Development of the Central Nervous System Study Notes

The human brain involves approximately $86$ billion neurons and trillions of connections.
Neurodevelopment is a precisely orchestrated sequence influenced by genetic, environmental, biochemical, and physical factors.
The first $1000$ days (conception to approximately $2.5$ years) are the most critical for maturation.
Environmental input is essential for guiding development, as seen in the Case of Genie, where extreme neglect led to profound growth and cognitive deficits.

Induction of the Neural Plate (3 weeks - 3 months): Development begins with the neural plate, which folds to form the neural tube. Failures in tube closure can lead to Encephalocele (day $24$ ) or Myelomeningocele/Spina Bifida (before day $26$ ).
Neural Proliferation (Peaks 3-4 months): Rapid cell division (neurogenesis) occurs, producing up to $4 \times 10^6$ cells per hour. Disruptions can cause Microcephaly (too few neurons) or Macrocephaly (too many neurons or fluid buildup). Teratogens like alcohol lead to Fetal Alcohol Spectrum Disorder (FASD).
Migration and Aggregation (Peaks 3-5 months): Cells travel to target locations via radial or tangential migration. This process follows an inside-out pattern in the cortex. Abnormalities include Lissencephaly (smooth brain), Polymicrogyria (too many small gyri), and Neuronal Heterotopia (misplaced neuron clumps).
Axon Growth and Synapse Formation: Growth cones with filopodia search for targets using guidance molecules. The Chemoaffinity Hypothesis suggests postsynaptic surfaces release chemical labels to attract axons. Agenesis of the Corpus Callosum is a primary disorder of axonal development.
Neuron Death and Synapse Rearrangement: Programmed cell death (Apoptosis) removes about $50 \%$ of produced neurons to increase efficiency. Neurotrophins, such as nerve growth factor, are required for survival. Microglia assist in pruning inactive connections.

Brain Size: Brain volume increases fourfold between birth and adulthood, primarily due to synaptogenesis, dendritic branching, and myelination rather than new neuron production.
Myelination: Performed by oligodendrocytes, this process continues into the third decade of life. Sensory and motor regions myelinate first, followed by higher-order association areas.
Prefrontal Cortex (PFC): The most prolonged period of development, governing working memory, restraint, and planning. Insufficient PFC development in infants leads to perseveration (A-not-B error).

Critical Periods: Specific windows where environmental exposure is mandatory for system development (e.g., binocular vision at 3-8 months).
Sensitive Periods: Windows where environmental input has a greater influence but systems remain somewhat flexible (e.g., language before 7 years).
Environmental Impact: Enrichment leads to thicker cortices and more dendritic spines, while deprivation (e.g., cataracts or social isolation) can cause permanent functional deficits.

Neurogenesis continues in adults within the Hippocampus (approximately $700$ per day) and the Striatum.
These processes are enhanced by exercise and cognitive challenges, allowing adaptation to complex environments.
Neuroplasticity in the adult cortex allows for functional reorganization based on extensive environmental exposure, such as enlarged representation areas in the somatosensory cortex of musicians.

Preterm Birth: Very preterm birth occurs during critical myelination phases, often leading to white matter loss and increased risk for cognitive impairment.
Neurodivergence: Conditions like ADHD and Autism are viewed as non-pathological variations in brain development involving atypical neurogenesis and synaptic organization. Disability is often defined by the lack of "environmental fit" rather than the divergence itself.
Paediatric Acquired Brain Injury (ABI): Early injury can disrupt critical periods but also benefits from high plasticity, offering significant potential for recovery and compensation depending on severity.