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PRE LECTURE QUIZ WILL BE PRE LECTURE QUIZ WILL BE GIVEN ON:GIVEN ON:
BRAIN DEVELOPMENTBRAIN DEVELOPMENT CRANIAL NERVESCRANIAL NERVES
The Human EmbryoThe Human Embryo
ZygoteZygote EmbryoEmbryo (first 8 weeks) (first 8 weeks)
– endoderm (bodily linings)endoderm (bodily linings)
– mesodermmesoderm (connective tissues)(connective tissues)
– ectoderm (nervous system)ectoderm (nervous system) FetusFetus (after 8 weeks) (after 8 weeks)
Emergence of the Nervous SystemEmergence of the Nervous System
Neural plate becomes neural tubeNeural plate becomes neural tube
……Emergence of the Nervous SystemEmergence of the Nervous System
Walls of neural tube Walls of neural tube become neurons and become neurons and glial cellsglial cells
Cavity of neural tube Cavity of neural tube becomes ventricular becomes ventricular systemsystem
……Emergence of the Nervous SystemEmergence of the Nervous System
Neural tube defectsNeural tube defects
– anencephalyanencephaly
– spina bifidaspina bifida
AnencephalyAnencephaly
cephalic disorder cephalic disorder that results from a that results from a neural tube defectneural tube defect
occurs when the cephalic (head) end of the neural tube fails to close usually between the 23rd and 26th day of pregnancy, resulting in the absence of a major portion of the brain, skull, and scalp.
Children with this disorder are born without Children with this disorder are born without a forebrain, the largest part of the brain a forebrain, the largest part of the brain consisting mainly of the cerebral consisting mainly of the cerebral hemispheres (which include the neocortex, hemispheres (which include the neocortex, which is responsible for higher-level which is responsible for higher-level cognition, i.e., thinking). cognition, i.e., thinking).
The remaining brain tissue is often exposedThe remaining brain tissue is often exposed—not covered by bone or skin.—not covered by bone or skin.
AnencephalyAnencephaly
A baby born with anencephaly is usually blind, A baby born with anencephaly is usually blind, deaf, unconscious, and unable to feel pain. deaf, unconscious, and unable to feel pain.
Although some individuals with anencephaly Although some individuals with anencephaly may be born with a main brain stem, the lack of may be born with a main brain stem, the lack of a functioning cerebrum permanently rules out a functioning cerebrum permanently rules out the possibility of ever gaining consciousness. the possibility of ever gaining consciousness.
Reflex actions such as breathing and responses Reflex actions such as breathing and responses to sound or touch occur.to sound or touch occur.
Spina bifidaSpina bifida developmental birth defect caused by the developmental birth defect caused by the
incomplete closure of the embryonic neural incomplete closure of the embryonic neural tubetube
cleft spine, which is an incomplete closure cleft spine, which is an incomplete closure in the spinal column.in the spinal column.
Some vertebrae overlying the spinal cord are not Some vertebrae overlying the spinal cord are not fully formed and remain unfused and open. fully formed and remain unfused and open.
If the opening is large enough, this allows a If the opening is large enough, this allows a portion of the spinal cord to protrude through the portion of the spinal cord to protrude through the opening in the bones. opening in the bones.
There may or may not be a fluid-filled sac There may or may not be a fluid-filled sac surrounding the spinal cord. surrounding the spinal cord.
Other neural tube defects include Other neural tube defects include anencephalanencephaly y encephalocele, which results when other parts of , which results when other parts of the brain remain unfused.the brain remain unfused.
The Developing BrainThe Developing Brain
Rostral part of neural tube becomes brainRostral part of neural tube becomes brain Caudal part of neural tube becomes spinal Caudal part of neural tube becomes spinal
cordcord Differentiation of brain into 3 parts:Differentiation of brain into 3 parts:
– prosencephalon (forebrain)prosencephalon (forebrain)– mesencephalon (midbrain)mesencephalon (midbrain)– rhombencephalon (hindbrain)rhombencephalon (hindbrain)
Further differentiation of these 3 partsFurther differentiation of these 3 parts
The Developing Spinal CordThe Developing Spinal Cord
2 major zones of cells:2 major zones of cells:– alar platealar plate (dorsal) (dorsal)
» sends sensory projections to brainsends sensory projections to brain
– basal platebasal plate (ventral) (ventral)» motor neuron developmentmotor neuron development
Formation of Neurons and Glial CellsFormation of Neurons and Glial Cells
Single layer of cells within Single layer of cells within neural tube…ventricular layerneural tube…ventricular layer
Daughter cellsDaughter cells
– Migration first to marginal Migration first to marginal (outer) layer(outer) layer
– Migration then to Migration then to intermediate layerintermediate layer
– Migration to cortical plate & Migration to cortical plate & subventricular layersubventricular layer
– Some remain in ventricular Some remain in ventricular layer (ependymal cells)layer (ependymal cells)
Cell Formation and MigrationCell Formation and Migration
NeurogenesisNeurogenesis: formation of : formation of new neuronsnew neurons
Neural migration guided by:Neural migration guided by:
– radial glial cellsradial glial cells
– glycoproteinsglycoproteins
Cell DifferentiationCell Differentiation
Creation of different cell Creation of different cell typestypes
2 types:2 types:– cell-autonomous cell-autonomous
differentiation (genetic)differentiation (genetic)– induction (dependent upon induction (dependent upon
influence of other cells)influence of other cells)
Glial Cell DevelopmentGlial Cell Development Begins in embryonic Begins in embryonic
periodperiod Most intense glial Most intense glial
production after birthproduction after birth Myelination (wrapping Myelination (wrapping
axon in myelin sheath)axon in myelin sheath)– spinal cord then spinal cord then
hindbrain then hindbrain then midbrain then forebrainmidbrain then forebrain
– sensory then motorsensory then motor
Formation of Neural ConnectionsFormation of Neural Connections
Axons grow towards target cellsAxons grow towards target cells Axon emerges from growth cone (containing Axon emerges from growth cone (containing
filopodia)filopodia) Filopodia are attracted to target cell through Filopodia are attracted to target cell through
chemical signalchemical signal– neurotrophinsneurotrophins
Abrupt changes in direction of growth due to Abrupt changes in direction of growth due to guidepost cellsguidepost cells
Importance of Neural ActivityImportance of Neural Activity
Axonal remolding (Shatz, 1992) Axonal remolding (Shatz, 1992)
– neurons grow to many neurons grow to many addressesaddresses
» correct address...remaincorrect address...remain
» incorrect incorrect address...eliminatedaddress...eliminated
Neurons that fire together wire Neurons that fire together wire togethertogether
Neuron Cell DeathNeuron Cell Death
Theories:Theories:– Competition to attain connections with target Competition to attain connections with target
cellscells– Need to receive certain qualitity of Need to receive certain qualitity of
neurotrophins for survivalneurotrophins for survival Examples:Examples:
– Motor neuron-muscle fiber connections (PNS)Motor neuron-muscle fiber connections (PNS)– Purkinje cells of cerebellum (CNS)Purkinje cells of cerebellum (CNS)
Failures of Neural DevelopmentFailures of Neural Development
Genetic defectsGenetic defects– Down Syndrome Down Syndrome - trisomy 21, is a chromosomal condition - trisomy 21, is a chromosomal condition
caused by the presence of all or part of an extra 21st chromosome.caused by the presence of all or part of an extra 21st chromosome.
– Phenylketonuria (PKU) - Phenylketonuria (PKU) - autosomal recessive metabolic autosomal recessive metabolic genetic disorder characterized by a deficiency in the hepatic enzyme genetic disorder characterized by a deficiency in the hepatic enzyme phenylalanine hydroxylase (PAH)phenylalanine hydroxylase (PAH)
» PAH : converts phenylalanine ('Phe') to the amino acid tyrosine.PAH : converts phenylalanine ('Phe') to the amino acid tyrosine.
» hyperphenylalaninemia (HPA) was associated with mental retardationhyperphenylalaninemia (HPA) was associated with mental retardation ..
– Fragile X Syndrome Fragile X Syndrome
Fragile X Syndrome Fragile X Syndrome genetic syndrome which results in a genetic syndrome which results in a
spectrum of characteristic physical and spectrum of characteristic physical and intellectual limitations and emotional and intellectual limitations and emotional and behavioral features which range from behavioral features which range from severe to mild in manifestation.severe to mild in manifestation.
expansion of a single trinucleotide gene expansion of a single trinucleotide gene sequence (sequence (CGG) CGG) on the on the X chromosome, and chromosome, and results in a failure to express the protein results in a failure to express the protein coded by the coded by the FMR1 gene, which is required ne, which is required for normal neural development.for normal neural development.
External factorsExternal factors– MalnutritionMalnutrition– Fetal Alcohol Syndrome - next slideFetal Alcohol Syndrome - next slide
Failures of Neural DevelopmentFailures of Neural Development
(Andrew Syndrome) is a pattern of mental (Andrew Syndrome) is a pattern of mental and physical defects that can develop in a and physical defects that can develop in a fetus when a woman drinks alcohol during fetus when a woman drinks alcohol during pregnancy. pregnancy.
Redevelopment of the Nervous SystemRedevelopment of the Nervous System
Damage to cell body of nerve...death; damage to axon...degenerative changes, Damage to cell body of nerve...death; damage to axon...degenerative changes, sometimes deathsometimes death
anterograde degeneration (from point of injury to axon terminals)anterograde degeneration (from point of injury to axon terminals) retrograde degeneration (between point of injury and cell body)retrograde degeneration (between point of injury and cell body) chromatolysis (degeneration of cell body)chromatolysis (degeneration of cell body) transneuronal degeneration (damage to connecting neurons)transneuronal degeneration (damage to connecting neurons)
...Redevelopment of the Nervous System...Redevelopment of the Nervous System
Regeneration occurs within PNS onlyRegeneration occurs within PNS only Collateral sprouting within CNSCollateral sprouting within CNS Research on recovery of CNS functioningResearch on recovery of CNS functioning
– collateral sproutingcollateral sprouting– suppression of growth-inhibiting chemicalssuppression of growth-inhibiting chemicals– fetal tissue transplantationfetal tissue transplantation
Compensatory strategies after CNS damageCompensatory strategies after CNS damage– rehabilitationrehabilitation