All of the following cranial nerves have nuclei in the medulla EXCEPT:

Explanation

## Brainstem Nuclei of Cranial Nerves **Key Point:** The **medulla oblongata** contains the motor and sensory nuclei of CN IX, X, XI, and XII. The **trochlear nerve (CN IV)** is unique: its nucleus is in the **midbrain** (mesencephalon), not the medulla. ### Medullary Cranial Nerve Nuclei | Cranial Nerve | Nucleus Location | Function | |---------------|------------------|----------| | **CN X (Vagus)** | Medulla (dorsal motor, nucleus ambiguus, solitary nucleus) | Motor (pharynx, larynx, soft palate); sensory (viscera) | | **CN XI (Accessory)** | Medulla + spinal cord (C1–C5) | Motor to sternocleidomastoid, trapezius | | **CN XII (Hypoglossal)** | Medulla (hypoglossal nucleus) | Motor to tongue muscles | | **CN IV (Trochlear)** | **Midbrain** (dorsal tegmentum) | Motor to superior oblique muscle | **High-Yield:** CN IV is the **only cranial nerve with a nucleus in the midbrain**. It also has the unique feature of **decussating (crossing) completely before exiting the brainstem**, making it the only CN with a contralateral nucleus. ### Anatomical Hierarchy of Brainstem CN Nuclei ```mermaid graph TD A[Brainstem CN Nuclei]:::outcome --> B[Midbrain]:::action A --> C[Pons]:::action A --> D[Medulla]:::action B --> E[CN III, IV]:::outcome C --> F[CN V, VI, VII, VIII]:::outcome D --> G[CN IX, X, XI, XII]:::outcome E --> H[CN IV: Trochlear<br/>Midbrain nucleus] H -->|Unique| I[Complete decussation<br/>before exit]:::urgent ``` **Clinical Pearl:** A **medullary lesion** (e.g., Wallenberg syndrome from PICA infarction) may affect CN IX, X, XI, and XII on the same side, but CN IV would be spared. Conversely, a **midbrain lesion** would affect CN III and IV.