Histological structure of Medulla 3 levels

🖇 Lecture Notes: Medulla Oblongata & CNS Pathways

📌 General Rules of Neuronal Pathways

• 3 neurons: First, Second, Third order
• 🟠 1st order → Ipsilateral (same side)
• 🟠 2nd order → does the Crossing (Decussation)
• 🟠 3rd order → Contralateral (opposite side, reaching cortex)

📌 Ascending (Sensory) Tracts

🟣 Proprioception & Medial Lemniscus

• Receptors: muscle spindle, tendon spindle, Pacinian corpuscle
• Gracile Tract → lower body (below T6, medial)
• Cuneate Tract → upper body (above T6, lateral)
• Relay in Gracile & Cuneate Nuclei (Medulla)
• 2nd order axons → Internal Arcuate Fibers → cross midline → Sensory Decussation
• Form Medial Lemniscus → Thalamus (VPL) → Cortex (Area 3,1,2)

🟣 Other Sensory Structures

• Spinal Nucleus of Trigeminal (face pain/temp)
• Equivalent to Substantia Gelatinosa of Rolando (SGR)

📌 Descending (Motor) Tracts

• 11 tracts = 2 pyramidal + 9 extrapyramidal
• Pyramidal (Corticospinal):
• • Origin: Area 4 (motor cortex)
  • Form pyramids in upper medulla
  • 🚨 Motor Decussation (lower closed medulla):
  • • 85% cross → Lateral Corticospinal tract
    • 15% uncrossed → Ventral Corticospinal tract
• Medial Longitudinal Fasciculus (MLF): formed by sulcomarginal + ventral vestibulospinal → reduces tracts to 10

📌 Regions of the Medulla

🟠 Closed Medulla (Lower)

• Contains central canal
• Level of Motor Decussation
• Small Gracile & Cuneate Nuclei

🟠 Closed Medulla (Upper)

• Level of Sensory Decussation
• Large Gracile & Cuneate Nuclei
• Accessory Cuneate Nucleus appears
• Pyramid still undivided

🟠 Open Medulla

• Central canal opens into 4th ventricle
• Features: Pyramid, Inferior Olivary Nucleus, Vestibular nuclei
• CN nuclei:
• • CN XII (Hypoglossal)
  • CN IX, X, XI complex
  • Vagus nuclei: Dorsal Motor, Nucleus Ambiguus, Nucleus Solitarius

🚨 Key Questions Emphasized

1. How many decussations in Medulla? → 2 (Motor & Sensory)
2. Motor Decussation: lower closed medulla (85% cross, 15% uncrossed)
3. Sensory Decussation: upper closed medulla (Internal Arcuate Fibers → Medial Lemniscus)
4. Spinal Nucleus of Trigeminal = SGR (face ↔ spinal cord analogy)
5. Number of long tracts: initially 8 sensory + 11 motor (changes as tracts merge/terminate)
6. General Rule: 1st order same side, 2nd order crosses, 3rd order opposite side