Brain Stem Study Notes

Introduction

  • Definition of Brain Stem Terminology
    • Dorsal: Posterior and towards the back. Example: Dorsal fin of a fish.
    • Ventral: Anterior or towards the front.
    • Rostral: Towards the top of the head.
    • Caudal: Towards the tail or away from the head.

Overview of the Brain Stem

  • Location: At the base of the brain, just above the spinal cord.
  • Components:
    • Medulla
    • Pons
    • Midbrain
  • Visual Reference: Mention of an MRI image illustrating the medulla, pons, and midbrain.

Structures within the Brain Stem

  • Contains sensory and motor fibers.
  • Hosts nuclei for cranial nerves.
  • Importance of knowing:
    • Locations of different cranial nerves at each level.
    • Tracks traveling between the brain and spinal cord.
    • Medial vs. lateral structures for identifying stroke syndromes (e.g., medial medullary syndrome, lateral pontine syndrome).

Brain Stem Sections Identification

Midbrain, Pons, and Medulla Differentiation

  • Medulla and Pons: Both have the fourth ventricle; visual identification guide provided.
  • Midbrain: Lacks the fourth ventricle; instead contains the cerebral aqueduct and the third cranial nerve resembling “spider legs”.
  • Inferolivary Nucleus:
    • Recognized by its shape, indicating the medulla; associated with the cerebellum.

Key Structures and Functions in Each Section

Midbrain

  • Spinothalamic Tract: Carries pain and temperature information.
  • Medial Lemniscus: Connected to posterior columns in spinal cord; conveys proprioception and vibration information.
  • Red Nucleus: Important for fine-tuning movements; lesions lead to tremor and ataxia.
  • Oculomotor Nerve (Cranial Nerve III): Associated with eye movement; lesions cause ocular motor palsy.
  • Cerebral Peduncle: Carries motor fibers; damage results in upper motor neuron paralysis of the face and lower extremities.
  • Medial Longitudinal Fasciculus: Damage yields problems with lateral gaze; condition called internuclear ophthalmoplegia.
  • Cerebral Aqueduct: Conduit for cerebrospinal fluid (CSF).
Midbrain Stroke Syndromes
  1. Benedict Syndrome: Damage to cranial nerve III, medial lemniscus, red nucleus; symptoms include ocular motor palsy, contralateral loss of proprioception/vibration, and tremor/ataxia.
  2. Weber Syndrome: Damage leading to loss of cranial nerve III, corticospinal tract, corticobulbar tract; symptoms include contralateral hemiparesis and pseudobulbar palsy.
  3. Paranoid Syndrome: Involves the posterior midbrain; leads to vertical gaze palsy and pseudo Argyll Robinson pupil.

Pons

  • Key Structures:
    • Vestibular Nucleus (Cranial Nerve VIII): Vestibular dysfunction symptoms (nausea, vomiting, vertigo, nystagmus).
    • Cranial Nerve VII (Facial Nerve): Involved in facial motion; damage results in facial droop and loss of corneal reflex.
    • Medial Lemniscus: Loss of proprioception and vibration.
    • Corticospinal Tract: Damage results in hemiparesis.
    • Abducens Nerve (Cranial Nerve VI): Issues moving the eye laterally.
    • Spinothalamic Tract: Loss of pain and temperature sensation.
    • Spinal Tract and Nucleus of Trigeminal Nerve (Cranial Nerve V): Sensation in the face; damage results in facial loss of sensation.
    • Fourth Ventricle: Present at the top.
    • Medial Longitudinal Fasciculus & Paramedian Pontine Reticular Formation: Important for lateral gaze and coordination.
Pons Stroke Syndromes
Medial Pontine Syndrome
  • Symptoms: Loss of corticospinal tract (contralateral hemiparesis), loss of cranial nerves VI (gaze palsy), and cranial nerve VII (facial droop).
Lateral Pontine Syndrome
  • Symptoms: Loss of vestibular nuclei, symptoms like nystagmus, vertigo, nausea; loss of spinothalamic tract results in contralateral pain/temperature loss; loss of cranial nerve V leads to ipsilateral facial pain/temp loss.

Medulla

  • Nucleus Solitarius & Dorsal Motor Nucleus of Vagus Nerve: Key for autonomic sensory information.
  • Nucleus Ambiguous: Shared motor nucleus for cranial nerves IX, X, and XI; impacts vagus nerve functionality.
  • Cranial Nerve XII (Hypoglossal Nerve): Controls tongue movement.
  • Medial Lemniscus: Carries proprioception/vibration information; lesion results in corresponding deficits.
  • Corticospinal Tract: Bulges called pyramids signify this structure; lesions cause pyramidal dysfunction.
  • Inferior Olivary Nucleus: Helps identify medulla structurally.
  • Spinothalamic Tract: Carries pain and temperature sensations.
  • Nucleus and Tract of Trigeminal Nerve: Affects facial sensation.
  • Hypothalamospinal Tract: Carries sympathetic fibers; damage results in Horner syndrome.
Medulla Stroke Syndromes
  1. Medial Medullary Syndrome: Involves corticospinal tracts, medial lemniscus, cranial nerve XII; symptoms include contralateral hemiparesis, loss of proprioception, and flaccid paralysis of the tongue to the side of the lesion.
  2. Lateral Medullary Syndrome (Wallenberg Syndrome): Loss of vestibular nuclei leading to nystagmus, loss of sympathetic pathways to cause Horner syndrome, spinothalamic tract loss leading to contralateral deficits; damage to the spinal nucleus of cranial nerve V results in ipsilateral deficits in facial sensations.

Rule of Fours

  • Cranial Nerve Localization:
    • Medulla: Four cranial nerves (9, 10, 11, 12).
    • Pons: Four cranial nerves (5, 6, 7, 8).
    • Midbrain: Four cranial nerves (3, 4, 6).
  • Midline Columns (M):
    1. Motor Nucleus
    2. Motor Pathway (Corticospinal Tract)
    3. Medial Longitudinal Fasciculus
    4. Medial Lemniscus
  • Lateral Columns (S):
    1. Sympathetic Tracts
    2. Spinothalamic Tract
    3. Sensory to Face (Trigeminal Nucleus)
    4. Spinocerebellar Tract
  • Importance of recognizing whether a lesion is medial (M) or lateral (S) based on affected tracts and cranial nerves.

Clinical Cases and Examples

  • Case Example 1: 75-year-old man with sudden left-sided weakness, tongue deviating to the right; identified as a right medial medullary syndrome (anterior spinal artery involvement).
  • Case Example 2: Right-sided weakness with left eye down and out; identified as Weber syndrome (left medial midbrain lesion).
  • Case Example 3: Loss of pain/temperature sensation, hoarseness, and palate elevation issues; identified as left lateral medullary syndrome (posterior inferior cerebellar artery involvement).