Biological Psychology - Genetics, Development, & Plasticity

Genetics and Behavior

• Both genes and environment interact to shape human behavior.
• The fundamental issue is how much a role each factor plays in shaping human behaviors.
• Examples:
  • Facial expressions
  • Psychological disorders
  • Weight gain
  • Personality
  • Sexual orientation

How DNA Controls the Development of an Organism

• Proteins determine the development of the body by:
  • Forming part of the structure of the body
  • Serving as enzymes, biological catalysts that regulate chemical reactions in the body

Sex-Linked and Sex-Limited Genes

• Autosomal genes: all other genes except for sex-linked genes
• Sex-linked genes: genes located on the sex chromosomes
• In mammals, the sex chromosomes are designated X and Y
  • Females have two X chromosomes (XX).
  • Males have an X and a Y chromosome (XY).

Mendelian Genetics—X and Y

• During reproduction:
  • Females contribute an X chromosome
  • Males contribute either an X or a Y chromosome that determines the sex of the child.
  • If an X chromosome is contributed by the male, the offspring is genetically female.
  • If a Y chromosome is contributed by the male, the offspring will be genetically male.

Epigenetics

• A field that is concerned with changes in gene expression without the modification of the DNA sequence.
• Some genes are active only at a certain point in one’s life, a certain time of day, and so on.
• Changes in gene expression are central to learning and memory.
• Epigenetic differences are a likely explanation for differences between monozygotic “identical” twins.

Heredity and Environment

• Almost all behaviors have both a genetic and an environmental component.
• Researchers:
  • Study monozygotic and dizygotic twins to infer contributions of heredity and environment
  • Study adopted children and their resemblance to their biological parents to infer hereditary influences
  • Identify specific genes linked to some behavior

Environmental Modification

• Traits with a strong hereditary influence can be modified by environmental intervention.
• For example, PKU: a genetic inability to metabolize the amino acid phenylketonuria.
• Environmental interventions can modify PKU.

How Genes Affect Behavior

• Genes do not directly produce behaviors.
• Genes produce proteins that increase the probability that a behavior will develop under certain circumstances.
• Genes can also have an indirect affect.
• Genes can alter your environment by producing behaviors or traits that alter how people in your environment react to you.

Behavior and Natural Selection

• Some behaviors are more debatable with regard to the influence of natural selection.
• Examples
  • Life span length
  • Gender differences in sexual promiscuity
  • Altruistic behavior: a behavior that benefits someone other than the actor
• Altruism is hard to find outside of humans.

Maturation of the Vertebrate Brain

• The human central nervous system begins to form when the embryo is approximately 2 weeks old.
• The dorsal surface thickens, forming a neural tube surrounding a fluid-filled cavity.
• The forward end enlarges and differentiates into the hindbrain, midbrain, and forebrain.
• The rest of the neural tube becomes the spinal cord.

Human Brain at Four Stages of Development

• At birth, the human brain weighs approximately 350 grams.
• By the first year, the brain weighs approximately 1000 grams.
• The adult brain weighs 1200–1400 grams.

The Development of Neurons

• The development of neurons in the brain involves the following processes:
  • Proliferation
  • Migration
  • Differentiation
  • Myelination
  • Synaptogenesis

1. Proliferation

• The production of new cells/neurons in the brain primarily occurring early in life
• Early in development, the cells lining the ventricles divide
• Some cells become stem cells that continue to divide.
• Others remain where they are or become neurons or glia that migrate to other locations.

2. Migration

• The movement of the newly formed neurons and glia to their eventual locations
• Some do not reach their destinations until adulthood.
• Occurs in a variety of directions throughout the brain
• Chemicals known as immunoglobulins and chemokines guide neuron migration.

3. Differentiation

• The forming of the axon and dendrites that gives the neuron its distinctive shape
• The axon grows first either during migration or once it has reached its target and is followed by the development of the dendrites.

4. Myelination

• The process by which glia produce the fatty sheath that covers the axons of some neurons
• Myelin speeds up the transmission of neural impulses.
• First occurs in the spinal cord and then in the hindbrain, midbrain, and forebrain
• Occurs gradually for decades

5. Synaptogenesis

• The final stage of neural development—the formation of the synapses between neurons
• Occurs throughout life as neurons are constantly forming new connections and discarding old ones
• Slows significantly later in the lifetime

New Neurons Later in Life (1 of 2)

• Originally believed that no new neurons were formed after early development
• Later research suggests otherwise
• Stem cells: undifferentiated cells found in the interior of the brain that generate “daughter cells” that can transform into glia or neurons
• New olfactory receptors also continually replace dying ones.
• Stem cells differentiate into new neurons in the adult hippocampus of mammals and facilitate learning.

The Life Span of Neurons

• Different cells have different average life spans.
• Skin cells are the newest; most are under a year old.
• Heart cells, on the other hand, tend to be as old as the person.
• Mammalian cerebral cortexes form few or no new neurons after birth.

Pathfinding by Axons

• Axons must travel great distances across the brain to form the correct connections.
• Sperry’s (1954) research with newts indicated that axons follow a chemical trail to reach their appropriate target.
• Growing axons reach their target area by following a gradient of chemicals in which they are attracted by some chemicals and repelled by others.

Competition among Axons as a General Principle

• When axons initially reach their targets, they form synapses with several cells.
• Postsynaptic cells strengthen connection with some cells and eliminate connections with others.
• The formation or elimination of these connections depends on the pattern of input from incoming axons.
• In this competition among synaptic connections, we initially form more connections than we need.
• The most successful axon connections and combinations survive while the others fail to sustain active synapses

Determinants of Neuronal Survival

• Levi-Montalcini discovered that muscles do not determine how many axons form; they determine how many survive.
• Nerve growth factor (NGF) is a type of protein released by muscles that promotes the survival and growth of axons.
• The brain’s system of overproducing neurons and then applying apoptosis enables the exact matching of the number of incoming axons to the number of receiving cells.
• Axons that are not exposed to neurotropins after making connections undergo apoptosis—a preprogrammed mechanism of cell death.
• Therefore, the healthy adult nervous system contains no neurons that failed to make appropriate connections.

Fetal Alcohol Syndrome (1 of 2)

• A condition that children are born with if the mother drinks heavily during pregnancy
• Marked by the following:
  • Hyperactivity and impulsiveness
  • Difficulty maintaining attention
  • Varying degrees of mental retardation
  • Motor problems and heart defects
  • Facial abnormalities

Fetal Alcohol Syndrome (2 of 2)

• The dendrites of children born with fetal alcohol syndrome are short with a few branches.
• Exposure to alcohol in the fetus brain suppresses glutamate and enhances the release of GABA.
• Many neurons consequently receive less excitation and exposure to neurotrophins than usual and undergo apoptosis.

Differentiation of the Cortex

• Neurons in different parts of the brain differ from one another in their shape and chemical components.
• Immature neurons transplanted to a developing part of the cortex develop the properties of the new location.
• Neurons transplanted at a later stage of development develop some new properties but retain some old properties.
• Example: ferret experiment

Changes in Dendritic Trees

• The gain and loss of spines indicate new connections, which relates to learning.
• Measurable expansion of neurons has also been shown in humans as a function of physical activity.
• As old neurons die by apoptosis and new ones form to take their place, there is improved learning and memory.

Experience and Dendritic Branching

• It was once believed that teaching a child a difficult concept (e.g., Greek, advanced math, etc.) would enhance intelligence in other areas.
• This concept is known as “far transfer.”
• Evidence shows that 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 their attention to touch and sound, based on practice.
• Touch information activated this occipital cortex area, which is ordinarily devoted to vision alone.
• The occipital lobe normally dedicated to processing visual information adapts to also process tactile and verbal information.
• People blind from birth are better at discriminating between objects by touch and have increased activation in their occipital cortex (visual cortex) while performing touch tasks.

When Brain Reorganization Goes Too Far

• Focal hand dystonia or “musicians cramp” refers to a condition where the reorganization of the brain goes too far.
• The fingers of musicians who practice extensively become clumsy, fatigue easily, and make involuntary movements.
• This condition is a result of extensive reorganization of the sensory thalamus and cortex so that touch responses to one finger overlap those of another.

Brain Development and Behavioral Development

• Adolescents tend to be more impulsive than adults.
• Impulsivity can be a problem when it leads to drinking, risky driving, sex, and so on.
• Adolescents tend to “discount the future.”
• The prefrontal cortex of adolescents is relatively inactive in certain situations, but this may or may not be the cause of impulsivity.
• Adolescents are not equally impulsive in all situations.
• Peers, amount of time to make decisions, and so on, affect their decisions.

Plasticity after Brain Damage

• Almost all survivors of brain damage show behavioral recovery to some degree.
• Some recovery relies on the growth of new branches of axons and dendrites.
• Possible causes of brain damage
  • Tumors
  • Infections
  • Exposure to toxic substances or radiation
  • Degenerative diseases
  • Closed head injuries

Brain Damage and Immediate Treatments

• A closed head injury refers to a sharp blow to the head that does not puncture the brain.
• One of the main causes of brain injury in young adults
• After a severe injury, recovery can be slow and incomplete.
• A stroke or cerebrovascular accident is temporary loss of blood flow to the brain.
• Common cause of brain damage in the elderly

Types of Strokes

• Ischemia: the most common type of stroke, resulting from a blood clot or obstruction of an artery
  • Neurons lose their oxygen and glucose supply.
• Hemorrhage: a less frequent type of stroke resulting from a ruptured artery
  • Neurons are flooded with excess blood, calcium, oxygen, and other chemicals.

Effects of Strokes

• Ischemia and hemorrhage cause:
  • Edema: the accumulation of fluid in the brain resulting in increased pressure on the brain and increasing the probability of further strokes
  • Disruption of the sodium–potassium pump leading to the accumulation of potassium ions inside neurons
• Edema and excess potassium trigger the release of the excitatory neurotransmitter glutamate.
• Excess positive ions in the neuron block metabolism in the mitochondria and kill the neuron.

Immediate Treatments for Stroke

• A drug called tissue plasminogen activator (tPA) breaks up blood clots and can reduce the effects of an ischemic strokes.
• Research has begun to attempt to save neurons from death by blocking:
  • Glutamate synapses; Calcium entry
• One of the most effective laboratory methods used to minimize damage caused by strokes is to cool the brain.
• Cannabinoids have also been shown to potentially minimize cell loss after a brain stroke.
• Research shows that they are most effective in laboratory animals when taken before the stroke.

Later Mechanisms of Recovery

• Following brain damage, surviving brain areas increase or reorganize their activity.
• Diaschisis: decreased activity of surviving neurons after damage to other neurons
• Because activity in one area stimulates other areas, damage to the brain disrupts patterns of normal stimulation.
• Drugs (stimulants) may stimulate activity in healthy regions of the brain after a stroke.
• Destroyed cell bodies cannot be replaced, but damaged axons do grow back under certain circumstances.

Regrowth of Axons

• Damaged axons do not readily regenerate in a mature mammalian brain or spinal cord.
• Scar tissue makes a mechanical barrier to axon growth.
• Neurons on the two sides of the cut pull apart.
• Glia cells that react to CNS damage release chemicals that inhibit axon growth.
• Research on building protein bridges may help.

Axon Sprouting

• Collateral sprouts are new branches formed by other non-damaged axons that attach to vacant receptors.
• Cells that have lost their source of innervation release neurotrophins that induce axons to form collateral sprouts.
• Over several months, the sprouts fill in most vacated synapses and can be useful, neutral, or harmful.

Reorganized Sensory Representations and the Phantom Limb

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

Learned Adjustments in Behavior

• Deafferentated limb: limbs that have lost their afferent sensory input
• Can still be used but are often not because use of other mechanisms to carry out the behavior are easier
• Has led to the development of therapy techniques to improve functioning of brain damaged people
• Focuses on what they are capable of doing