Ch.8 Adult & Aging Brain

THE ADULT BRAIN

• Size & composition
  • Loses gray matter through synaptic pruning; rate slows by late 20s
  • Gains white matter through myelination; peaks ~40 yrs
  • Shift from locally-organized childhood networks ➔ widely distributed adult networks
• Prefrontal Cortex (PFC)
  • Last major region to become fully “wired”
  • Governs planning, problem-solving, decision-making, cognitive control (impulse suppression)
  • Adult wiring ➔ decisions less dominated by emotion/reward/social pressure
• Intelligence trajectories
  • Overall peak: early–middle adulthood (≈ 25–60 yrs)
  • Fluid intelligence (pattern-finding, novel problem-solving): peaks ≈ 30
  • Crystallized intelligence (vocabulary, factual knowledge): rises until ≈ 50
  • No single age maximizes every cognitive skill

DEFINING & DISTINGUISHING AGING

• Aging = dynamic, gradual; can feature resilience & health
• Normal aging ≠ dementia/pathology
  • Dementia: cognitive decline that interferes with daily life
  • Profound, widespread neuron loss is pathological, not normal
• Healthy aging research examines lifestyle (diet, exercise, etc.) that supports cognition

COGNITIVE CHANGES WITH AGE

• Memory
  • Declarative (episodic + semantic): declines
  • Non-declarative (procedural): largely spared
  • Working memory: capacity & manipulation efficiency fall; decline may start ~30
• Fluid-intelligence components
  • Processing speed ↓
  • Problem-solving agility ↓
• Attention
  • Selective attention: harder to filter distractions (e.g., noisy restaurant)
  • Divided attention: multitasking (talking while driving) more challenging

STRUCTURAL CHANGES

• Global brain volume
  • Starts decreasing in 30s–40s; accelerates ≈ 60
• Regional shrinkage (greatest ➔ lesser)
  • PFC, cerebellum, hippocampus show biggest losses
• Neuron-level contributors
  • Smaller soma, dendritic retraction, myelin loss
  • Contrasts with adolescence where volume loss = pruning + cell death
• Cortical thinning
  • Follows volume pattern; frontal & temporal lobes thinnest
  • “Last-in, first-out” theory: regions that mature last deteriorate first
• White-matter trajectories
  • Projection fibers (early-maturing) age better
  • Association fibers (late-maturing, intra-hemispheric) deteriorate fastest

NEURONAL & SYNAPTIC CHANGES

• Dendritic alterations
  • Branch complexity ↓; dendritic spines lost, esp. “thin” spines (highly plastic)
  • Potential link to working-memory decline (unconfirmed)
• Neurogenesis
  • Continues life-long in olfactory bulbs & dentate gyrus, but rate ↓ with age (steep in mice; moderate decline in humans)
  • Enhancement strategies boost cognition in rodents

CHEMICAL CHANGES

• Neurotransmitters
  • Dopamine synthesis ↓; fewer receptors ➔ impacts motivation, cognition
  • Evidence suggests serotonin may also ↓

WHY DOES THE BRAIN AGE? (MECHANISMS)

• Gene-expression shifts
  • Synaptic-plasticity genes under-expressed; exhibit more DNA damage
• Oxidative stress & DNA damage
  • Mitochondria produce free radicals → damage lipids, proteins, DNA
  • Antioxidant defenses weaken with age
  • Observed ↑ mitochondrial DNA damage in aging & Alzheimer’s brains; rodent oxidative damage correlates with memory impairments
  • Brain uses ≈ 20\% of body’s energy ➔ high metabolic vulnerability; glucose uptake & mitochondrial efficiency decline
• Immune dysfunction
  • Microglia become hyper-reactive: chronic inflammation, ↓ anti-inflammatory factors, synapse remodeling deficits
  • Excessive microglial activity linked to cognitive impairments in mice
• Impaired protein recycling
  • Autophagy & proteasome systems less efficient ➔ abnormal protein accumulation
  • Neurons’ long lifespan (limited replacement) amplifies vulnerability
• Systemic contributors
  • Cardiovascular deterioration may drive/accelerate neural aging changes

HEALTHY AGING STRATEGIES

• Diet
  • Cardiovascular risk factors (hypertension, high LDL) ⇒ cognitive risk
  • Plant-rich diets (Mediterranean, DASH) correlate with lower dementia incidence
  • Specific nutrients: antioxidants (vitamins C & E, flavonoids), omega-3 fatty acids show observational benefit; supplement trials yield mixed results
  • Caloric restriction (energy intake ↓ without malnutrition) linked to cognitive benefits & longevity
• Exercise
  • Aerobic activity improves learning, memory, slows dementia progression, reduces cortical thinning & hippocampal shrinkage
  • Mechanisms: ↑ neuroplasticity, ↑ neurogenesis, ↑ neurotrophic factors, better cerebral blood flow
  • Rodent evidence: running wheels → higher hippocampal neurogenesis + better memory
  • Earlier adoption likely yields stronger neuroprotection, but late-life start still beneficial
• Mental stimulation & social engagement
  • Enriched environments in mice (toys, mazes, social interaction) → better late-life cognition, ↑ neurogenesis, ↑ neurotrophic factors
  • Humans: cognitively demanding jobs, reading, puzzles, music correlate with reduced cognitive decline
  • Large social networks/active social life bolster cognition

PRACTICAL & PHILOSOPHICAL IMPLICATIONS

• Aging brain’s plasticity suggests interventions (lifestyle, societal supports) can meaningfully extend cognitive health span
• Ethical dimension: ensuring equitable access to exercise facilities, nutritious foods, social opportunities for older adults
• Public-health planning: anticipate diverse aging trajectories; distinguish normal decline from pathological dementia for early detection & resource allocation
• Lifelong perspective: neuroprotective habits adopted in youth/midlife compound benefits, reinforcing preventive-medicine paradigms

KEY NUMERICAL & STATISTICAL REFERENCES

• Peak white-matter volume: ≈ 40 yrs
• Adult intelligence peak range: 25–60 yrs; fluid \approx 30, crystallized \approx 50
• Brain consumes 20\% of total body energy

CONNECTIONS TO OTHER CHAPTERS/FOUNDATIONAL PRINCIPLES

• Chapter 4 memory taxonomy: declarative vs. non-declarative informs aging-memory patterns
• Chapter on myelination: same principles explain white-matter growth & later degeneration
• Neuroplasticity concept underlies benefits of exercise, cognitive stimulation

SUMMARY TAKEAWAYS

• Maturation continues into third decade; decline begins subtly by fourth
• Normal aging entails selective, not wholesale, neuronal/synaptic loss
• Lifestyle plays decisive role in modulating trajectory; brain health is malleable across lifespan