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Cranial Nerves and Brainstem Anatomy
Apr 10, 2025
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Neurology: Section Three
Overview
Section divided into two parts:
Introduction to cranial nerves
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.
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