CNS Development

Embryological Origin of the CNS

• The entire central nervous system (CNS)—brain + spinal cord—derives from the surface ectoderm of the embryo.
• A specialized thickened region of this ectoderm, the neural plate, marks the future CNS.
  • Orientation terminology: Head (rostral) end vs Tail (caudal) end.

Neurulation: Four Key Morphogenetic Steps

• Step 1 – Neural-plate formation
  • Flat sheet of ectoderm designated to become the nervous system.
• Step 2 – Neural-groove formation
  • Plate sinks along the embryo’s dorsal midline, creating a neural groove flanked by elevated neural folds.
• Step 3 – Zipping/closure begins
  • Deepening groove causes folds to converge and “zip” cranially and caudally.
• Step 4 – Neural-tube closure
  • Folds fuse to form a hollow neural tube (future spinal cord & brain).
  • Overlying ectoderm reseals and later becomes the skin.
  • Tube detaches from surface, then migrates ventrally into the embryonic axis.

Neural-Tube Defect Highlight

• Spina bifida
  • Etiology: Neural tube fails to drop completely into vertebral column → unfused vertebral arches.
  • Consequences: Exposed spinal tissue → neurological deficits (motor, sensory, bladder).
  • Modern management: Prenatal (in-utero) surgical repair now feasible.

Primary Brain Regions Derived from the Neural Tube

• Forebrain (prosencephalon)
  • Cerebrum
  • Thalamus & Hypothalamus
• Midbrain (mesencephalon)
• Hindbrain (rhombencephalon)
  • Cerebellum
  • Pons
  • Medulla oblongata
• Continuous caudal extension = Spinal cord.

Chronological Milestones in Brain Development

• Week 5
  • Distinct forebrain, midbrain, hindbrain vesicles visible.
• Week 13
  • Rapid expansion of the cerebrum begins to overgrow diencephalon & brain-stem structures.
• Week 26
  • Surface gyri & sulci start to appear; cerebellum enlarges.
• Birth (≈ week 40)
  • Adult-like regional arrangement: massive cerebrum overlying thalamus; cerebellum tucked beneath occipital lobes; fully segmented brain stem.

Adult Brain & Major Components (Anatomical Recap)

• Cerebrum – higher cognition, voluntary motor control.
• Thalamus – relay/processing gateway for sensory input.
• Hypothalamus – autonomic & endocrine regulation, homeostasis.
• Midbrain – auditory/visual reflex centers, motor tracts.
• Pons – bridge connecting cerebellum with cerebrum & spinal cord; respiratory modulator.
• Medulla oblongata – cardiovascular, respiratory nuclei; decussation of pyramids.
• Cerebellum – coordination, balance, motor learning.
• Spinal cord – two-way conduction pathway + reflex center.

Adult Spinal Cord & Spinal Nerves

• Cord terminates around vertebral level L2 (conus medullaris).
• Regional spinal nerves
  • Cervical, Thoracic, Lumbar, Sacral.
  • Each region innervates corresponding body segments.

Functional & Clinical Connections

• Proper neural-tube closure is prerequisite for normal CNS development; failure at cranial end → anencephaly (fatal), at caudal end → spina bifida (variable severity).
• Early CNS patterning sets stage for later cortical specialization, myelination, and synaptogenesis—processes continuing into postnatal life.