Genetics, Development, & Plasticity

Genetics and Behavior

• Genes and environment interact to shape human behavior.
• The fundamental question is the extent to which each factor influences behavior.
  • Examples include facial expressions, psychological disorders, weight gain, personality, and sexual orientation.

How DNA Controls Development

• Proteins, determined by DNA, control body development by:
  • Forming structural components.
  • Acting as enzymes (biological catalysts) to regulate chemical reactions.

Sex-Linked and Sex-Limited Genes

• Autosomal genes: All non-sex-linked genes.
• Sex-linked genes: Genes located on sex chromosomes.
• Mammals: Sex chromosomes are X and Y.
  • Females: XX
  • Males: XY

Mendelian Genetics—X and Y

• Reproduction:
  • Females contribute an X chromosome.
  • Males contribute either X or Y, determining the offspring's sex.
  • Male X contribution: Genetically female offspring.
  • Male Y contribution: Genetically male offspring.

Epigenetics

• Field studying changes in gene expression without DNA sequence modification.
• Gene activity varies based on life stage, time of day, etc.
• Changes in gene expression are crucial for learning and memory.
• Epigenetic differences may explain variations in monozygotic twins.

Heredity and Environment

• Nearly all behaviors have genetic and environmental components.
• Research methods:
  • Twin studies (monozygotic and dizygotic).
  • Adoption studies.
  • Identification of specific genes linked to behaviors.

Environmental Modification

• Traits strongly influenced by heredity can be modified by environmental intervention.
• Example: PKU (phenylketonuria), a genetic inability to metabolize phenylalanine.
  • Environmental interventions can modify PKU's effects.

How Genes Affect Behavior

• Genes do not directly cause behaviors.
• Genes produce proteins that increase the likelihood of a behavior developing under certain conditions.
• Genes can indirectly influence behavior by altering the environment through traits or behaviors that affect how others react to an individual.

Behavior and Natural Selection

• Influence of natural selection is debated for some behaviors.
• Examples:
  • Life span length.
  • Gender differences in sexual promiscuity.
  • Altruistic behavior: Benefits others at the actor's expense; rare outside humans.

Maturation of the Vertebrate Brain

• Human central nervous system formation begins around 2 weeks of embryonic age.
• The dorsal surface thickens, forming the neural tube with a fluid-filled cavity.
• The forward end of the neural tube enlarges and differentiates into the hindbrain, midbrain, and forebrain.
• The remaining neural tube becomes the spinal cord.

Human Brain Development Stages

• At birth: Approximately 350 grams.
• By the first year: Approximately 1000 grams.
• Adult brain: 1200–1400 grams.

Development of Neurons

• Development involves:
  • Proliferation
  • Migration
  • Differentiation
  • Myelination
  • Synaptogenesis

1. Proliferation

• Production of new cells/neurons, mainly early in life.
• Early in development, cells lining ventricles divide.
  • Some become stem cells that continue to divide.
  • Others become neurons or glia that migrate.

2. Migration

• Movement of new neurons and glia to final locations, sometimes continuing into adulthood.
• Occurs in various directions throughout the brain.
• Guided by chemicals like immunoglobulins and chemokines.

3. Differentiation

• Neuron develops axon and dendrites, forming its distinctive shape.
• Axon grows first, either during migration or after reaching its target, followed by dendrite development.

4. Myelination

• Glia produce a fatty sheath (myelin) covering some axons.
• Myelin speeds up neural impulse transmission.
• Occurs first in the spinal cord, then hindbrain, midbrain, and forebrain.
• Gradual process lasting for decades.

5. Synaptogenesis

• Formation of synapses between neurons; the final stage of neural development.
• Occurs throughout life as neurons form new connections and discard old ones.
• Slows significantly later in life.

New Neurons Later in Life

• Initial belief: No new neurons formed after early development.
• Later research: Suggests otherwise.
• Stem cells: Undifferentiated cells in the brain's interior can transform into glia or neurons.
• New olfactory receptors continually replace dying ones.
• Stem cells differentiate into new neurons in the adult hippocampus, facilitating learning.

The Life Span of Neurons

• Different cells have varying average life spans.
• Skin cells are relatively new: most under a year old.
• Heart cells tend to be as old as the person.
• Mammalian cerebral cortex forms few or no new neurons after birth.

Pathfinding by Axons

• Axons must travel long distances to form correct connections.
• Sperry’s (1954) research: Axons follow chemical trails to reach targets.
• Growing axons are attracted to some chemicals and repelled by others, following a chemical gradient.

Specificity of Axon Connections

• Axons regrow and attach to the same target neurons as before.

Competition Among Axons

• Initially, axons form synapses with multiple cells.
• Postsynaptic cells strengthen some connections and eliminate others.
• Formation/elimination depends on input from incoming axons.
• More connections are initially formed than needed.
• Successful axon connections survive; others fail.

Determinants of Neuronal Survival

• Levi-Montalcini: Muscles determine neuron survival, not formation.
• Nerve growth factor (NGF): Protein released by muscles promotes axon survival and growth.
• The brain overproduces neurons, then uses apoptosis to match incoming axons to receiving cells.
• Axons not exposed to neurotrophins undergo apoptosis (preprogrammed cell death).
• Healthy adult nervous system contains no neurons that failed to make appropriate connections.

Fetal Alcohol Syndrome

• Condition in children born to mothers who drank heavily during pregnancy.
• Marked by:
  • Hyperactivity and impulsiveness.
  • Difficulty maintaining attention.
  • Varying degrees of mental retardation.
  • Motor problems and heart defects.
  • Facial abnormalities.
• Dendrites are short with few branches.
• Alcohol suppresses glutamate and enhances GABA release in the fetal brain.
• Neurons receive less excitation and exposure to neurotrophins, leading to apoptosis.

Differentiation of the Cortex

• Neurons in different brain parts vary in shape and chemical components.
• Immature neurons transplanted to a developing cortex adopt properties of the new location.
• Neurons transplanted later develop some new properties but retain some old ones.
• Example: Ferret experiment.

Changes in Dendritic Trees

• Gain and loss of spines indicate new connections related to learning.
• Measurable neuron expansion occurs with physical activity.
• As old neurons die (apoptosis) and new ones form, learning and memory improve.

Experience and Dendritic Branching

• Old belief: Teaching difficult concepts enhances intelligence in other areas (far transfer).
• Evidence: Skills associated with the practiced task transfer, but not other skills.
• The brain cannot be “exercised” like a muscle.

Effects of Special Experiences

• Blind people improve attention to touch and sound with practice.
• Touch information activates the occipital cortex, normally for vision.
• The occipital lobe adapts to process tactile and verbal information.
• People blind from birth are better at discriminating objects by touch and have increased occipital cortex activation during touch tasks.

When Brain Reorganization Goes Too Far

• Focal hand dystonia or “musicians cramp” results from excessive brain reorganization.
• Musicians' fingers become clumsy, fatigue easily, and make involuntary movements.
• Caused by extensive reorganization of the sensory thalamus and cortex, leading to overlapping touch responses between fingers.

Brain Development and Behavioral Development

• Adolescents are more impulsive than adults.
• Impulsivity can lead to risky behaviors.
• Adolescents tend to “discount the future.”
• The prefrontal cortex is relatively inactive in certain situations.
• Impulsivity varies based on peers and decision-making time.

Plasticity after Brain Damage

• Most brain damage survivors show some degree of behavioral recovery.
• Recovery involves growth of new axon and dendrite branches.
• Possible causes of brain damage:
  • Tumors
  • Infections
  • Exposure to toxic substances or radiation
  • Degenerative diseases
  • Closed head injuries

Brain Damage and Immediate Treatments

• Closed head injury: A sharp blow to the head that does not puncture the brain; common in young adults.
• Recovery can be slow and incomplete after severe injury.
• Stroke (cerebrovascular accident): Temporary loss of blood flow to the brain; common in the elderly.

Types of Strokes

• Ischemia: Most common; caused by a blood clot or artery obstruction, leading to oxygen and glucose deprivation.
• Hemorrhage: Less frequent; caused by a ruptured artery, flooding neurons with excess blood, calcium, oxygen, and other chemicals.

Effects of Strokes

• Ischemia and hemorrhage cause:
  • Edema: Fluid accumulation in the brain, increasing pressure and stroke probability.
  • Disruption of the sodium–potassium pump: Leading to potassium accumulation inside neurons.
  • Edema and excess potassium trigger glutamate release.
  • Excess positive ions block mitochondrial metabolism and kill the neuron.

Immediate Treatments for Stroke

• Tissue plasminogen activator (tPA): Breaks up blood clots, reducing ischemic stroke effects.
• Research focuses on blocking glutamate synapses and calcium entry to save neurons.
• Cooling the brain is effective in minimizing damage.
• Cannabinoids may minimize cell loss, most effective if taken before the stroke (in animal studies).

Later Mechanisms of Recovery

• Surviving brain areas increase or reorganize activity.
• Diaschisis: Decreased activity of surviving neurons after damage to other neurons.
• Damage disrupts normal stimulation patterns.
• Stimulant drugs may activate healthy brain regions after a stroke.
• Destroyed cell bodies cannot be replaced, but damaged axons can regrow under certain circumstances.

Regrowth of Axons

• Damaged axons do not readily regenerate in a mature mammalian brain or spinal cord.
• Scar tissue creates a mechanical barrier.
• Neurons on either side of the cut pull apart.
• Glia cells release chemicals inhibiting axon growth.
• Research on protein bridges may help.

Axon Sprouting

• Collateral sprouts: New branches from non-damaged axons attach to vacant receptors.
• Cells that lost innervation release neurotrophins that induce collateral sprouts.
• Sprouts fill vacated synapses over months; can be useful, neutral, or harmful.

Reorganized Sensory Representations and the Phantom Limb

• Phantom limb: Continued sensation of an amputated body part.
• The cortex reorganizes itself after amputation, becoming responsive to other body parts.
• Original axons degenerate, leaving vacant synapses into which others axons sprout.

Sources of Phantom Sensation

• Homunculus: Representation of the body in the somatosensory and somatomotor cortex.

Learned Adjustments in Behavior

• Deafferentated limb: Limbs that have lost afferent sensory input.
• Can still be used but are often not because using other mechanisms is easier.
• Therapy techniques improve functioning in brain-damaged people by focusing on what they can do.