Cranial Nerves and Brainstem Anatomy

Apr 10, 2025

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.