Download Chapter 5 Development

Chapter Five
Genetics and Development of the
Human Brain
© Cengage Learning 2016
© Cengage Learning 2016
Genetics and Behavior
• The interaction between genetics
(“nature”) and the environment (“nurture”)
can influence physical and behavioral
traits
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Heritability
• The contribution of genetics to the
variation of a trait observed in a population
– Heritability always refers to a population, not
to individuals
• Heritability cannot be assessed without
taking the environment into account
• Twin and adoption studies
– Minnesota Study of Twins Reared Apart
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Epigenetics
• Reversible genetic change
– Not a DNA sequence change
– Influenced by environmental factors
• Determines gene expression patterns
– Histone modifications
– DNA methylation
• Presence of methyl groups associated with gene
silencing
• Abnormal DNA methylation associated with
disease
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DNA Methylation and Histone Modification
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DNA Methylation and Child Maltreatment
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Building a Brain: Prenatal Development
• Zygote forms from fusion of egg and
sperm
• Early differentiation
– Cell germ layers – ectoderm, mesoderm, and
endoderm
– Neural plate, neural groove, neural tube
– Formation of prosencephalon,
mesencephalon, and rhombencephalon
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Division of the Neural Tube
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Nervous System Development:
Neurogenesis and Migration
• Neurogenesis
– Formation of neurons and glia
– Originate from cells in the ventricular zone
– Progenitor cells divide by mitosis
• Cell migration
– Guided by radial glia
– Cells in cerebral cortex arrive in an inside-out
fashion
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Neurogenesis
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Radial Glia Guide the Migration of New
Cells
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Nervous System Development:
Differentiation
• Differentiation of neural tube along the
dorsal and ventral halves and along the
rostral-caudal axis
– Influenced by differentiation-inducing factors
(DIFs)
– Organization of cerebral cortex affected by
intrinsic and extrinsic factors
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Nervous System Development:
Axon and Dendrite Growth
• Axons and dendrites arise from neurites
• Developing axons and dendrites end in
growth cones
• Filapodia and lamellipodia
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Growth Cones Guide Axons to Their Targets
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Growth Cones Respond to a Variety of
Cues
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Nervous System Development:
Formation of Synapses
• Synaptic specificity – neurite identification
of the correct target cell
• Movement of receptors to the synaptic site
is guided by presynaptic and postsynaptic
structures
• Interaction with target cells influences the
type of neurotransmitter released by the
presynaptic cell
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Nervous System Development:
Cell Death
• Apoptosis = programmed cell death
• Access to neurotrophins influence the
survival of a neuron
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Steps in the Formation of a Synapse at the
Neuromuscular Junction
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Nervous System Development:
Synaptic Pruning
• Reduces the number of functional
synapses
• Influenced by neutrophins and functionality
of the synapse
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Nervous System Development:
Myelination
• Occurs in rostral direction starting with the
spinal cord, then hindbrain, midbrain, and
forebrain
• Begins at 24 weeks postconception, with a
burst around the time of birth
• Prefrontal cortex not completely
myelinated until early adulthood
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Synaptic Rearrangement over the Lifespan
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Effects of Experience on Development
• Plasticity
– The ability to rearrange synaptic connections
• Experience and the visual system
– Early in development, cells of LGN and
primary visual cortex receive input from both
eyes
– Experience with sensory information
influences segregation of ocular dominance
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Input Influences the Development of the
Optic Tectum
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Input from Both Eyes Competes for the
Control of Target Cells in the LGN
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Early Experience Affects the Organization of
Ocular Dominance Columns
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Effects of Experience on Development
(cont’d.)
• Experience and social behavior
– Imprinting in several species of birds
– Romanian children and social deprivation
• Ending a critical period
– Conclusion of growth spurt in myelin
coincides with reduced abilities to learn
additional languages
– Presence or absence of neurotrophins may
influence timing of critical periods
– Epigenetics may play a role
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Effects of Enriched Environments
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Disorders of Nervous Development
• Neural tube defects
– Anencephaly
– Spinal bifida
• Genetic disorders
– Down syndrome
– Fragile-X syndrome
– Phenylketonuria (PKU)
• Environmental toxins
– Fetal alcohol syndrome
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Fragile X Syndrome
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Fetal Alcohol Syndrome Produces Physical
and Intellectual Abnormalities
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The Brain in Adolescence and Adulthood
• Puberty
– Surge of gray matter development and
pruning
– Thickening of cortex; frontal lobe
– Amygdala matures first
• Explains teen risky behavior
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White Matter Disruption and Binge Drinking
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The Adult Brain
• Brain is fully mature at age 25; weight of
brain starts to decrease at age 45
• Neurogenesis
– Adult learning and memory
– Decline of neurogenesis associated with
cognitive decline
– Multiple reserves help resist loss of function
• Important to distinguish between healthy
aging and disease conditions
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Adult Neurogenesis
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