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Re: (meteorobs) Aids for night vision?

On 4 Jan 2001, at 19:32, Gary W. Kronk wrote:

> For what its worth, I was always curious about the eye drops used by 
> optometrists to dilate the pupil. A few years ago I suffered an eye 
> injury that had me going back fairly often for checkups. I finally 
> began scheduling them for the evening, to avoid missing work. On 
> several occasions, my pupils were still fully dilated when I got home 
> and the skies were dark AND clear. Where I expected to see fainter 
> stars than ever before, I saw a sky full of fuzzy blobs. So the 
> lesson here is that fully dilated pupils are not good for observing, 
> unless, of course, the drops were causing something else to happen in 
> addition to the pupil dilation.

The autonomic functions of the eye (those NOT under concious control - 
including pupillary dilation and constriction) can be a bit confusing, so I would 
like to make an attempt to explain them here - I hope its not too far off topic Lew! 

The innervation of the eye's internal structures is via the autonomic nervous 
system. There are two types of autonomic nerves: sympathetic (adrenergic - 
noradrenalin is the neurotransmitter) and parasympathetic (cholinergic - 
acetylcholine is the neurotransmitter). These tend to operate in opposing 
fashion, balancing the responses to external stimuli.

There is parasympathetic innervation of BOTH the iris sphincter and the 
ciliary muscle.  Activation of the iris sphincter causes the pupil to constrict 
(miosis). The ciliary muscle controls accommodation (focusing near and far). 
Thus, any drug which affects parasympathetic transmission will affect BOTH 
of these muscles.

On the other hand, there is sympathetic innervation only of the dilator muscles 
of the iris. Thus, sympathetic activation results just in pupil dilation (the fight 
or flight response).  

The two most common topical mydriatics (pupillary dilator drugs) are 
Tropicamide (Mydriacyl) and phenylephrine (Neosynephrine). They operate by 
separate mechanisms. Tropicamide is a cholinergic blocker, hence it affects 
both activation of the iris sphincter and the ciliary muscle. This prevents 
pupillary constriction AND affects the ability to focus. Phenylephrine, on the 
other hand, is an adrenergic stimulant. It causes activation of the dilator 
muscle directly without affecting the ability to focus.

I suspect you were given Tropicamide, because it is the most common 
mydriatic since Phenylephrine is not as effective used alone. But,unfortunately
it has the side-effects of blurring your vision more than the adrenergics. 
It lasts about 5 hours or so. There is also the issue of a larger peripheral 
area of your lens becoming involved in imaging if you dilate the pupil beyond 
its normal bounds. This may contribute to the blurring as well.

It would be interesting if you wore a specially designed corrective lens which 
fixes these aberrations, thereby allowing a sharp focused image at these 
significantly greater pupillary diameters! You may be able to gain nearly a 
full magnitude for LM??

Mike.


 
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