Muscles spared during injury to cranial root of accessory nerve
**Core Concept**
The cranial root of the accessory nerve supplies the muscles of the pharynx and larynx, which are crucial for swallowing and speech. Injury to this nerve can lead to weakness or paralysis of these muscles, affecting various functions.
**Why the Correct Answer is Right**
The cranial root of the accessory nerve joins the vagus nerve and contributes to the formation of the pharyngeal plexus. This plexus supplies the muscles of the pharynx, including the stylopharyngeus, which is the only muscle to receive a separate nerve supply from the cranial root of the accessory nerve. The other muscles of the pharynx and larynx receive their innervation from branches of the vagus nerve. Therefore, the stylopharyngeus muscle is the only one spared during injury to the cranial root of the accessory nerve.
**Why Each Wrong Option is Incorrect**
* **Option A:** The sternocleidomastoid and trapezius muscles, which are supplied by the spinal root of the accessory nerve, are not spared during injury to the cranial root.
* **Option B:** The muscles of the larynx, including the cricothyroid, thyroarytenoid, and arytenoid muscles, receive their innervation from branches of the vagus nerve and are affected by injury to the cranial root of the accessory nerve.
* **Option C:** The pharyngeal constrictor muscles, including the superior, middle, and inferior constrictors, receive their innervation from branches of the vagus nerve and are affected by injury to the cranial root of the accessory nerve.
**Clinical Pearl / High-Yield Fact**
The stylopharyngeus muscle is the only muscle to receive a separate nerve supply from the cranial root of the accessory nerve, making it the muscle spared during injury to this nerve.
**Correct Answer:** A. Stylopharyngeus