Which pair of letters are BOTH labelling a muscle or group of muscles innervated by lateral branches of dorsal (posterior) rami?

Question

BlackTom AI · Accepted Answer

Lateral branches of the dorsal (posterior) rami supply the intermediate and lateral columns of the intrinsic (deep) back muscles—principally the erector spinae group (iliocostalis laterally and longissimus intermediately) and the overlying skin. In contrast, the medial branches of dorsal rami supply the most medial/deep muscles (multifidus, semispinalis, rotatores, interspinales) and the skin immediately adjacent to the midline.

Step-by-step reasoning:
1. Identify which muscles the labeled letters point to. In the image, the labels B and E correspond to the vertically oriented, laminated muscle columns lateral and intermediate to the spinous processes (the errector spinae mass). B points to the lateral-most column (iliocostalis) and E to the intermediate column (longissimus).
2. Apply the innervation rule: iliocostalis and longissimus receive innervation from the lateral branches of the dorsal rami. Therefore both B and E label muscles innervated by lateral branches.
3. Check the other labels to rule out alternative choices: A (upper left bracket) marks muscles in the most medial/superior deep region (semispinalis/splenius region). These medial/deep muscles are primarily supplied by medial branches of dorsal rami (so A is not a lateral-branch target). D and C label more lateral shoulder/neck or superficial muscles (e.g., trapezius/levator/scapular region) that are not supplied by lateral branches of dorsal rami (trapezius is cranial nerve XI, levator/dorsal scapular is from ventral rami), so they do not meet the criterion.

Why the correct answer is correct:
- Both B and E identify components of the erector spinae (iliocostalis and longissimus), and these are classically innervated by lateral branches of the posterior rami. Thus the pair “E and B” satisfies the condition that both are muscles innervated by lateral branches of dorsal rami.

Why other options are incorrect:
- D and A: at least one (A) is a medial/deep muscle innervated by medial branches, and D is not a lateral-branch target.
- A and C: A is medial-branch innervated; C is not part of the lateral-branch-innervated erector spinae.
- B and A: A is not innervated by lateral branches.

Conclusion: E and B is the correct choice, matching the provided answer.

Which pair of letters are BOTH labelling a muscle or group of muscles innervated by lateral branches of dorsal (posterior) rami?单项选择题

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