Neurology: Section Three

Overview

• Section divided into two parts:
  1. Introduction to cranial nerves
  2. Focus on examples and integration of material

Part 1: Introduction to Cranial Nerves

Corticobulbar Tract

• Definition: Cranial nerve pathway connecting upper motor neurons (UMN) to lower motor neurons (LMN).
• Origin: UMN originate in the primary motor cortex.
• Pathway:
  • Axons descend through the internal capsule (genu of the internal capsule).
  • Axons decussate above each LMN in the brainstem (midbrain, pons, medulla).

Brainstem Anatomy

• Brainstem Regions:
  • Midbrain
  • Pons
  • Medulla
• Cranial Nerve Nuclei: Located within the brainstem (cranial nerve nuclei).
• Example: Trigeminal cranial nerve nucleus spans pons, medulla, and cervical spinal cord.

Decussation

• Axons from UMN decussate directly superior to the cranial nerve nucleus they innervate.
• Applies to all cranial nerve nuclei receiving motor information.

Complexity and Lesions

• Corticobulbar tract is complex.
• Lesions are unpredictable; focus on understanding the general pathway.

Part 2: Cross-Sections of the Brainstem

Midbrain (Figure 8.16)

• Identification: Recognized by cerebral peduncles ('two legs') and substantia nigra (dark pigment).
• Cranial Nerve Nuclei:
  • CN III (Oculomotor) and CN IV (Trochlear) nuclei are present.
  • CN nuclei are typically near the cerebral aqueduct.

Pons (Figure 8.18)

• Identification: Identified by transverse pontine fibers.
• Cranial Nerve Nuclei:
  • CN V to VIII emanate from the pons.
  • Importance of the solitary nucleus for sensory information (CN VII, IX, X).

Nucleus Locations

• Use the sulcus limitans as a guide:
  • Nuclei lateral to sulcus are sensory.
  • Nuclei medial to sulcus are motor.

Microscopic Cross-Sections

• Importance: Familiarity with myelin stains of brainstem segments.
• Recognition: Recognize what segments are being viewed and identify major landmarks.

Key Terms

• Corticobulbar Tract: Connects UMN to LMN.
• Decussation: Crossing of axons above LMN.
• Cranial Nerve Nuclei: Location of LMN in the brainstem.
• Sulcus Limitans: Divides sensory and motor nuclei in brainstem.