Presentation on theme: "Brain Development & Plasticity"— Presentation transcript:
1Brain Development & Plasticity Dr. Elizabeth SheppardDevelopmental Cognitive Neuropsychology(C8CLDC)Child Clinical Neuropsychology(C8DCHN)
2Learning objectivesConsider the role of brain development in the study of childhoodcognitive disordersGeneral principles of brain developmentInfluences on brain developmentPrenatal brain developmentStructural featuresCellular basis (proliferation, migration, differentiation)Disruptions to prenatal developmentPostnatal brain developmentDendritic aborisationSynaptogenesisMyelinationSpecialisation or functional plasticity?
3Developmental Cognitive Neuropsychology: The “Neuro” Dimension Why look at brain development?DCN: how are cognitive functions may be disordered during development?Adult Cognitive Neuropsychology has shown there to be a close relationship between brain and behaviour (e.g., localisation of function). Assumes breakdown within a stable or ‘static’ system.But development = dynamic process. Complex interaction between neurological, cognitive, and psychosocial factors.Need to understand the neural mechanisms involved in brain development to fully appreciate the relationship between the developing brain and cognitive functions.
4Developmental Cognitive Neuropsychology: The “Neuro” Dimension Key aspects of CNS development for DCN:1.Is there a relationship between changes in brain structure/function and cognitive development?2.Recovery of impaired cognitive functions: Higher during certain periods of brain development? Is neural plasticity simply a response to insults or a driving mechanism in development?3.What is the role of environmental influences on brain maturation? Can changes in environment influence cognitive development and rehabilitation? What are the negative consequences of disadvantaged environments?
5Brain Development: General Principles 1.) Protracted period of brain development:The CNS starts to develop early in gestation and continues throughinfancy and childhood to adolescence and into adulthood.Prenatal development structural formationPostnatal development elaboration of CNS(connectivity)
6Brain Development: General principals 1.) Protracted period of brain development:Ongoing process throughout gestation and childhood.Unique to humans.Fastest rate occurs prenatally. Approx. 250,000 new brain cells formed every minute (Papalia & Olds, 1992).Structural morphology of brain complete at birth but growth continues postnatally (birth approx. 400 g; early adulthood (peaking years) approx g; gradual decline)Postnatal increase in brain weight due to differentiation, growth and maturation of existing neurons (not formation of new neurons).
7Brain Development: General principals Stages of human brain development throughout gestation.About day 40 of embryonic life CNS begins to develop.Around day 100, brain is recognisable in its mature form.(from Johnson, 1997)
8Developmental Cognitive Neuropsychology: The “Neuro” Dimension 1.) Protracted period of brain development2.) Properties of brain development:Nature of brain development is believed to be:hierarchical (cerebellar/brain stem areas, then posterior areas, and lastly anterior regions, especially frontal cortex)stepwise (growth spurts in weeks gestation (completion of neuronal generation), early infancy (dendritic & synaptic development & myelination), then again at 7-10 years, and in early adolescence)stage-like (follows a series of precise and genetically pre-determined stages; partially pre-requisite sequence of complicated and over-lapping processes).
9Brain Development: General principals 1.) Protracted period of brain development2.) Properties of brain development3.) Two major processes operate:Process of additionongoing accumulation or growthE.G.1. myelination (stage-like progression)E.G.2. dendritic aborisation (continual progression)Process of regressioninitial overproduction followed by elimination of redundant elementsE.G.1. Number of neurons prenatally is in excess of number required by mature brain. Redundant neurons die off during stage of differentiation.E.G.2. Number of synapses formed postnatallyNot considered detrimental. Fine-tuning of system.
10Brain Development: General principals 1.) Protracted period of brain development2.) Properties of brain development3.) Two major processes operate4.) Critical or “sensitive” periods:Stage in developmental sequence during which a behavioural function experiences major progressionIf progression does not occur appropriately then it may never occur.E.G. Visual deprivation during critical periods results in irreversible effects on ongoing maturation of particular visual processes (Blakemore, 1974).E.G.2. In humans, removal of cataracts after early infancy affects particular visual processes (e.g., face processing, LeGrand et al., 2003)
11Brain Development: Influences Various influences can impact on brain development. These include:Direct CNS injury or insult (e.g., stroke, tumour, trauma)Environmental factors (e.g., malnutrition, sensory deprivation)Environmental toxins (e.g., lead, radiation)Psychosocial factors (e.g., quality of mother-child relationship, level of available stimulation, social support structures, access to resources etc.)
12Brain Development: Influences Impact may not be static. Cascading influences on brain maturation may occur. For example:meningitis or febrile convulsions hippocampal sclerosis epilepsy (Ounstead et al., 1966)cranial irradiation (treatment for cerebral tumour orleukaemia) delayed cerebral pathology, especially cerebralcalcifications and other white matter pathology (Matsumoto et al., 1995; Paakko et al., 1992)
13Prenatal CNS Development: Structural features Prenatal brain development resembles that of other vertebrates.Soon after conception, the fertilized cell undergoes process of rapid cell division cluster of proliferating cells called the blastocyst.Within a few days, blastocyst differentiates into three-layered structure called the embryonic disc.
14Prenatal CNS Development: Structural features Embryonic disc further differentiates into major organic systems:Endoderm (inner layer) internal organs (e.g., digestive etc)Mesoderm (middle layer) skeletal & muscular structures- Ectoderm (outer layer) skin surface & nervous systemFrom
15Prenatal CNS Development: Structural features CNS begins with a process called neurolation.Portion of ectoderm folds in on itself hollow cylinder called the neural tube.Disruption serious structural abnormalities:incomplete closure of spinal cord(myelomeningocele) spina bifidaincomplete closure of neural tube(anencephaly) absent skull vault i.e.no brain (incompatible with life)Spina Bifida Association - Wisconsin
16Prenatal CNS Development: Structural features Neural tube differentiates along three dimensions:Length major subdivisions of CNS (forebrain & midbrain, & spinal cord)Circumference sensory & motor systemsRadius different layering patterns & cell types
17Prenatal CNS Development: Structural features Disruption of neural tube differentiation failure of formation of structural divisions. Include:- E.G.1. Failure to form two cerebral hemispheres (holoprosencephaly)- E.G.2. Incomplete fusion of the skull (craniosynostosis)Holoprosencephaly: AlobarFrom:Lobar holoprosencephalyFrom:
18Prenatal CNS Development: Cellular basis CNS contains two main classes of cells:Neurons produced by division of neuroblastsGlial cells produced by division of glioblastsNeurons:Basic functional (computational) unit of the CNSTransmit impulses within complex network of interconnecting brain cellsEnormous variety of neurons, depending on function - all with similar basic structure
20Prenatal CNS Development: Cellular basis Structure of neurons comprise four primary components:the cell body (metabolic functions of neuron, holds RNA & DNA)the axon, long projection from cell body (conducts nerve impulses away from cell body. Mature axon covered by coating of myelin rapid neural transmission)the dendrites, branch off from cell body (receive and conduct impulses from other neurons towards cell body. Dendritic spines locus of the synapse information is transmitted between neurons)the presynaptic terminals (neurotransmitters are stored and released, cross the synaptic cleft activate neurons at postsynapse)
21Prenatal CNS Development: Cellular basis Glial cells:Supportive and nutrient roleNine times as many glial cells as neuronsLack axonsSeveral subtypes, including:Astrocytes (form blood-brain barrier, support cellular structure of brain, direct migration of neurons, clean up and plug injury sites)Oligodendrocytes (speed up neural transmission by coating axons with myelin)Microglia (clean up tissue around injury sites, primarily in grey matter)Relatively immature in early stages of brain development. Continue to generate with increased CNS maturity.
22Prenatal CNS Development: Cellular basis Prenatal brain development follows a genetically predetermined sequence involving three major mechanisms:Proliferation - cell generationMigration - young neurons move to their permanent locations.Two forms: (i) Passive cell displacement - oldest cells pushed away from newer cells outside-to- inside spatiotemporal gradient.(ii) Active migration - young cells move pastpreviously generated cells inside-out gradient.Differentiation - complex process in which cells become committed to specialised systems. Involves: (i) development of cell bodies; (ii) selective cell death; (iii) dendritic and axonal growth; (iv) formation of synaptic connections
23Prenatal CNS Development: Cellular basis Major developmental processes occurring prenatal brain development.Each successive process commences prior to the completion of the previous one.Final processes are heterochronous across cortical areas.(from Anderson et al.,2001)NeurogenesisMigrationDifferentiation &maturationCell death &synaptic pruning
24Brain Development: Influences Risk factors affecting prenatal brain development include:Maternal stress and ageMaternal health (e.g., history of infection, rubella, AIDS, herpes simplex)Maternal drug and alcohol addiction (smoking, alcohol, marijuana, cocaine, heroin)Environmental toxins (lead, radiation, trauma)
25Prenatal CNS Development: Disruptions Interruptions to the major developmental processes of prenatal brain development can have severe consequences for ongoing development (including cognitive development).Timing of the insult may be more important to outcome than the nature and severity of the insult during prenatal development.Earlier disruption impact on gross cerebral morphologyLater disruption impact on migrational activity & neuronal differentiation
26Prenatal CNS Development: Disruptions Examples of differences in timing of insult on prenatal brain development include:Induction (dorsal) weeks 3-4Myelomeningocele (spina bifida). Failure of closure of the spinal cord. Arises from genetic or nutritional factors. Results in motor & perceptual deficits.Induction (ventral) weeks 5-6Holoprosencephaly. Failure to form two hemispheres. Often genetic origin. Usually incompatible with life.Proliferation 2-5 monthsMicroencephaly. Early cessation of cell division abnormally small head. Genetic or trauma factors, e.g., infection, fetal alcohol syndrome. Associated with low intellectual abilities.
27Prenatal CNS Development: Disruptions Examples of differences in timing of insult on prenatal brain development include:Migration 2-5 monthsLissencephaly, Schizencephaly, DysplasiasDifferentiationPorencephaly. Large cystic lesions, usually bilateral. Occurs at5-7 months gestation. Usually of traumatic/vascular/infectious origin. Often results in retardation and epilepsy.
28Prenatal CNS Development: Malformations of cortical development RightLeftClassical lissencephaly:smooth gyral pattern andthickened cortexmigrational disorderbetween weeks 11-13severe mental retardation,seizures, neuromotordisorders.(from Anderson et al., 2001)
29Prenatal CNS Development: Malformations of cortical development RightLeftunilateral schizencephaly:grey matter-lined cleft inright posterior frontal lobecommunicating with rightlateral ventriclemigration disorder at 8weeksmental retardation,seizures, neuromotordisorders.(from Anderson et al., 2001)
30Prenatal CNS Development: Malformations of cortical development RightLeftFocal cortical dysplasia:evidence of poor grey-white matter differentiationand low white matter signal inthe right hemispheremigrational disorder withmultiple originsresults in epilepsy, learningdisability, schizophrenia.(from Anderson et al., 2001)
31Prenatal CNS Development: Malformations of cortical development RightLefthemimegencephaly:markedly abnormal lefthemisphere with thickened,irregular cortex, excessivewhite matter, heterotopicgrey matter, and a dilated,dysmorphic lateral ventrical(from Anderson et al., 2001)
32Postnatal Development: CNS elaboration Protracted process. Occurs throughout childhood and into adolescence.Brain quadruples in size from birth to adulthood. Occurs not because of increase in number of neurons (which is established at birth) but because of three processes of elaboration (additive):dendritic aborisationsynaptogenesismyelination
33Postnatal Development: CNS elaboration Dendritic aborisation:Additive process, no evidence of regression or pruning of dendrites (e.g., Huttenlocher, 1996)Dendritic branching begins as early as gestation and continues until birth.Major changes occur postnatally, including increased length and branching.Most dramatic development occurs between postnatal weeks Adult levels at 5-6 months (Becker et al., 1984).Development in the frontal areas may continue until age 7 (Huttenlocher, 1996).Environmental stimulation/deprivation can increase/hinder the process (e.g., Kolb, 1995).
34Postnatal Development: CNS elaboration Cellular structure of visual cortex from birth to 6 months. Shows increased connectivity in brain during this period. (From Johnson, 1997).Newborn1 month old3 months old6 months old
35Postnatal Development: CNS elaboration Synaptogensis:Synaptic connections increase from birth, with bursts of rapidgrowth at various stages within different cerebral regions- V1: peak in density between 4-12 months (150% of adult)- A1 (Heschl’s gyrus): similar- Prefrontal cortex: density increase is much slower, peak only after first year- Begins in 2nd trimester of gestation (Molliver et al., 1973)Most development is postnatalRegressive process (initial over-production then reduction)Synapses initially unspecified in function (Huttenlocher, 1994)As neural circuits emerge synapses become utilised in thesefunctional systemsUnspecified synapses regress, starting after 1 year
36Postnatal Development: CNS elaboration Synaptogensis:Relatively immune to environmental stimulation/deprivation(Goldman-Rakic et al., 1997)Parallel pattern of development of neurotransmitter levels(Huttenlocher, 1994) i.e. although counter-intuitive, there issome consistency in this rise-and-fall pattern of developmentRedundancy of synapses may be associated with functionalplasticity (Huttenlocher, 1994).
37Postnatal Development Additive/Regressive Processes:Rise and fall of synaptic density for visual (open circles), auditory (filled circles) and prefrontal cortex (crossbars)From Huttenlocher (2002)
38Postnatal Development: CNS elaboration Myelination:Mostly postnatal process, with rapid development in first 3years but continuing, at a slower pace, into second decade(Valk & Van der Knapp, 1992)Hierarchical progression (e.g., Fuster, 1993):proximal before distalsensory before motorprojection before associationcentral before polesposterior before anteriorGradual increase in thickness of myelin sheathsRate varies across cerebral regions, with frontal lobesbecoming myelinated lastDisruption to process leads to reduced speed of response,attention, processing capacity, IQ.
39Brain Development: Influences Risk factors affecting postnatal brain development include:Birth complications (e.g., anoxia, prematurity)NutritionCerebral infectionEnvironmental toxins (lead, radiation, trauma)Environment & experience (e.g. normal sensory experiences vs. sensory deprivation)
40Human brain development: Postnatal influences Sensory deprivation of input affects synaptic density in kittensFrom Huttenlocher (2002)
41Specialisation or functional plasticity of the cerebral cortex How do different brain areas specialise?Two major opposing views on functional specialisation within thecerebral cortex:Prespecified functional organisation:cortical differentiation begins prenatally with cortical structure and function established prior to postnatalexperience (Rakic, 1988), by intrinsic factors.neuronal proliferation & migration suggest neurons arepreprogrammed to form particular cerebral structuresthat subsume particular functions (Johnson, 1997).
42Specialisation or functional plasticity of the cerebral cortex Undifferentiated cortex:cortex is initially undifferentiated but becomes increasingly specialised in function throughout postnatal period (e.g., Killackey, 1990; O’Leary, 1989) due to extrinsic factors like input from other parts of brainsuggests cortical regions could subsume a variety of functions depending on the sensory input they receive.if cerebral damage occurs before specialisation is complete functional localisation may be permanently altered.
43Specialisation or functional plasticity of the cerebral cortex Considerable disagreement over these two viewpoints e.g. Temple vs. Johnson:Temple - plasticity is response to brain damage but not a driving force in development. Maturational account (“preformist – nativist”): Areas come ‘online’ at different points in development, according to a genetically specified plan.- Johnson argues middle-ground position whereby large scale regions are prespecified, while establishment of small-scale functional areas require activity-dependent processes. Interactive Specialisation Account (“neuroconstructivist”): Experience is necessary to build functional long-range connections between areas that earlier in development are not connected as effectively. These connections drive specialisation mutually across areas.
44Background ReadingAnderson, V., Northam, E., Hendy, J., & Wrennall, J. (2001). Developmental Neuropsychology: A Clinical Approach. Hove: Psychology Press. Chapter 2.Johnson, M.H. (2000). Developmental Cognitive Neuroscience. Oxford:Blackwell Publishers Ltd. Chapter 2.Johnson, M.H., Munakata, Y., & Gilmore, R.O. (Eds). (2002). BrainDevelopment and Cognition: A Reader. Oxford: Blackwell Publishing. Part II.General principles of CNS development: Nowakowski, R.S. & Hayes, N.L.Intrinsic and extrinsic determinants of neocortical parcellation: A radial model: Rakic, P.Positrom Emission Tomography study of human brain functional development: Chugani, H.T., Phelps, M.E. & Mazziotta, J.C.Morphmetric study of human cerebral cortex development: Huttenlocher, P.R.
45ReferencesAnderson, V., Northam, E., Hendy, J., & Wrennall, J. (2001). Developmental Neuropsychology: AClinical Approach. Hove: Psychology Press.Becker, L., Armstrong, D., Chan, F., & Wood, M. (1984). Dendritic development in human occipitalcortical neurons. Developmental Brain Research, 13,Blakemore, C. (1974). Development of functional connections in the mammalian visual system. BritishMedical Bulletin, 30,Fuster, J. (1993). Frontal lobes. Current Opinion in Neurobiology, 3,Goldman-Rakic, P.S. (1997). Development of cortical circuitry and cognitive functions. ChildDevelopment, 58,Huttenlocher, P.R. (1994). Synaptogenesis in human cerebral cortex. In G. Dawson & K. Fischer (Eds.),Human behaviour and the developing brain. New York: Guilford Press.Huttenlocher, P.R. (1996). Morphometric study of human cerebral cortex development. In M. Johnson(Ed.), Brain development and cognition: A reader. Cambridge, MA: Blackwell.Johnson, M.H. (1997). Developmental Cognitive Neuroscience. Oxford: Blackwell Publishers Ltd.Killackey, H. (1990). Neocortical expansion: An attempt towards relating phylogeny and ontogeny.Journal of Cognitive Neuroscience, 2, 1-17.Kolb, B. (1995). Brain plasticity and behavior. Hillsdale, NJ: Lawrence Erlbaum Associates Inc.Kolb, B. & Wishaw, Q. (1996). Fundamentals of human neuropsychology (4th ed.). New York: W.H.Freeman.
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