Sympathomimetic Effects

by Pravin Shukle, MD

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    Okay. Let's go back to the eye. So what are the effects of drugs that are sympathomimetics on the eye? Well, first of all, you get contraction of the radial or dilator muscles, so you open up the iris. That's called mydriasis. Now, accommodation is rarely affected in this situation. But just remember that when you're running away from that saber-toothed tiger as a caveman or a cavewoman, you want to be able to see in the dark. That's a sympathomimetic effect, and so sympathomimetic drugs are going to cause pupillary dilatation. Nonselective alpha agonists reduce intraocular pressure, and that's because you increase drainage of aqueous humour. Alpha 2 selective agonist also reduce intraocular pressure, and that's because there's reduced synthesis of aqueous humour, and examples are listed here. Okay, now I'm confused. What is going on? And I know you're saying this "You just finished telling me that those other drugs, the cholinomimetics do the same thing. and the beta blocker do the same thing. Well, how come all of these drugs reduce intraocular pressure?" Well, let's just go over it really quickly. Beta blockers reduce secretion from the ciliary epithelium. Cholinomimetics contract the ciliary muscles and they open up trabecular. So, normally the trabecular are closed, but as the ciliary muscle contracts, they open up like this. And what you end up having is increased outflow of the aqueous humour, and therefore you reduced pressure. Alpha agonists increased outflow through the veins. So, in the veins in the back of the eye and inside the retina, you have veins that are sealed up with tight gap junctions, and when the veins relax because of the alpha agonists, the gap junction open up, fluid from the eye can get absorbed into the vein, and so the vein carries away...

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    The lecture Sympathomimetic Effects by Pravin Shukle, MD is from the course ANS - Pharmacology.

    Author of lecture Sympathomimetic Effects

     Pravin Shukle, MD

    Pravin Shukle, MD

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