (http://rosavisionenglish.blogspot.com/2008/05/little-bit-of-basic-ocular-anatomy-eye.html). As these cells are the ones responsible to catch the details, they work better with well-light conditions. So, some activities where these cells are used are for instance, when reading or writing.
- Rod cells, in the other hand, are found mainly in the peripheral area. As we get away from the macula (14), the quantity of cones decreases and the quantity of rods increases. In the peripheral area the information about the clarity or the color is not so important, but detecting the orientation or the movement of the object that we look at. Therefore, these cells are stimulated with low-light conditions. Also, these cells are very sensitive to changes in contrast even at low-light level.
These photoreceptors (A) at the same time, stimulate certain ganglion cells (C); that is, each type of photoreceptor stimulates one type of ganglion cell, so each cell takes a certain information; both information elements travel parallelly to different areas of the brain, and once there, all information is mixed. The brain gives the meaning of the world surrounding us: where the object is, what it is, how big is, what color it is, how far it is,… It takes all information from the world and puts it together, in order to find similarities and differences, compare, discriminate and so on.
Consequently, in one hand, CONES (A) send information to PARVO ganglion cells (C), which take the information about the shape, the color and the detail, that is, what the object is; they help us identify and take out a meaning; help to see that object clearly (the Visual Acuity) and they work better if the object is stopped.
In the other hand, RODS (A) send information to MAGNO ganglion cells (C), which take information about the movement, the space and the orientation; that is, they report on the movement direction, its speed, calculate distances, where the object is, where I am and three-dimensionality. They help us move ourselves inside a dark room avoiding that we hit ourselves with other objects, or avoiding that we crash with the things that we do not look at directly (for example, with the doorframe when we go through it without looking at it), and so on.
As you can see, we do not just “see” the objects, or rather, we do not just see them clear or blurred; what a retina catches from an object, that image, is not only a picture, goes along with much more information and it all begins to be processed in the retina. But the rest of this complex but amazing processing of visual information that happens in the brain, will be explained it in the next post…
A little bit of basic ocular anatomy… Eye or Ocular Globe
A little bit of basic ocular anatomy… What is the eye surrounded by?
Sunday, June 08, 2008
As I wrote in the last post, in this one I am going to briefly explain those structures surrounding the eye; they are as important as the eye itself, because if any of them are not in perfect condition, the visual information can not be adequately processed.
LACRIMAL SYSTEM AND EYELIDS
EYELIDS protect the eye against any element that “wants” to get in. There is a reflex that cause that, when we simply touch the eyelashes, the eyelid closes. This is a “little inconvenience” when we want to position contact lenses onto the cornea (1) or simply when we need to put some drops on the eyes.
Also, they cover the eye when we sleep and, along with the pupil, control the quantity of light that gets into the eye.
If blinkings are not frequent (they are different in each people, but, the average frequency might be 1 blinking for every 5 seconds), the tear is not totally extended by all the cornea (1) and thus the cornea is not correctly lubricated, causing problems of clear vision, reddening and stinging of eye, discomfort with the contact lenses, and so on.
But all these problems are also caused, when the eyelids are not completely closed in each blink, that is, when we blink and the eyelids margins do not touch. Many people blink that way, and they do not know it. In fact, my eyelids blinked wrongly before I started my degree; one day, as I was doing my practice, one colleage let me know it.
People using computers in a frequent basis, usually suffer these problems and in general, everybody that work many hours doing tasks that involve looking at near distance. These people concentrate so much on their tasks, that they “forget” to blink. This paper shows an interesting guide about how to blink consciously the correct way so to automate it and thus to avoid present or future ocular problems.
The TEAR serves to protect the cornea, cleaning and moisturizing our eyes. The Lacrimal Glands (in upper eyelids) produce tears that flow over the cornea (1). The tears drain into two small openings at the inside corner of the upper and lower eyelids called the Lacrimal Puncta. The tears drain into the tear ducts (Canaliculus) and then into the Lacrimal Sac and finally into the back of your nose and throat (Nasolacrimal Duct). Now you can understand why when we cry, “we cry with our nose, too”.
If many tears are produced and they are not correctly drain (Epiphora), the tears will drain down the face rather than through the nasolacrimal system. It is the feeling that the eye is always watery, with many tears.
Sometimes an obstruction is present in any of these ducts owing to a infection, this causes an inflammation of Lacrimal Sac (Dacryocysititis). Some babies suffer this infection (20-30 percent) and some adults, mainly women, because of aging.
Besides the Lacrimal Gland, there are some sebaceous glands in the eyelids, that produce the lipid layer of the tear. If any of them gets blocked, it might cause the following disorders:
- Stye: It is a red lump in the eyelid margin, very painful. It is caused by an infection of the sebaceous glands at the base of the eyelashes, with more or less depth. If it is deep, its treatment is more difficult.
- Blepharitis: It is an inflammation and irritation of the margins of the eyelids, due to an allergic, infectious, seborrheic, irritable or mixed reason. It usually occurs in both eyes at the same time, and it is recurrent.
- Chalazion: It is a hard and painless inflammation of some little sebaceous glands in the eyelid margin. It usually disappears in a few months, but it sometimes remains, develops into a cyst and its size increases. When this occurs, it causes aesthetic problems and, what is worse, might compress the cornea and modify vision. If they are small, they usually just need a corticoids injection, but if they are big, sometimes a little surgery is required to extirpate them.
In one hand, six EXTRAOCULAR MUSLES (EOM) are surrounding the eyeball and anchor it to the orbit. The extraocular muscles control eye movement and allow to lead them wherever we want (while reading, practicing sports, driving,…).
The Superior (2) (top) and Inferior (3) (bottom) Rectus Muscles control the eye’s vertical movement (up and down).
The Medial Rectus (4) and Lateral Rectus Muscles (5) control the eye’s lateral movement (from side to side).
The Superior Oblique (6) and Inferior Oblique Muscles (8) help rotate the eyes inward and outward in order to balance the sideways tilts of the head (they cause an opposite movement to eye).
All six of these extraocular muscles work together to move the eye. They coordinate so that the eyes are always aligned.
Any trauma in any orbit bone may cause a partial or total paralysis of any of these six muscles:
- If it is a partial paralysis we are before a Paresis or partial loss of movement owing to the weakness of one of them.
- If it is a total paralysis we are before a Paralysis or complete loss of the muscle function that causes restricted movement.
In any of these previous conditions, the eye movements in both eyes are not synchronous and thus cross-eyed or strabismus may appear and, consequently, double vision (but I will explain this later on).
In the other hand, we also have PALPEBRAE MUSCLES, which give eyes their shape and allow to open or close our eyes voluntary or involuntary manner.
If any of the muscles described is altered, it may cause the following conditions:
- The upper eyelid is dropped (Ptosis)
- When the previous condition happens or when there is a recession of the eyeball, the eyes seem smaller (Enophthalmos).
- Or, alternatively, when the eyes are more opened that what is usual or the eyeball bulges anteriorly out of the orbit, they seem bigger (Exophthalmos).
- The lower eyelid folds inward (Entropion), and this causes discomfort because the eyelashes rub against the cornea constantly (Trichiasis).
- Or, alternatively, the lower eyelid folds outward (Ectropion), drying the conjunctiva and the cornea, as the eye can not close totally (Lagophtalmos).
Well, my intention with this post is not for you learn these odd scientific concepts, but that these problems are familiar to you, and as with the previous post, if for whatever reason someone mentions these terms, you have where you can look them up to know what they talked about.
In the upsets of the palpebrae muscles I have preferred not to show directly the pictures just in case they are disgusting to any of you.
A little bit of basic ocular anatomy… Eye or Ocular Globe
A little bit of basic ocular anatomy… The Retina.