Introduction to Neurodevelopment

Introduction to Neurodevelopment

1. What is Neurodevelopment?

• Definition: Neurodevelopment refers to the growth, differentiation, and organisation of the nervous system from conception to adulthood.

• Key Aspects:

  • Formation of the neural tube (early brain and spinal cord).

  • Neurogenesis (birth of neurons).

  • Synaptogenesis (formation of connections between neurons).

  • Myelination (insulating neurons to improve communication).

  • Plasticity (how experiences shape the brain).

• Why It Matters: Abnormal neurodevelopment can lead to disorders like autism, schizophrenia, ADHD, and learning disabilities.

2. Key Facts About the Developing Brain

• At birth, the brain weighs ~350g; by adulthood, it reaches ~1300g.

• The mature brain contains ~85 billion neurons and trillions of synapses.

• During peak neurogenesis (before birth), the brain generates ~250,000 new neurons per minute!

• The brain continues developing into early adulthood, with major refinements in adolescence.

3. Stages of Prenatal Brain Development

Prenatal brain development occurs in three major stages:

Germinal Stage (Weeks 1-2)

• Fertilisation occurs → formation of a zygote.

• At 12 hours, the zygote begins cleavage (cell division).

• After several divisions, it forms a morula (a cluster of identical cells).

• The morula develops into a blastocyst (200-300 cells).

• Around week 2, the blastocyst implants into the uterus → transition to embryonic stage.

4. Formation of the Nervous System: Embryonic Stage (Weeks 3-8)

Gastrulation (Formation of Germ Layers)

• The embryo consists of three germ layers:

  • Ectoderm → Forms the nervous system and skin.

  • Mesoderm → Forms muscles, bones, and blood vessels.

  • Endoderm → Forms internal organs (lungs, liver, gut).

Neurulation (Weeks 3-4) – How the Brain Begins

• The ectoderm thickens, forming the neural plate.

• The neural plate folds in on itself, creating the neural tube.

• The neural tube later develops into:

  • Forebrain (Cortex, Thalamus, Hypothalamus)

  • Midbrain (Tectum, Tegmentum)

  • Hindbrain (Cerebellum, Pons, Medulla)

  • Spinal Cord

5. Neural Tube Defects (NTDs)

What Happens If the Neural Tube Doesn't Close Properly?

• Folic acid is crucial for proper neural tube closure (hence why pregnant women take it).

6. Stages of Brain Development (Neurodevelopment Timeline)

Neurogenesis (Weeks 4-20)

• Neurons proliferate rapidly in the ventricular zone of the brain.

• Some neurons migrate to the cortex, while others remain in deeper brain structures.

Cell Migration (Weeks 6-24)

• Neurons move along radial glial cells to their correct positions.

• Errors in migration are linked to autism and schizophrenia.

Synaptogenesis & Synaptic Pruning

• Neurons begin forming synapses with other neurons.

• By age 2, a single neuron can have ~15,000 synapses!

• Synaptic pruning: Unused connections are removed to increase efficiency.

• Example: Language-learning toddlers have excess synapses, but pruning refines speech patterns.

Myelination (Birth → Early Adulthood)

• Myelin sheaths form around axons to speed up neural communication.

• Begins prenatally and continues into early adulthood.

• Example: The prefrontal cortex (involved in decision-making) isn’t fully myelinated until ~25 years old!

7. The Role of Experience in Brain Development

• Neurodevelopment is not just genetic – experience shapes brain structure!

Two Types of Brain Plasticity

• Example: Enriched vs. Deprived Environments

  • Rats raised in enriched cages (toys, social interactions) develop MORE synapses.

  • Rats raised in isolation have FEWER synapses and impaired cognition.

8. Critical Periods in Neurodevelopment

• Definition: A window of time when the brain is especially sensitive to input.

• If key experiences don’t happen during this period, brain development may be permanently altered.

Examples of Critical Periods

1. Vision (First 6 months) – If infants don’t receive visual input, their vision won’t develop properly.

2. Language (~Birth to 7 years) – Children deprived of language input struggle to develop fluency later.

3. Attachment & Social Skills (~First 2 years) – Deprivation (e.g., orphanages) can cause emotional dysregulation.

9. Implications for Neurodevelopmental Disorders

1. Autism Spectrum Disorder (ASD)

• Linked to excess synapses due to reduced synaptic pruning.

• Differences in neurogenesis, migration, and connectivity.

• Early intervention helps reshape neural circuits.

2. Schizophrenia

• Linked to abnormal synaptic pruning in adolescence.

• Disruptions in dopamine and glutamate systems.

• Genetic and environmental interactions play a role.

3. ADHD

• Delayed cortical maturation (prefrontal cortex develops slower).

• Differences in dopamine regulation.

• Stimulant medications (Ritalin) help improve prefrontal function.

Conclusion

• Neurodevelopment is a complex interaction of genes, environment, and experience.

• Early experiences shape the brain through plasticity.

• Understanding neurodevelopment helps us diagnose and treat disorders like ASD, schizophrenia, and ADHD.