Which structure is highlighted and indicated by the leader line? Multiple Choice Vestibulocochlear n. (VIII) Hypoglossal n (XII) Spinal accessory n. (XI) Glossopharyngeal n. (IX) Vagus n. (CN X)
The Correct Answer and Explanation is:
ChatGPT said:
The correct answer is: Spinal accessory nerve (XI)
Explanation
The spinal accessory nerve (cranial nerve XI) is one of the twelve cranial nerves, and it plays a crucial role in motor innervation to specific muscles. It has two distinct roots: a cranial root and a spinal root. The spinal root arises from the upper cervical segments of the spinal cord (C1–C5/C6), ascends through the foramen magnum into the cranial cavity, and then exits the skull through the jugular foramen, along with cranial nerves IX (glossopharyngeal), X (vagus), and the cranial root of XI.
After exiting the skull, the spinal accessory nerve typically courses posteriorly and inferiorly, passing deep to the sternocleidomastoid muscle (SCM), which it innervates. It then crosses the posterior triangle of the neck to innervate the trapezius muscle. These two muscles are primarily responsible for shoulder shrugging and head rotation.
The nerve is purely motor and does not carry sensory fibers. Its superficial location in the posterior triangle makes it vulnerable to iatrogenic injury during surgical procedures like lymph node biopsies or neck dissections.
In anatomical diagrams, the spinal accessory nerve can be identified as a slender nerve descending from the base of the skull (near the jugular foramen) and coursing obliquely across the neck muscles, often appearing in relation to the internal jugular vein. It is distinct from the vagus nerve (CN X), which continues more vertically within the carotid sheath, and from cranial nerves IX and XII, which have different courses and functions (IX being mixed sensory and motor, and XII being motor to the tongue).
Because of its unique path and function, the spinal accessory nerve is commonly highlighted in anatomy images that focus on cranial nerve exit points and neck dissections. Hence, it is the correct identification in this context.
