Take the 2-minute tour ×
Biology Stack Exchange is a question and answer site for biology researchers, academics, and students. It's 100% free, no registration required.

I had laser eye surgery a decade ago, but in recent years my eyesight has become significantly myopic. I consulted an ophthalmologist to see if this was eye strain because I work at computers a lot, or part of a natural degradation of the eye over time, or both. My ophthalmologist seems to believe that in my case I'm youngish enough that the natural degradation with age is minimal, and that it's mostly eye strain that is my problem. She believes I can get my eyesight back to roughly 90% of my post-surgery sight ability, if I can reduce eye strain. She gave me some eye drops to help with dryness, and recommended various ways to for me to help my eyes recuperate.

I decided to look into eye strain to learn more about what conditions cause it and what can alleviate it. What I learned is that the lens of the eye needs to be flatter to accommodate focusing on far objects, and rounded to focus on near objects. The way the lens becomes flat is by using spring-like connective tissue called choroids that pull it taught. Attached to these choroids are muscles called ciliary muscles that stretch the choroids out when they contract. This action causes the choroids to stop pulling on the lens, and the lens will retract into a more rounded shape. So, when the ciliary muscles are relaxed, you can see far. When the ciliary muscles are contracted, you can see close up. This diagram from the York University website was the clearest explanation I have come across:

ciliary muscles and choroids

Thus, the reason for my current inability to focus on far objects is that so much focusing on close objects, mainly computer monitors, is straining my eyes. In order to regain the ability to focus on far objects, I need to reduce strain and allow the muscles to relax. If they relax, the choroids can pull the eye to the flatter shape needed to see far.

However, I can't reconcile that model with how I understand the mechanics of the other muscles in my body. If I go to the gym and run or lift weights, or in any way expose my muscles to work, they respond by getting stronger without sacrificing the ability to stop contracting. The muscles in my body don't lose the ability to relax, no matter how much I train them. I have never heard of anyone who worked out too hard or too long such that their bicep would remain in a permanent state of contraction.

In fact, in my experience, after a hard workout, it's impossible to prevent my muscles from relaxing and resisting doing more work. When I do a bicep curl at the gym, and I do it to the point that I can't lift the weight anymore, my muscle gives up and I drop the weight. Similarly, if I've spent a long time looking at close up objects, shouldn't my ciliary muscles give up, allowing the choroids to take over, making clear far vision the unavoidable outcome?

The idea that my ciliary muscles need to relax in order to see far also seems to contradict my personal anecdotal experience. Sometimes I am able to see far away, but I can't hold it for more than a few seconds. If I try to maintain focus on far objects for too long, I get an uncomfortable feeling in my eyes that is hard to describe, but it's a form of pain that forces me to give up. My vision goes blurry, and I can only see close objects again. If my bicep worked the same way, it would be as if it hurt to let my arm hang straight down with a weight, and the only way to alleviate it would be to raise the weight, which makes no sense. I feel like the effort is in seeing far, and that when I'm tired, I can only see close up.

It's not that the case that I think that all medical research on the eye has it backwards, it must be that there is some aspect of this that I am not seeing (pun intended).

How can it be that the ciliary muscles, unlike other muscles, lose their ability to relax?

Why is it that my ciliary muscles don't become exhausted and allow the choroids to take over by default?

share|improve this question
    
Regarding focusing on far away objects - perhaps that is unrelated to the distance? I imagine you would "stare" at the objects with intense concentration, rather than gazing casually, and would probably blink less and move your gaze direction less, fatiguing the retina. –  Superbest Apr 21 at 20:49
add comment

1 Answer 1

up vote 3 down vote accepted
+100

First of all, I should correct some points that were misunderstood. Don't change the question because this will lead to confusion.

"The way the lens becomes flat is by using spring-like connective tissue called choroids that pull it taught."

In classic ophthalmology you don't need to think about choroid in direct relation to accommodation: choroid is a sponge-like layer between the sclera and retina and in general it is consisted of blood vessels. The anterior part of the choroid continues anteriorly to became a ciliary body which in turn contains ciliary muscle - one circular muscle per eye. From the ciliary body/muscle spread zonules (zonule fibers) and they are fixed on the lens equator.

Physiology: contraction of ciliary muscle causes zonules to became loosen and "free" the lens to became more convex and move the focus anteriorly (not choroid contracts itself). If the ciliary muscle relaxes then zonules are tighten up and the lens consequently becomes more flat (less convex) moving the focus posteriorly. In other words you can say it in the terms of depth of focus - convex lens gives less depth, less convex gives more depth of focus.

Thus, the classical choroid layer does not perform any action (look at your choroid related link - there are almost nothing about accommodation).

"Permanent state of contraction" can be physiological (=normal) as well as abnormal one, and it is very common in some conditions (muscles spasms). One example is priapism, where corporal smooth muscle contraction causes permanent and dangerous penile erection which can be medical emergency (priapism is by far more complex, so take the explanation like a metaphor).

If we refer to "accommodation spasm" there is analogy to "muscle spasm" (and partially to priapism), but I should state we believe that spasm of ciliary muscle exists - since we don't see it directly. Probably (and take this sentence as speculation, since I cannot give you reference right now) the causes of this is not a muscle spasm itself, but the state of zonular fibers which cannot come back to their base state. I like the example with iron rod - if you will contract it fast and many times, at some point it can be "loosen" as well as fractured (and probably it does happens to zonules too). Probably (I say "probably" to underline the point we do not exactly know this), the "accommodation spasm" is partially misname and in future the investigations will clarify that.

Probably, you will learn some interesting facts from definition of "pseudoexfoliation" syndrome, but I do not explain it here because it is not related directly to the question. From wiki "has been known to cause a weakening of structures within the eye which help hold the eye's lens in place, called lens zonules"

Another example for analogy for continued "spasm" is the situation when one should care something heavy for a long distance without releasing the grasp - finally one can get not only spastic contraction but also severe ischemic damage to the fingers.

Considering your case, you should know about pathological (degenerative) myopia where the eye expands posteriorly and consequently the focus is before the retina which should be corrected by minus lenses. It is well known fact that myopic eyes have longer axial length then normal eyes. Probably, it is your case.

So, as you can see, the answer to your question is not a clear cut, but assumption. The ciliary muscle does can relax, but probably the problem is more complex then only ciliary muscle related issue.

PS The image you reposted is a little bitconfusing one and is not exact. This one is classical and gives better understanding of the anatomy -

enter image description here

share|improve this answer
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.