Regarding the midbrain cross-section at the level of the superior colliculus, all of the following structures are present EXCEPT:

Explanation

## Midbrain Cross-Section Anatomy at Superior Colliculus Level ### Key Anatomical Components **Key Point:** The midbrain at the level of the superior colliculus contains several critical structures arranged in a rostrocaudal organization. | Structure | Location in Midbrain | Function | |-----------|----------------------|-----------| | Superior colliculus | Dorsal tectum | Visual reflexes, eye movements | | Oculomotor nucleus | Periaqueductal gray (ventral to aqueduct) | CN III motor innervation | | Substantia nigra | Ventral tegmentum | Dopamine production, motor control | | Red nucleus | Central tegmentum | Motor coordination, rubrospinal tract | | Medial lemniscus | **Dorsal midbrain/upper pons** | Proprioception, fine touch | | Spinothalamic tract | Lateral tegmentum | Pain and temperature | ### Why the Medial Lemniscus is NOT at This Level **High-Yield:** The medial lemniscus is a **dorsal brainstem structure** that ascends through the medulla and lower pons. By the midbrain level, it has already **decussated and shifted laterally**, becoming part of the **ventral posterior thalamus (VPL/VPM)**. At the superior colliculus level, the medial lemniscus is NOT present in the ventral midbrain; instead, it is found in the **dorsal pons** at lower levels. **Clinical Pearl:** A lesion at the midbrain level (Weber syndrome) affects the oculomotor nerve and corticospinal tract, NOT the medial lemniscus. Medial lemniscus damage occurs with medullary lesions (medial medullary syndrome). ### Correct Structures at This Level 1. **Substantia nigra** — ventral tegmentum, dopaminergic neurons 2. **Red nucleus** — central tegmentum, receives cerebellar input 3. **Oculomotor nuclei** — periaqueductal gray, CN III origin 4. **Spinothalamic tract** — lateral tegmentum (NOT medial lemniscus) **Warning:** Do not confuse the **medial lemniscus** (dorsal, lower brainstem) with the **spinothalamic tract** (lateral, present at all brainstem levels).