Eye anatomy question

[acciddropping]

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Hey guys -

In FA 2013, it says that when ciliary muscle tightens, it relaxes the suspensory ligaments (zonular) fibers and therefore lens become more convex. - It's a process of how eyes accommodate when focusing on near objects.

My question is wouldn't the tightening (contraction) of the ciliary muscles pull the zonular fibers more? I don't get how zonula gets relaxed.

Thanks

 

[BlueJay25]

This one is a hard concept to grasp at first, I know. But contraction of the cilliary muscles actually allow relaxation of the zonular fibers. The way I understand it, is that when the ciliary muscle contracts, it takes the tension off of the zonules, so the zonular fibers are no longer taut, but rather slack.

Just remember if the muscle is contracted, the zonules will be relaxed and if the muscle is relaxed, the zonules will be taut.

Sympathetic activation of the muscle's B2 receptors cause relaxation of the ciliary muscles and increase in ciliary body size. This tautens the zonular fibers and the lens is stretched out, making it good for far vision.

Parasympathetic activation of the M3 receptors causes ciliary muscle contraction, the effect of contraction is to decrease the diameter of the ring of ciliary muscle. The zonular fibers relax and the lens becomes more spherical, making it good for near vision.

 

[acciddropping]

This one is a hard concept to grasp at first, I know. But contraction of the cilliary muscles actually allow relaxation of the zonular fibers. The way I understand it, is that when the ciliary muscle contracts, it takes the tension off of the zonules, so the zonular fibers are no longer taut, but rather slack.

Just remember if the muscle is contracted, the zonules will be relaxed and if the muscle is relaxed, the zonules will be taut.

Sympathetic activation of the muscle's B2 receptors cause relaxation of the ciliary muscles and increase in ciliary body size. This tautens the zonular fibers and the lens is stretched out, making it good for far vision.

Parasympathetic activation of the M3 receptors causes ciliary muscle contraction, the effect of contraction is to decrease the diameter of the ring of ciliary muscle. The zonular fibers relax and the lens becomes more spherical, making it good for near vision.

Thanks for this! Would you mind helping integrate this concept into ARP? In ARP, accommodation reflex is intact and what you explained above mechanism is therefore intact. I am not sure what it means when pupillary reflex is not present - In pupilary reflex, CN2 and CN3 are involved - but I am assuming that CN3 is affected in ARP?

 

[syma]

Your afferents are CN 2.

 

[BlueJay25]

By ARP I'm assuming you're referring to Argyll Robertson Pupil.

As far as I know: the pupil will constrict to accommodate for near vision, but not for bright light. The pathophys is unknown.

Just for your knowledge below I gave you the afferent and efferent branches of the Pupillary light reflex.
Pupillary light reflex:
CN 2 - afferent
CN 3 - efferent