Ch. 7 Infant, Child & Adolescent Brain

Developmental Neurobiology: Core Challenge

• Central question: How do billions of brain cells form, specialize, migrate, and wire into adaptive networks?

• Significance:

  • Illuminates origins of learning abilities and vulnerabilities to disease.

  • Re‐frames adult disorders (e.g., schizophrenia, autism) in developmental terms.

  • Informs regenerative medicine by revealing how early wiring principles might be re-engaged after injury.

Prenatal Foundations ➜ Newborn Snapshot

• Gestation length considered: 40 weeks (three trimesters).

• Average newborn brain mass: 370\,\text{g} (\approx13 oz; a bit <1 lb).

• Adult comparison: 3 lb brain containing 86\,\text{billion} neurons.

• Thus, birth ≠ endpoint—major construction continues post-natally.

Post-Natal Growth Rate & Volume Milestones

• Immediate post-birth whole-brain growth: 1\%/day.

• By \text{age}=3 months, slows to 0.4\%/day.

• At 90 days: overall volume ↑ 64\% vs. birth.

• Cerebellum: fastest-growing region; >\times2 volume within same 90-day window.

  • Function: dense neuron packing; essential for early motor learning (grasping, feeding).

• Brain size timeline:

  • 5 yrs: \approx90\% adult volume.

  • 2 yrs: \approx80\% adult volume yet 50 % more synapses than adult cortex.

Cellular & Microstructural Changes

• Proliferation: glia + neurons multiply rapidly.

• Differentiation: cells assume region-specific phenotypes.

• Migration: neurons relocate to cortical/subcortical destinations.

• Cortex-specific neuron number ↑ 23$–$30\% in first 3 months.

• Synaptogenesis: dendrites & axons elongate; countless new synapses add bulk.

• Myelination:

  • Oligodendrocytes wrap axons; white matter looks "white" due to myelin.

  • Supports faster conduction & metabolic efficiency.

Synaptic Density & Pruning ("Rose-Bush" Metaphor)

• Two-year-old cortex: hyper-connected (50 % more synapses than adult) ➜ energetically unsustainable.

• Early childhood = massive synaptic pruning:

  • Weaker, low-use synapses eliminated.

  • Strong, frequently-activated synapses stabilized.

  • Analogy: pruning roses directs nutrients to productive branches, enabling new blooms.

• Process guided by experience + activity‐dependent signals.

Comparative Perspective & Critical Period Logic

• Humans born neurologically immature vs. many species (e.g., squirrel monkeys reach adult brain size by 6 months).

• Advantage: extended critical periods allow culture-specific shaping (language, social cues).

• Sensory, motor, and emotional inputs (faces, voices, touch) sculpt circuits through combined gene × environment forces.

• Cell death + pruning coincide with heightened learning (running, multi-language acquisition).

Adolescence: Second Critical Period

• Brain metaphor: "big ball of clay"—highly moldable yet disorganized.

• Key mechanistic changes:

  • Competitive elimination intensifies; only strongest synapses survive.

  • Dendritic branching & myelination surge, especially in frontal lobes.

• MRI findings:

  • White-matter volume ↑ notably in corpus callosum ➜ enhanced inter-hemispheric communication & learning.

  • Structural re-balancing among frontal (control) and limbic (emotion/reward) regions drives risk taking, sensation seeking, and robust learning.

• Longitudinal designs track individuals ➜ link early environment to teen outcomes (education, disease risk).

Addiction Vulnerability in Teens

• Enhanced plasticity + reward sensitivity → double-edged sword.

• Addiction framed as an acquired learning disorder (shared circuitry with learning & memory).

• Imaging evidence:

  • DTI: alcohol/drug users show ↓ white-matter integrity & ↓ gray-matter volume.

  • fMRI: adolescent binge drinkers exhibit lower task-related activity, reduced sustained attention, poorer working-memory performance.

Emerging Adulthood (≲30 yrs)

• Overall neurodevelopment continues until ~30 yrs.

• Region-specific trends:

  • Gray-matter density generally declines, except left temporal lobe (language/memory) which ↑ until \approx30.

  • Myelination trajectory shifts:
    • Childhood: visual, auditory, limbic cortices.
    • 20s: frontal & parietal neocortices → supports working memory & higher cognition.

• Frontal lobe = final maturation site → explains teen impulsivity, brief attention spans, forgetfulness.

Plasticity: Overarching Theme

• Defined: brain’s ability to modify circuits in response to environment.

• Required for critical periods; not unique to humans but exceptionally robust in our species.

Experience-Expectant Plasticity

• Brain "expects" universal inputs during narrow windows (faces, language, caregiving touch).

• Absence of input → abnormal maturation (e.g., finches failing to learn song without early adult model exposure).

Experience-Dependent Plasticity

• Ongoing, lifelong remodeling driven by idiosyncratic experiences (e.g., violin practice ➜ enlarged cortical map for left-hand fingers).

• Two-photon imaging in animals shows new spines & synapses forming even in adults after skill learning.

Therapeutic & Ethical Implications

• Goal: harness or re-open plasticity in adulthood to:

  • Treat developmental mis-timing disorders (autism, schizophrenia).

  • Improve recovery after traumatic brain injury.

  • Address learning disabilities & age-related decline.

• Strategies under exploration: pharmacological modulators, behavioral therapies, circuit "rewiring" protocols.

• Ethical notes:

  • Manipulating critical periods demands caution to avoid unintended cognitive/behavioral consequences.

  • Longitudinal data essential for evaluating lifespan outcomes.

Key Numbers & Formulas (Quick Reference)

• Newborn brain mass: 370\,\text{g}

• Adult brain mass: 3\,\text{lb}\; (\approx1.36\,\text{kg})

• Neuron count (adult): 86\times10^{9}

• Post-natal growth rate: 1\% \rightarrow 0.4\%/day (birth ➜ 3 months).

• Volume change at 90 days: +64\%.

• Cerebellar volume at 90 days: >\times2 initial.

• Cortical neuron increase (0–3 mo): 23\text{–}30\%.

• Synaptic surplus at 2 yrs: +50\% vs. adult.

• Brain size at 5 yrs: \approx90\% adult volume.

• Maturation complete ≈ 30 yrs.

These bullet-point notes encapsulate every major and minor detail, examples, metaphors, statistics, and implications presented in the transcript, delivering a comprehensive standalone study guide on infant, child, and adolescent brain development.