BR Chapter 1 Passage 8 Questions 51 and 52: Retinal Projecitons

[deleted647690]

Could someone explain the eyes to me?
51. "As part of a routine eye exam, the following is noticed: if light is shone directly into the patient's left eye, the patient exhibits a consensual but not a direct response. Which of the following is likely?
A. The optic nerve of the left eye is intact, but the efferent limb of the left eye is damaged.

B. The ON of the LE is damaged, but the efferent of the LE is intact.

C. The ON of the RE is intact, but the efferent limb of the RE is damaged.

D. THe ON of the RE is damaged, but the efferent limb of the RE is intact."


I thought it was D, but the correct answer is A.
Since the patient's right eye is working but not his left, I thought his right ON would be damaged. I thought the right ON controlled the left eye, while the left ON controlled the RE.
I don't really know what it means by "efferent limb"






52. "In another patient, the following is observed: Light shone into the RE does not elicit a response in either pupil. Light shone directly into the LE causes a direct and consensual response. It can be concluded that there is a lesion in the:
A. Left ON

B. Right ON

C. Left preganglionic parasympathetic neuron

D. Right preganglionic parasympathetic neuron"


I thought that since the left ON controls right eye, and right ON controls left eye, the left optic nerve had to be damaged since the right eye does not work upon light exposure. So I thought A.
However, the answer was B.

So I guess the left ON controls left eye, and right ON controls right eye. So what is it that is contralateral in the eye then? Where does decussation occur?

Could someone link me to a helpful diagram?

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[deleted647690]

edit:


I figured it out from this video.

There was a passage that provided an explanation for this, but I was so confused by the passage. There is no way I would be able to understand this without a diagram, based on the way it was described in the passage......

While I read the passage, I just kept thinking about what I knew from my physiology class about the decussation of the optic nerve. I had no idea this whole thing existed.

 

[BerkReviewTeach]

That is a really cool post. Very helpful video.