Neurology Section Three: Cranial Nerves and Corticobulbar Tract

Overview

• This section is divided into two parts:
  • Part 1: Introduction to cranial nerves with examples.
  • Part 2: Integration of remaining material focusing on examples.

Corticobulbar Tract

• Definition: A cranial nerve pathway connecting upper motor neurons (UMNs) to lower motor neurons (LMNs).
• Origin: All motor information originates from UMNs in the primary motor cortex.
• Pathway:
  • UMNs originate in the primary motor cortex.
  • Axons descend through the brain via the internal capsule.
  • Pass through the genu of the internal capsule.
  • Axons decussate (cross over) above each LMN in the brainstem.
• Brainstem Regions:
  • Midbrain, Pons, Medulla.
• Cranial Nerve Nuclei:
  • Located within the brainstem, also known as cranial nerve nuclei.
  • Some, like the trigeminal nerve, span large brainstem segments.

Important Principles

• Axons from UMNs decussate directly superior to the cranial nerve nucleus they innervate.
• Lesions in the corticobulbar tract are unpredictable; focus on understanding the general pathway.

Brainstem Cross-Sections

Midbrain

• Figure 8.16: Transverse section of the midbrain.
• Identification Features:
  • Cerebral peduncles (looks like legs).
  • Substantia nigra (dark pigmented region).
  • Red nucleus.
• Cranial Nerve Nuclei:
  • Nuclei for cranial nerves III (oculomotor) and IV (trochlear) are located near the cerebral aqueduct.

Pons

• Figure 8.18: Transverse section of the pons.
• Identification Features:
  • Transverse pontine fibers.
• Cranial Nerve Nuclei:
  • Cranial nerves V through VIII emanate from the pons.
  • Nuclei are adjacent to the fourth ventricle.
  • Solitary Nucleus: Spans much of the brainstem, contains sensory information for cranial nerves VII, IX, and X.
• Sulcus Limitans: Divides sensory (lateral) from motor (medial) nuclei; a key guide for identifying functions.

Important Notes

• Familiarity with myelin stains of brainstem cross-sections is critical for recognizing segments and landmarks.
• Use the sulcus limitans as a guide to differentiate sensory and motor nuclei.
• Understand the general layout and features of the brainstem to answer related questions effectively.