How does the human eye lens work?

[mishrashubham]

Ok, I know that the human eye lens adjusts its focal length depending on the object distance to obtain a sharp image on the retina because the image distance has to remain constant. What I am not able to understand is how do the ciliary muscles and the suspensory ligaments contract in order change the shape of the lens. What exactly is the mechanism by which muscles and the ligaments work?

What type of muscle is the ciliary muscle? Wikipedia confuses me with

The ciliary muscle (pronounced /ˈsɪli.ɛəri/) is a ring of striated smooth muscle in the eye's middle layer (vascular layer) that controls accommodation...

and then saying

Smooth muscle is an involuntary non-striated muscle...

In what pattern are the muscle fibers arranged? (Radial, concentric, or some other complex structure)

The dilemma is that muscles are supposed to provide a force by contracting i.e. becoming smaller in size(Am I right?). But in a textbook I saw a diagram where the ring of muscles is expanding its width and exerting an inwards force on the lens (which does not look like contracting).

Also is the eye lens elastic i.e. is it constantly applying an inward elastic force on the ciliary muscles?

Thank You

 

[Andy Resnick]

The lens is a small fraction of the optical power of your eye- most of the refractive power is in the air-cornea interface (which is why PRK/LASIK is so effective).

But, to your questions: The lens is composed of epithelial tissue with a high concentration of proteins called [itex]\alpha[/itex]-crystallins. The lens is a concentric structure, like an onion, and is one of the oldest tissues in your body- it was formed before you were born- and there is no cell turnover.

The lens is supported around the edge by zonal fibers, which keep the lens stretched and flat- this means at rest, you are accommodated for distance viewing (say, 7 m). To view close-in, ciliary muscles attached to the zonal fibers contract, which releases tension in the zonal fibers, allowing the lens to become rounder.

http://img.medscape.com/pi/emed/ckb/ophthalmology/1189694-1219573-471.jpg

Geometrically, this can happen if the muscle contracts in the anterior-posterior direction; contraction will 'thicken' the ciliary body, allowing the zonal fibers to relax.

The ciliary body is appears to be smooth muscle:

http://vision.about.com/od/eyeanatomy/g/Ciliary_Body.htm

but I couldn't find too much detailed information about it. Recall, smooth muscle is less understood than striated muscle in general (the main physiological difference I know of is that smooth muscle can remain contracted without expending much energy).

 

[mishrashubham]

Thanks Andy for explaining...
I did some searching around and have managed to find an image which I have attached. Is that what you are talking about?

However Wikipedia says there are three theories about the mechanism of accommodation. It seems that scientists have not yet confirmed the mechanism, so am I to trust the image?

 

[Andy Resnick]

I am not surprised there is still much unknown about the physiology of vision. I don't know about 'trusting' schematics, but I haven't found any images of an actual eye/ciliary muscles in different states of accommodation.

 

[epenguin]

The lens is composed of epithelial tissue with a high concentration of proteins called [itex]\alpha[/itex]-crystallins. The lens is a concentric structure, like an onion, and is one of the oldest tissues in your body- it was formed before you were born- and there is no cell turnover.

All I knew about this was that crystallin surprisingly was found to be nothing but lactate dehydrogenase, the enzyme that in the muscles catalyses the last step of glycolysis (the process that occurs in muscle during rapid exertion giving you energy without overall oxidation) the reduction of pyruvate to lactate. To add to my ignorance I had always assumed that it was the accumulation of lactic acid that gave you cramp during exercise but minutes ago it turned out that what I always assumed is rubbish; however in my defence "Medical textbooks skirt the topic... avoiding any explanation." http://www.nytimes.com/2008/02/14/health/nutrition/14BEST.html

It turns out however that my other knowledge was too narrow - maybe that was the first enzyme to be found to have such a role but there are a number of equally surprising other ones. You may find these articles (which I shall now have to read myself) of interest - since I see you are at beginnings but curious do not worry if you do not understand all detail, but it will give you a sense of some interesting stuff that you may come back to later. In particular the crystallins are useful models to study for the protein folding theme. You can understand why protein folding is important in the lens. The properly folded proteins are like egg white, but 'denaturing' or misfolding proteins is like what you get as you heat egg white!

http://en.wikipedia.org/wiki/Alpha_crystallin

http://icb.oxfordjournals.org/content/43/4/481.full

http://pdbbeta.rcsb.org/pdb/static.do?p=education_discussion/molecule_of_the_month/pdb127_1.html

 

[mishrashubham]

Thanks Andy and epenguin

 

[Andy Resnick]

All I knew about this was that crystallin surprisingly was found to be nothing but lactate dehydrogenase, the enzyme that in the muscles catalyses the last step of glycolysis (the process that occurs in muscle during rapid exertion giving you energy without overall oxidation) the reduction of pyruvate to lactate.

That's cool- I had no idea:

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC282134/

 

[nismaratwork]

Hmmm... I wonder if this explains why eyeballs are so prized as foodstuffs in some cultures...

 

[FtlIsAwesome]

Hmmm... I wonder if this explains why eyeballs are so prized as foodstuffs in some cultures...

 

[nismaratwork]

I hear you, I'd previously attributed it to the isolation of island cultures, but... if they're similar to eggs when cooked, it might make more sense.

 

[pergradus]

I once tried a fish eyeball that had been cooked. (the fish had been steamed whole, the eyeball wasn't meant to be eaten but it was there so i tried it).

It was hard, tasteless and really didn't seem very edible.

 

[nismaratwork]

I once tried a fish eyeball that had been cooked. (the fish had been steamed whole, the eyeball wasn't meant to be eaten but it was there so i tried it).

It was hard, tasteless and really didn't seem very edible.

...And just like that, I throw up a little in my mouth...

 

[mishrashubham]

...And just like that, I throw up a little in my mouth...

Is it really supposed to be that gross?

 

[nismaratwork]

Is it really supposed to be that gross?

Well, from what I understand the eyeballs of large fish (tuna for instance) are usually cooked to remain... oh, let's just say, "plump".

I can't believe that a small shriveled eyeball is the ideal cooking method, but... as open minded as I can be, I'd only eat an eye if would offend my host to refuse it, or if I were starving.