Chapter 4

Modified: 2023-09-21 10:43 PM CDST

This chapter wraps up our look at human neurophysiology with examinations of genetics, evolution, brain development, plasticity and brain damage.

Module 4.1 Genetics and Evolution of Behavior (p. 104)

• 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

• Example: facial expressions

• Other examples: psychological disorders, weight gain, personality, and sexual orientation

Genetic Contribution to Facial Expression (p. 104)

Note the similarity between sighted (Left) and blind relative (Right) making facial expressions. Research implies a genetic contribution to facial expressions.

Mendelian Genetics

• 19th century monk Gregor Mendel demonstrated that inheritance occurs through discrete units of heredity, called genes

• Prior to Mendel, it was commonly believed that inheritance was a blending process of the properties of the egg and sperm (like paint)

• Genes come in pairs, called alleles, and are aligned along chromosomes

• Human karyotype definition, below the 23 pairs of human genes arranged by size, except for X Y, making this a male karyotype

Mendelian Genetics – DNA and RNA

• A gene is defined as a portion of a chromosome and is composed of deoxyribonucleic acid (DNA)

• DNA serves as a model for the synthesis of ribonucleic acid (RNA)

• RNA is a single strand chemical that can serve as a template/model for the synthesis of proteins (messenger RNA)

• Gene: actual material that crosses generations (in sexually reproducing species partners sacrifice half of their genes. In sexually reproducing species offspring's genetic complement comes from both parents. (See below for more)

• Meme: cultural transmission ('viral' information transfer, does not require transfer of physical material from generation to generation)

Mendelian Genetics –Proteins

• 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

  ---

  Genetics Review

  • Mendel
    • Austrian monk
    • Father of genetics
    • Worked out inheritance in English peas
  • Genes
    • Genes are DNA or RNA nucleotides that code for proteins
    • The collection of an individual's genes is called the genotype
    • The physical expression of those genes is called the phenotype
    • The modern synthesis of biology unites evolution with the study of genetics (see more below)
  • Modern Synthesis-genetics and evolution
  • Human Genome Project-example of "big science" with pure and applied results
    • Identified the 20,500 human genes
    • Determined the sequence of the 3 billion human DNA base pairs
    • Created databases to share this information
    • Addressed the legal and ethical issues surrounding genetic research and practice
  • OMIM-Online Mendelian Inheritance in Man
  • OMIM is one of the genetic databases mentioned above (its name is not very politically correct :-(
  • OMIM is "a continously update catalog of human genes, and genetic disorders and traits"
    • Victor McKusick obituary in journal Cell
      • Author of Mendelian inheritance in man, he proposed mapping the human genome in 1969
      • In 2003, the Human Genome project succeeded in doing so
      • BTW, I heard McKusick speak while I was an undergraduate student at the University of Baltimore. I thought he was old then!

How DNA Controls the Development of an Organism (p. 105)

Mendelian Genetics – Heterozygous and Homozygous Genes

• Being homozygous for a gene means that a person has an identical pair of genes on the two chromosomes

• Breed of dogs and other animals are homozygous, that is why they "breed true" (link for reference only, not on test)

• Being heterozygous for a gene means that a person has an unmatched pair of genes on the two chromosomes

Mendelian Genetics – Dominant and Recessive Genes

• Genes are either dominant, recessive, or intermediate

• Examples: eye color, ability to taste PTC

• A dominant gene shows a strong effect in either the homozygous or heterozygous condition

• A recessive gene shows its effect only in the homozygous condition

• An intermediate gene occurs in a phenotype where there is incomplete dominance in the heterozygous condition

Four Equally Likely Outcomes of a Mating Between Heterozygous Tasters

Note the 3:1 ratio of phenotypes in this Punnett Square (3 dominant, 1 recessive)

• Before molecular genetics looking at such ratios was the only way to assess genetic contributions. Today modern methods can directly assess genotype. PS refuse the police's offer of a drink from a glass while being interrogated.

Mendelian Genetics – Gene Expression

• Examples such as PTC and hair color can be misleading

• Implies that a single gene combination completely controls a characteristic, but this is not always true

• Some genes are only expressed partly: in some cells and not others, or only under certain circumstances

Types of 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 & Y

• Females have two X chromosomes (XX)

• Males have an X and a Y chromosome (XY)

• Sex determined at time of conception, X bearing sperm and Y bearing sperm, ovum is always X bearing

• Sex determination is not X and Y in many species

• Honeybee genetics: Queen creates females via fertilizing eggs, males come from unfertilized eggs. More? Click HERE

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

Mendelian Genetics – Sex-Linked and Sex-Limited Genes

• The human Y chromosome has genes for far fewer proteins than the X chromosome

• Thus, sex-linked genes usually refer to X-linked genes: e.g., red-green color deficiency

• Sex-limited genes are genes that are present in both sexes but mainly have an effect on one sex (chest hair, breast size, prostate (males) or ovarian cancer (females), etc.)

Genetic Changes

• Genes change in several ways

• Mutation: a heritable change in a DNA molecule (e.g., FOXP2 gene) FYI, mutations are usually deleterious

• Microduplication/microdeletion: part of a chromosome that might appear once might appear twice or not at all

• Example: some researchers believe schizophrenia might be a result of microduplications and microdeletions of brain-relevant genes

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, etc.

• Changes in gene expression are central to learning and memory

• Epigenetic differences are a likely explanation for differences between monozygotic “identical” twins

Activation of Certain Genes (p. 108)

One mechanism for exposing and "turning on" a gene, "blurs" the distinction between genetic and environmental effects

Epigenetic Effects

• What you do at any moment not only affects you now, but produces epigenetic effects that alter gene expressions for a longer period of time

• Experiences alter the activity of genes

Heritability

• Refers to how much characteristics depend on genetic differences

• Researchers have found evidence for heritability in almost every behavior they have tested

• Heritability of a certain trait is specific to a given population

• Strong environmental influences may cause genetic influences to have less of an effect

• Positive heritability effects have been demonstrated for: loneliness, neuroticism, TV watching, childhood misbehavior, social attitudes, cognitive performance, educational attainment, and speed of learning a second language. Religious choice does not show heritability effects.

• Plomin, et al. (2016), Top ten replicated findings from behavioral genetics. (Article for those who wish to venture further, ten points below for everyone in class.)

  1. All psychological traits show significant and substantial genetic influence: including intelligence, schizophrenia, political beliefs, autism, and hyperactivity

  2. No traits are 100% heritable: researchers must account for genetics in studies (see: 7, 8, and 9 below)

  3. Heritability is caused by many genes of small effect: seems to be a given. Animal breeding and selection studies with selected traits (maze-bright vs maze dull or low open field vs high open field) show "After 30 generations of such selective breeding, a 30-fold average difference in activity had been achieved, with no overlap between the activity of the low and high lines." Graph

  4. Phenotypic correlations between psychological traits show significant and substantial genetic mediation: "Phenotypic covariance between traits is significantly and substantially caused by genetic covariance, not just environmentally driven covariance."

  5. The heritability of intelligence increases throughout development: in identical twins, the heritability of intelligence increases with age. No one yet knows why. Graph

  6. Age-to-age stability is mainly due to genetics: age to age stability in behavior (e.g., personality, intelligence) is genetic, age to age change is primarily environmental

  7. Most measures of the ‘environment’ show significant genetic influence: e.g., neighborhoods, schools, and work environments.

  8. Most associations between environmental measures and psychological traits are significantly mediated genetically: Chicken and egg type problem here: are associations mediated genetically or environmentally?

  9. Most environmental effects are not shared by children growing up in the same family: "the basic finding that most environmental effects are not shared by children growing up in the same family" (Very important finding)

  10. Abnormal is normal: learning disabilities, schizophrenia, autism, and hyperactivity are more common than "rare single-gene disorders" (e.g., PKU)

They conclude: "What we like best about some of these findings is that they are counterintuitive. For example, who would have thought that the heritability of intelligence increases throughout development (Finding 5) or that environmental measures show genetic influence (Finding 7) or that the abnormal is normal (Finding 10)? Another feature of these findings is that each is falsifiable. For example, if major-gene effects on complex traits and common disorders are found, they would falsify the hypothesis that heritability is caused by many genes of small effect (Finding 3)."

 

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

• Video: Using Twin Studies to Determine Heritability (watch)

Environmental Modification

• Traits with a strong hereditary influence can by modified by environmental intervention

• e.g., PKU: a genetic inability to metabolize the amino acid phenylketonuria

• Environmental interventions can modify PKU: special diet, even in adulthood (low in phenylalanine)

• Demonstrates that heritable does not mean unmodifiable

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

Examples

Behavior Genetics

• Genetics affects behavior, the question is by how much

  • Twin Studies

    • The study of identical twins raised apart has yielded much information about that question

    • Minnesota Twins Raised Apart Study

      • The Jim Twins were remarkably similar:

        • Both twins are married to women named Betty and divorced from women named Linda.

        • One has named his first son James Alan while the other named his first son James Allan.

        • Both twins have an adopted brother whose name is Larry.

        • Both named their pet dog "Toy."

        • Both had some law-enforcement training and had been a part-time deputy sheriff in Ohio.

        • Each did poorly in spelling and well in math.

        • Each did carpentry, mechanical drawing, and block lettering.

        • Each vacation in Florida in the same three-block-long beach area.

        • Both twins began suffering from tension headaches at eighteen, gained ten pounds at the same time, and are six feet tall and 180 pounds.

        • Most such identical twins are not so similar, but are more similar to each other than regular siblings are

    • My take: My wife and I have three children, none are twins, but all are very different in behavior and personality from birth even though they were raised in the same home.

The Evolution of Behavior

• Evolution refers to a change in the frequency of various genes in a population over generations

• Regardless of whether the change is helpful or harmful to the species

• Evolution attempts to answer two questions:

• How did some species evolve? (e.g., Natural History)

• How species evolved is based on inferences from fossils/comparisons of living species

• How do species evolve? (Evolutionary mechanisms)

How Do Species Evolve?

• How species evolve rests upon some assumptions:

• Offspring generally resemble their parents for genetic reasons

• Mutations, recombination, and microduplications of genes introduce new heritable variations

• Certain individuals successfully reproduce more than others

Artificial Selection

• Refers to choosing individuals with desired traits and making them parents of the next generation

• According to Darwin, nature also selects, and successful individuals’ genes will be prevalent in later generations

• Race horses, plants (Cavendish banana), dogs

Common Misconceptions about Evolution

• Lamarckian evolution: “The use or disuse of some structure or behavior causes an increase or decrease in that behavior” (near the end of his career even Darwin felt pressure to include Lamarckian evolution in his later editions of his book.)

• Circumcision (in Judaism, going on for thousands of years, boys still are born uncircumcised)

• “Humans have stopped evolving” (fails to consider differential reproduction, not differential survival)

• “Evolution means improvement” (improves "fitness" but change in environment could disrupt)

• “Evolution acts to benefit the individual or the species” (this the argument for "group selection" and there is no mechanism that will confer "fitness" to a group. Evolution acts at level of genes. Think of organisms as vessels for collections of genes.

Brain Evolution

• One explanation is that our ancestors managed to get enough nutrition to provide a big brain with all the fuel it needs

• Cooking food made it easier to digest

• Group hunting was more efficient

• Humans have better capacity for glucose transport

Evolutionary Psychology

• Focuses upon functional and evolutionary explanations of how behaviors evolved

• Assumes that behaviors characteristic of a species have arisen through natural selection and provide a survival advantage

• Examples: differences in peripheral/color vision, sleep mechanisms in the brain, eating habits, temperature regulation (color vision, sleep, hibernation, piloerection, grasp reflex)

Behavior and Natural Selection

• Some behaviors are more debatable with regard to the influence of natural selection

• Examples

• Life span length: varies by species (tarantulas-females 20 years, males 1 year and die after sexual reproduction)

• Gender differences in sexual receptivity see HERE

  • Opposite gender confederates asked three questions:

    • Would you go out?

    • Will you come over to my apartment?

    • Would you go to bed with me?

  • Unsurprisingly, no females agreed with last question but 75% in the first study and 69% of the males did. Of course, the study stopped at that point.

  • Evolutionary psychologists point to reproductive mechanism differences here, males improve their reproductive success via multiple matings, but females improve their reproductive success by selecting a male partner with better genes.

• Altruistic behavior: a behavior that benefits someone other than the actor genetically. The ultimate example being losing one's life (and future potential reproductive success) to save another unrelated person (and save their future reproductive success)

• Altruism is hard to find outside of humans

Group and Kin Selection

• A gene:

• Only spreads if individuals with it reproduce more than individuals without it

• That benefits the species but not the individual dies out with that individual

• Group selection: controversial hypothesis that states that altruistic groups survive better than less cooperative ones

• Kin selection: more plausible; selection for a gene benefits the individual’s relatives

  A. Inclusive Fitness and Altruism

  • Working for relatives is not really altruistic because of shared genes

  • Reciprocal altruism may be truly altruistic (I'll save you now and you'll save me later--maybe.)

  • Eugene XXXX, this obituary appeared in Milwaukee paper in 1978. Notice his inclusive fitness. The r values are the percentage of genes shared.

    XXXX, Eugene Nov. 7, 1978. Born Sept. 4, 1871 West Point, Miss. Age 107 years. Beloved husband of Viola, dear father of W. C.(Louise), Robert (Lorine) of Arkansas, Johnnie (Corine) of Michigan, Doris (Paul) Boozer of Ohio, Mary Black and Mary (Roth) Bryant both of Iowa, Rosie Reed of Michigan, Alice (Abraham) Crawford. Also surviving 40 grandchildren, 55 great grand- children, 3 great great grand- children, other fond relatives. In state Fri. after 1 p.m. O'BEE FUNERAL HOME. 2400 W. Center St. Vigil 7-8 p.m. Funeral at Sat. 1 p.m. O'Bee Chapel, Burial Evergreen Cemetery.

    Eugene's inclusive fitness*:

    Relatives	r value	Product
    7 sons and daughters	1/2	3.5
    40 grandchildren	1/4	10
    55 great grandchildren	1/8	6.875
    3 great great grandchildren	1/16	0.1875
    Total		20.5625
    Replacement
    2 sons and daughters	1/2	1
    4 grandchildren	1/4	1
    8 great grandchildren	1/8	1
    16 great great grandchildren	1/16	1
    Total		4

Notice that Eugene far outstripped the proportion of genes needed for replacement.

Reciprocal Altruism

• The idea that individuals help those that will return the favor

• Building a reputation for helpfulness only works if others are willing to cooperate with you

Module 4.2 Development of the Brain (p. 117)

• Brain development depends upon:

• Maturation

• Learning

• Video

• We can refine this understanding by learning how:

• Experience modifies development

  • ¹⁴C concentration acts as "clock"

  • No new neurons in cortex

  • 2% replacement/year of neurons in hippocampus

  • Some replacement in basal ganglia

  • Both important for learning

Maturation of the Vertebrate Brain

• The human central nervous system begins to form when the embryo is approximately two 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

Early Development of the Human Central Nervous System (p. 117)

Human Brain at Four Stages of Development (p. 118)

Cerebrospinal Fluid

• The fluid-filled cavity becomes the central canal of the spinal cord and the four ventricles of the brain

• This fluid is the cerebrospinal fluid

Brain Weight

• 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

• Brain weighs less in old age

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 the life as neurons are constantly forming new connections and discarding old ones

• Slows significantly later in the lifetime

New Neurons Later in Life

• 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

• Development of new neurons also occurs in other brain regions

• Example: songbirds have a steady replacement of new neurons in the singing area of the brain

• 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

Specificity of Axon Connections (p. 120)

Nerve Connections in Newts (p. 121)

Chemical Gradients (p. 121)

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

Neural Darwinism

• Some theorists refer to the idea of the selection process of neural connections as neural Darwinism

• 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. Pruning a bush is an analog.

Neurotropins

• Chemicals that promote the survival and activity of neurons (i.e., NGF)

• 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

Neuronal Death

• The elimination of massive cell death is part of normal development and maturation

• After maturity, the apoptotic mechanisms become dormant

• The visual cortex is actually thicker in blind people due to a lack of visual stimuli

• It cannot prune out ineffective neurons

The Vulnerable Developing Brain

Early stages of brain development are critical for normal development later in life

A mutation on one gene can lead to many defects

Chemical distortions in the brain during early development can cause significant impairment and developmental problems

Fetal Alcohol Syndrome (FAS)

• 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

• The dendrites of children born with fetal alcohol syndrome are short with 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

Video: FAS

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

A Ferret with a Rewired Temporal Cortex (p. 125)

Fine-Tuning by Experience

• The brain has some ability to reorganize itself in response to experience

• Axons and dendrites continue to modify their structure and connections throughout the lifetime

• Dendrites continually grow new spines

• The gain and loss of spines indicates new connections, which relates to learning

Changes in Dendritic Trees of Two Mouse Neurons (p. 126)

Effect of a Stimulating Environment (p. 126)

a) isolated jewelfish, b) jewelfish reared with others

• Experience and Dendritic Branching

• Rats raised in an enriched environment develop a thicker cortex, increased dendritic branching and improved learning

  • Rosenzweig and Bennett (1996) Abstract environment affected brain size and weight in rats

• 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

• 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

• Luminosity and "brain training"

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

Brain Adaptations

• 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

• Further research using magnetic stimulation to inactivate brain areas demonstrated that blind people use the occipital cortex to discriminate between tactile stimuli and Braille symbols but sighted people do not.

Music Training

• MRI studies reveal the following:

• The temporal lobe of professional musicians in the right hemisphere is 30% larger than non-musicians

• Thicker gray matter in the part of the brain responsible for hand control and vision of professional keyboard players

• Results suggest that practicing a skill reorganizes the brain to maximize performance of that skill

Brain Correlates of Music Practice (p. 128)

Special Training in Adulthood

• Adult experiences can also modify brain anatomy

• However, research is needed to determine whether the effects are strong enough to be observed with MRI or similar technology

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

When Brain Reorganization Goes Too Far (2 of 3, p. 129)

When Brain Reorganization Goes Too Far (3 of 3, p. 130)

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, etc.

• Adolescents tend to “discount the future”

Brain Development and Adolescents

• Adolescents are not equally impulsive in all situations

• Peers, amount of time to make decisions, etc., affect their decisions

• The prefrontal cortex of adolescents is relatively inactive in certain situations, but this may or may not be the cause of impulsivity

Brain Development and Old Age

• Some neurons lose their synapses, and the remaining synapses change more slowly than before in response to experiences

• Brain structures begin to lose volume

• Research underestimates older people:

• People vary in respect to intellectual decline

• Older people have a greater base of knowledge and experience, and many find ways to compensate for losses

Module 4.3 Plasticity after Brain Damage (p. 136)

• 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

• Understanding the processes of recovery will give us new and improved therapies

Plasticity After Brain Damage – Recovery

• Most survivors of brain damage show some degree of behavioral recovery

• Some of the mechanisms of recovery include those similar to the mechanisms of brain development such as the new branching of axons and dendrites

Brain Damage and Short-Term Recovery

• Possible causes of brain damage

• Tumors

• Infections

• Exposure to toxic substances or radiation

• Degenerative diseases

• Closed head injuries

• 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 elderly

Three Examples of Damaged Human Brains (p. 137)

a) stroke followed by immediate death, b) stroke followed by long life (note cavities on left side), c) gunshot wound and immediate death

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 also 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 triggers the release of the excitatory neurotransmitter glutamate

• The overstimulation of neurons leads to sodium and other ions entering the neuron in excessive amounts

• 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

• Cooling protects the brain after ischemia by reducing overstimulation, apoptosis, and inflammation

• Cannabanoids have also been shown to potentially minimize cell loss after a brain stroke

• Benefits are most likely due to cannabinoids anti-inflammatory effects

• Research shows that they are most effective in laboratory animals when taken before the stroke

Later Mechanisms of Recovery from Brain Damage

• 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

• If an axon in the peripheral nervous system is crushed, it follows its myelin sheath back to the target and grows back toward the periphery at a rate of about 1 mm per day

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

Collateral Sprouting (p. 139)

Denervation Supersensitivity

• Postsynaptic cells deprived of synaptic inputs develop increased sensitivity to the neurotransmitter to compensate for decreased input

• Denervation supersensitivity: the heightened sensitivity to a neurotransmitter after the destruction of an incoming axon

• Can cause consequences such as chronic pain

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

• The phantom limb can lead to the feeling of sensations in the amputated part of the body when other parts of the body are stimulated

• e.g., a touch on the face can bring about the experience of a phantom arm

• Use of an artificial limb can reduce the likelihood of experiencing phantom limb

Somatosensory Cortex of a Monkey After a Finger Amputation (p. 140)

Sources of Phantom Sensation (p. 141)

Stimulation where marked produced sensations in digits 1 to 5

Learned Adjustments in Behavior (Taub and PETA)

• 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

Cross-Section Through the Spinal Cord (p. 142)

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