Neurology - Embryology

Question 1

Neural development

• Notochord
• Neural plate
• Orientation
  
  • Alar plate
  • Basal plate

Answer 1

• Notochord
  
  • Induces overlying ectoderm to differentiate into neuroectoderm and form the neural plate.
  • Notochord becomes nucleus pulposus of the intervertebral disc in adults.
• Neural plate
  
  • Gives rise to the neural tube and neural crest cells.
• Same orientation as spinal cord.
  
  • Alar plate (dorsal): sensory
  • Basal plate (ventral): motor

Question 2

Regional specification of developing brain (444)

• For each primary vescile
  
  • Other name
  • Secondary vesicles
• Forebrain
• Midbrain
• Hindbrain

Answer 2

• Forebrain (prosencephalon)
  
  • Telencephalon
  • Diencephalon
• Midbrain (mesencephalon)
  
  • Mesencephalon
• Hindbrain (rhombencephalon)
  
  • Metencephalon
  • Myelencephalon

Question 3

Regional specification of developing brain (444):

• For each secondary vesicle
  
  • Adult derivatives of walls
  • Adult derivatives of cavities
• Telencephalon
• Diencephalon
• Mesencephalon
• Metencephalon
• Myelencephalon

Answer 3

• Telencephalon
  
  • Walls: Cerebral hemispheres
  • Cavities: Lateral ventricles
• Diencephalon
  
  • Walls: Thalamus
  • Cavities: Third ventricle
• Mesencephalon
  
  • Walls: Midbrain
  • Cavities: Aqueduct
• Metencephalon
  
  • Walls: Pons & Cerebellum
  • Cavities: Upper part of fourth ventricle
• Myelencephalon
  
  • Walls: Medulla
  • Cavities: Lower part of fourth ventricle

Question 4

CNS/PNS origins

• Neuroectoderm
• Neural crest
• Mesoderm

Answer 4

• Neuroectoderm
  
  • CNS neurons
  • Ependymal cells (inner lining of ventricles, make CSF)
  • Oligodendroglia
  • Astrocytes.
• Neural crest
  
  • PNS neurons
  • Schwann cells.
• Mesoderm
  
  • Microglia (like Macrophages, originate from Mesoderm).

Question 5

Neural tube defects

• Definition
• Associations
• Confirmatory test

Answer 5

• Definition
  
  • Neuropores fail to fuse (4th week) Ž–> persistent connection between amniotic cavity and spinal canal.
• Associations
  
  • Associated with low folic acid intake before conception and during pregnancy.
  • Elevated α-fetoprotein (AFP) in amniotic fluid and maternal serum. 
• Confirmatory test
  
  • Increased acetylcholinesterase (AChE) in amniotic fluid is a helpful confirmatory test (fetal AChE in CSF transudates across defect into the amniotic fluid).

Question 6

Neural tube defects

• Spina bifida occulta
• Meningocele
• Meningomyelocele

Answer 6

• Spina bifida occulta
  
  • Failure of bony spinal canal to close, but no structural herniation.
  • Usually seen at lower vertebral levels.
  • Dura is intact.
  • Associated with tuft of hair or skin dimple at level of bony defect.
• Meningocele
  
  • Meninges (but not the spinal cord) herniate through spinal canal defect.
  • Normal α-fetoprotein (AFP).
• Meningomyelocele
  
  • Meninges and spinal cord herniate through spinal canal defect.

Question 7

Forebrain anomalies

• Anencephaly
• Holoprosencephaly

Answer 7

• Anencephaly
  
  • Malformation of anterior neural tube resulting in no forebrain, open calvarium (“frog-like appearance”).
  • Clinical findings
    
    • Increased α-fetoprotein (AFP)
    • Polyhydramnios (no swallowing center in brain).
  • Associated with maternal diabetes (type I).
  • Maternal folate supplementation decreased risk.
• Holoprosencephaly
  
  • Failure of left and right hemispheres to separate
  • Usually occurs during weeks 5–6.
  • Complex multifactorial etiology that may be related to mutations in sonic hedgehog signaling pathway.
  • Moderate form has cleft lip/palate, most severe form results in cyclopia.

Question 8

Posterior fossa malformations

• Chiari II (Arnold-Chiari malformation)
• Dandy-Walker

Answer 8

• Chiari II (Arnold-Chiari malformation)
  
  • Significant herniation of cerebellar tonsils and vermis through foramen magnum with aqueductal stenosis and hydrocephalus.
  • Often presents with lumbosacral myelomeningocele and paralysis below the defect.
• Dandy-Walker
  
  • Agenesis of cerebellar vermis with cystic enlargement of 4th ventricle (fills the enlarged posterior fossa).
  • Associated with hydrocephalus and spina bifida.

Question 9

Syringomyelia

• Definition
• Findings

Answer 9

• Definition
  
  • Cystic cavity (syrinx) within the spinal cord [A] (if central canal Ž–> hydromyelia).
  • Syrinx = tube, as in syringe
  • Most common at C8–T1
• Findings
  
  • Crossing anterior spinal commissural fibers are typically damaged first.
  • Results in a “cape-like,” bilateral loss of pain and temperature sensation in upper extremities (fine touch sensation is preserved).
  • Associated with Chiari I malformation
    
    • > 3–5 mm cerebellar tonsillar ectopia
    • Congenital, usually asymptomatic in childhood, manifests with headaches and cerebellar symptoms

Question 10

Tongue development

• Branchial arches
• Muscles of the tongue are derived from…
• Innervation
  
  • Taste
  • Pain
  • Motor

Answer 10

• Branchial arches
  
  • 1st and 2nd branchial arches form anterior 2/3 (thus sensation via CN V3, taste via CN VII).
  • 3rd and 4th branchial arches form posterior 1/3 (thus sensation and taste mainly via CN IX, extreme posterior via CN X).
• Muscles of the tongue are derived from…
  
  • Occipital myotomes.
• Innervation
  
  • Taste—CN VII, IX, X (solitary nucleus).
  • Pain—CN V3, IX, X.
  • Motor—CN XII.