Chapter 05: Development & Plasticity of the Brain


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Growth and differentiation of the brain as well as brain damage and recovery.

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Chapter 05: Development & Plasticity of the Brain

  1. 1. Development of The Brain
  2. 2. Growth of the Brain <ul><li>CNS Begins Forming in the Embryo at About 2 Weeks </li></ul><ul><li>The Neural Tube forms around a fluid-filled cavity </li></ul><ul><li>Eventually it sinks under the skin & develops into the hindbrain, midbrain, & forebrain </li></ul><ul><li>The fluid-filled cavity becomes the central canal & the 4 ventricles </li></ul>
  3. 3. Neurogenesis <ul><li>5 Steps in Neuron Development </li></ul><ul><li>Proliferation </li></ul><ul><li>Production of New Cells </li></ul><ul><li>Migration </li></ul><ul><li>Movement of Primitive Neurons & Glia to Final Destination </li></ul><ul><li>Differentiation </li></ul><ul><li>Neurons Develop Axons & Dendrites </li></ul><ul><li>Myelination </li></ul><ul><li>Glial Cells Produce Myelin Sheaths Around Axons </li></ul><ul><li>Synaptogenesis </li></ul><ul><li>Formation of Synapses </li></ul>
  4. 4. Neuronal Survival <ul><li>Nerve Growth Factor </li></ul><ul><li>Promotes Neuronal Survival </li></ul><ul><li>Axons not Receiving Enough NGF Degenerate & Die (Apoptosis) </li></ul><ul><li>Neurotrophin </li></ul><ul><li>NGF-like Chemical the Promotes Neuronal Survival Activity </li></ul><ul><li>Work in Several Ways: </li></ul><ul><li>Prevent Apoptosis Early in Development </li></ul><ul><li>Increases Axonal Branching </li></ul><ul><li>Increases Regrowth of Axons after Damage </li></ul>
  5. 5. Developing Nervous System <ul><li>Developing Nervous System Produces 2-3X as Many Neurons as Needed </li></ul><ul><li>Allows for Errors to be Corrected </li></ul><ul><li>Compensates for Unpredictable Body Size Variations </li></ul>
  6. 6. Competition Among Axons <ul><li>Postsynaptic Cells Strengthen Synapses of Some Cells & Weaken Others </li></ul><ul><li>Neural Darwinism </li></ul><ul><li>During development, synapses form randomly before a selection process keeps some & rejects others </li></ul><ul><li>Synapse formation is also influenced by chemical guidance & trophic factors </li></ul>Signalling Molecules >
  7. 7. Fine-tuning by Experience <ul><li>Plasticity of the Brain </li></ul><ul><li>The brain can redesign itself (within limits) through experience </li></ul><ul><li>Experience Creates Dendritic Branches </li></ul><ul><li>Enriched environments create more dendrites and improved performance </li></ul><ul><li>Exercise releases neurotrophins </li></ul><ul><li>Generation of New Neurons </li></ul><ul><li>Adult vertebrate brain develops new neurons </li></ul><ul><li>Stem cells can form new glia or neurons </li></ul>
  8. 8. Effects of Experience <ul><li>Practice </li></ul><ul><li>Practice makes one more adept at a particular skill </li></ul><ul><li>Brain changes identified with expertise at a skill </li></ul><ul><li>Chemical & Experiential Effects </li></ul><ul><li>2 stage process for making synaptic connections </li></ul><ul><li>1. Axons find approximate targets by following chemical gradients </li></ul><ul><li>2. Experience strengthens or discards some connections </li></ul><ul><li>Exceptions: </li></ul><ul><li>Some axons produce spontaneous action potentials when reaching destination which are important for deciding what synaptic connections are appropriate in some areas of the brain </li></ul>
  9. 9. Brain Growth <ul><li>Human Brain is Similar to Other Species </li></ul><ul><li>Nearly all neurotransmitters found in the human brain are found throughout the animal kingdom </li></ul><ul><li>Ion channels are much the same in all other species & bacteria </li></ul><ul><li>Brain structures are in same locations & have similar functions </li></ul><ul><li>Size </li></ul><ul><li>The human brain is larger than most mammals, but smaller than whales, dolphins, & elephants </li></ul>
  10. 10. The Primate Brain <ul><li>Primates Have a Larger Cerebral Cortex in Comparison to the Brain of Insects </li></ul><ul><li>Some Differences in Brain Size are Due to the Way of Life </li></ul><ul><li>Development of Any Brain Depends on 2 Factors: </li></ul><ul><li>How long the embryonic development of the brain lasts </li></ul><ul><li>The number of new neurons produced per day </li></ul>
  11. 11. Vulnerability of the Developing Brain <ul><li>Developing Brain is Vulnerable to Malnutrition, Toxic Chemicals, & Infections </li></ul><ul><li>Fetal Alcohol Syndrome </li></ul><ul><li>Decreased alertness, hyperactivity, facial abnormalities, mental retardation, motor problems, & heart deficiencies </li></ul><ul><li>Cocaine Exposure </li></ul><ul><li>Lower IQ scores & decreased language skills </li></ul><ul><li>Cigarette Exposure </li></ul><ul><li>Low birthweight. SIDS, ADHD, intellectual deficits, impaired immune system, delinquency & crime later in life </li></ul>
  12. 12. Causes of Brain Damage <ul><li>Closed Head Injury </li></ul><ul><li>Sharp blow to the head that does not actually puncture the brain </li></ul><ul><li>Most common cause of brain damage in young people </li></ul><ul><li>Damages the brain because of rotational forces driving brain tissue against the inside of the skull </li></ul>
  13. 13. Causes of Brain Damage <ul><li>Stroke </li></ul><ul><li>Strokes destroy neurons by killing cells in the immediate vicinity of the stroke immediately and those in the surrounding area die over the next few days </li></ul><ul><li>Ischemia: Most common type of stroke caused by loss of blood flow due to a clot or other obstruction </li></ul><ul><li>Hemmorage: Bleeding due to rupture of an artery </li></ul>
  14. 14. Recovery from Brain Damage <ul><li>The Kennard Principle </li></ul><ul><li>Recovery from brain damage earlier in life is better than the same damage later </li></ul><ul><li>Exceptions: Younger brains are more vulnerable to forces that interfere with its development & organization </li></ul><ul><li>In essence, the elderly do not recover as well as younger adults </li></ul>Normal Exercising Damaged Limb
  15. 15. Mechanisms of Recovery <ul><li>Some Behavioral Improvement Seen After Damage </li></ul><ul><li>Due to structural changes in the surviving neurons & learned changes in behavior </li></ul><ul><li>Learned Adjustments in Behavior </li></ul><ul><li>Much of the recovery after damage is learned </li></ul><ul><li>Learn to make better use of unimpaired abilities </li></ul><ul><li>May also learn to use that at first appear lost but were only impaired </li></ul>
  16. 16. Mechanisms of Recovery <ul><li>Diaschisis </li></ul><ul><li>Decreased activity of surviving neurons after other neurons are destroyed </li></ul><ul><li>Drugs may sometimes improve behavioral deficits </li></ul><ul><li>Regrowth of Axons </li></ul><ul><li>Damaged axons rarely regenerate in CNS </li></ul><ul><li>Myelin in PNS secretes chemicals stimulating growth </li></ul>
  17. 17. Mechanisms of Recovery <ul><li>Collateral Sprouting </li></ul><ul><li>Newly formed branches from an uninjured axon attaching to a synapse vacated when the original axon was destroyed </li></ul><ul><li>Reorganization of Sensory Representations </li></ul><ul><li>(Phantom Limb) Amputation of a limb results in axonal sprouts forming not only in the cortex but also in the spinal cord, brainstem, & thalamus </li></ul>
  18. 18. Therapies <ul><li>Behavioral Interventions </li></ul><ul><li>Consists mainly of supervised practice of impaired behaviors </li></ul><ul><li>Drugs </li></ul><ul><li>New drugs aided recovery with animals: Mimodipine, gangliosides, & pregesterone </li></ul><ul><li>Brain Grafts </li></ul><ul><li>Replacing dead cells with healthy donor cells </li></ul>