AI

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.