Chapter 7 Infant child and adolescent brain

Developmental neurobiology

Field that studies how brain cells form, specialize, migrate, and connect to build neural networks

Developmental origin of neurological disorders

Many disorders such as schizophrenia or autism may arise from abnormal brain development early in life

Average weight of a newborn brain

About 370 grams (13 ounces)

Average weight of an adult brain

About 3 pounds (≈1.4 kg)

Number of neurons in the adult human brain

About 86 billion

Brain growth rate immediately after birth

About 1 percent per day

Brain growth rate by 3 months of age

About 0.4 percent per day

Brain volume increase by 90 days old

About 64 percent larger than at birth

Fastest growing brain region in infancy

Cerebellum

Function of the cerebellum

Coordination of movement and motor learning

Processes contributing to early brain growth

Cell proliferation, differentiation, migration, synaptogenesis, and myelination

Increase in cortical neurons during first 3 months

About 23–30 percent increase

Synaptogenesis

Formation of synaptic connections between neurons

Glia

Support cells that grow, multiply, and help myelinate axons

Oligodendrocytes

Glial cells that produce myelin in the central nervous system

White matter

Color caused by myelin-wrapped nerve fibers

Brain size by age 5

About 90 percent of adult size

Synaptic density in a 2-year-old

About 50 percent more synapses than an adult

Reason the brain prunes synapses

Synapses require energy and resources to maintain

Synaptic pruning

Process of eliminating weaker synapses while strengthening frequently used connections

Experience-dependent pruning

Connections that are frequently used are strengthened while unused ones are removed

Human brain maturity compared to other animals

Human brains develop more slowly and continue developing longer after birth

Example of faster brain maturation in animals

Squirrel monkeys reach adult brain size around 6 months

Advantage of prolonged human brain development

Greater ability for environment and experience to shape neural circuits

Critical periods

Limited developmental windows when environmental input strongly influences brain wiring

Examples of early sensory experiences shaping the brain

Seeing faces, hearing voices, physical contact

Genes vs environment in development

Both strongly influence neural circuit formation during critical periods

Longitudinal studies

Research studies that follow the same individuals over long periods of time

Purpose of longitudinal studies

Understand how early life events influence later development or disease risk

Adolescence in brain development

A second critical period with major neural remodeling

Synaptic changes in adolescence

Increased synaptic pruning and strengthening of important connections

Competitive elimination

Process where stronger neural connections replace weaker ones

Dendritic branching during adolescence

Neurons extend dendrites to strengthen connections

Myelination during adolescence

Increases, especially in frontal lobes

White matter change in adolescence

White matter volume increases

Corpus callosum

Large bundle of myelinated fibers connecting the two cerebral hemispheres

Importance of corpus callosum growth

Improves communication between hemispheres and learning capacity

Teenage brain characteristics

Increased risk taking, sensation seeking, and learning capacity

Brain systems contributing to teenage behavior

Reward system changes and imbalance between frontal cortex and limbic system

Addiction hypothesis

Addiction may represent a type of learned behavior disorder

Effect of frequent drug use during adolescence

Damage to brain regions responsible for memory, attention, and executive function

Brain imaging technique for structure

MRI (Magnetic Resonance Imaging)

Brain imaging technique for white matter integrity

Diffusion Tensor Imaging (DTI)

Brain imaging technique for brain activity

fMRI (functional MRI)

Effect of alcohol use on adolescent brain

Reduced gray matter volume and reduced white matter integrity

Brain development completion age

Brain continues developing until about age 30

Gray matter density change with age

Generally declines with age

Exception to gray matter decline

Left temporal lobe gray matter increases until about age 30

Brain regions myelinated earlier

Visual, auditory, and limbic cortices

Brain regions myelinated later

Frontal and parietal neocortex

Frontal lobe

Last brain region to fully mature

Functions of the frontal lobe

Executive functions such as planning, attention, impulse control, and decision making

Executive functioning

Cognitive processes including attention, response inhibition, organization, and planning

Brain plasticity

Ability of the brain to change and adapt in response to experience

Importance of plasticity

Allows the brain to respond to environmental inputs and learning

Experience-expectant plasticity

Brain development requiring common environmental stimuli during critical periods

Examples of experience-expectant inputs

Language exposure, visual input, social contact

Example of experience-expectant learning in animals

Finches must hear adult songs to develop proper singing

Experience-dependent plasticity

Brain changes resulting from unique individual experiences

Example of experience-dependent plasticity

Violinists developing larger cortical representation for left-hand fingers

Two-photon imaging

Microscopy technique that allows scientists to observe living neurons and their growth

Plasticity in adulthood

The brain continues to change and adapt throughout life

Potential medical applications of plasticity

Treatments for brain injury, neurological disorders, and learning disabilities

Goal of developmental neuroscience

Understanding brain development to design age-specific therapies