Chapter 05: Development & Plasticity of the Brain

Growth of the Growth of the BrainBrain

CNS Begins Forming CNS Begins Forming in the Embryo at About in the Embryo at About 2 Weeks2 WeeksThe Neural Tube forms around a fluid-The Neural Tube forms around a fluid-filled cavityfilled cavity

Eventually it sinks under the skin & Eventually it sinks under the skin & develops into the hindbrain, midbrain, develops into the hindbrain, midbrain, & forebrain& forebrain

The fluid-filled cavity becomes the The fluid-filled cavity becomes the central canal & the 4 ventriclescentral canal & the 4 ventricles

NeurogenesisNeurogenesis5 Steps in Neuron 5 Steps in Neuron DevelopmentDevelopmentProliferationProliferationProduction of New Cells

MigrationMigrationMovement of Primitive Neurons & Glia to Final Destination

DifferentiationDifferentiationNeurons Develop Axons & Dendrites

MyelinationMyelinationGlial Cells Produce Myelin Sheaths Around Axons

SynaptogenesisSynaptogenesisFormation of Synapses

Neuronal Neuronal SurvivalSurvival

Nerve Growth Nerve Growth FactorFactorPromotes Neuronal SurvivalPromotes Neuronal Survival

Axons not Receiving Enough NGF Axons not Receiving Enough NGF Degenerate & Die (Apoptosis)Degenerate & Die (Apoptosis)

NeurotrophinNeurotrophinNGF-like Chemical the Promotes NGF-like Chemical the Promotes Neuronal Survival ActivityNeuronal Survival Activity

Work in Several Ways:Work in Several Ways:

Prevent Apoptosis Early in Prevent Apoptosis Early in DevelopmentDevelopment

Increases Axonal BranchingIncreases Axonal Branching

Increases Regrowth of Axons after Increases Regrowth of Axons after DamageDamage

Developing Nervous Developing Nervous SystemSystem

Developing Nervous System Produces Developing Nervous System Produces 2-3X as Many Neurons as Needed2-3X as Many Neurons as NeededAllows for Errors to be CorrectedAllows for Errors to be CorrectedCompensates for Unpredictable Body Compensates for Unpredictable Body Size VariationsSize Variations

Competition Among Axons

Postsynaptic Cells Strengthen Synapses of Some Cells & Weaken Others

Neural DarwinismDuring development, synapses form randomly before a selection process keeps some & rejects othersSynapse formation is also influenced by chemical guidance & trophic factors

Fine-tuning by Experience

Plasticity of the BrainThe brain can redesign itself (within limits) through experience

Experience Creates Dendritic BranchesEnriched environments create more dendrites and improved performanceExercise releases neurotrophins

Generation of New NeuronsAdult vertebrate brain develops new neuronsStem cells can form new glia or neurons

Effects of Effects of ExperienceExperience

PracticePractice makes one more adept at a particular skillBrain changes identified with expertise at a skill

Chemical & Experiential Effects2 stage process for making synaptic connections1. Axons find approximate targets by following chemical gradients2. Experience strengthens or discards some connectionsExceptions: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

Brain GrowthBrain GrowthHuman Brain is Similar to Other SpeciesNearly all neurotransmitters found in the human brain are found throughout the animal kingdomIon channels are much the same in all other species & bacteriaBrain structures are in same locations & have similar functions

SizeThe human brain is larger than most mammals, but smaller than whales, dolphins, & elephants

The Primate The Primate BrainBrain

Primates Have a Larger Cerebral Cortex in Comparison to the Brain of InsectsSome Differences in Brain Size are Due to the Way of LifeDevelopment of Any Brain Depends on 2 Factors:How long the embryonic development of the brain lastsHow long the embryonic development of the brain lastsThe number of new neurons produced per dayThe number of new neurons produced per day

Vulnerability of the Vulnerability of the Developing BrainDeveloping Brain

Developing Brain is Developing Brain is Vulnerable to Vulnerable to Malnutrition, Toxic Malnutrition, Toxic Chemicals, & Chemicals, & InfectionsInfectionsFetal Alcohol Fetal Alcohol SyndromeSyndromeDecreased alertness, hyperactivity, facial abnormalities, mental retardation, motor problems, & heart deficiencies

Cocaine ExposureCocaine ExposureLower IQ scores & decreased language skills

Cigarette ExposureCigarette ExposureLow birthweight. SIDS, ADHD, intellectual deficits, impaired immune system, delinquency & crime later in life

Causes of Brain Causes of Brain DamageDamage

Closed Head InjuryClosed Head InjurySharp blow to the head that does Sharp blow to the head that does not actually puncture the brainnot actually puncture the brain

Most common cause of brain Most common cause of brain damage in young peopledamage in young people

Damages the brain because of Damages the brain because of rotational forces driving brain rotational forces driving brain tissue against the inside of the tissue against the inside of the skullskull

Causes of Brain Causes of Brain DamageDamage

StrokeStrokes destroy neurons by killing cells in the immediate vicinity of the stroke immediately and those in the surrounding area die over the next few daysIschemia: Most common type of stroke caused by loss of blood flow due to a clot or other obstructionHemmorage: Bleeding due to rupture of an artery

Recovery from Recovery from Brain DamageBrain Damage

The Kennard PrincipleThe Kennard PrincipleRecovery from brain damage earlier in life is better than the same damage Recovery from brain damage earlier in life is better than the same damage laterlaterExceptions: Younger brains are more vulnerable to forces that interfere with Exceptions: Younger brains are more vulnerable to forces that interfere with its development & organizationits development & organizationIn essence, the elderly do not recover as well as younger adultsIn essence, the elderly do not recover as well as younger adults

Mechanisms of Mechanisms of RecoveryRecovery

Some Behavioral Improvement Seen After Some Behavioral Improvement Seen After DamageDamageDue to structural changes in the surviving neurons & Due to structural changes in the surviving neurons & learned changes in behaviorlearned changes in behavior

Learned Adjustments in BehaviorLearned Adjustments in BehaviorMuch of the recovery after damage is learnedMuch of the recovery after damage is learned

Learn to make better use of unimpaired abilitiesLearn to make better use of unimpaired abilities

May also learn to use that at first appear lost but were May also learn to use that at first appear lost but were only impairedonly impaired

Mechanisms of Mechanisms of RecoveryRecovery

DiaschisisDiaschisisDecreased activity of surviving neurons after other neurons are destroyedDrugs may sometimes improve behavioral deficits

Regrowth of AxonsRegrowth of AxonsDamaged axons rarely regenerate in CNSMyelin in PNS secretes chemicals stimulating growth

Mechanisms of Mechanisms of RecoveryRecovery

• Collateral Collateral SproutingSproutingNewly formed branches from an uninjured axon attaching to a synapse vacated when the original axon was destroyed

• Reorganization of Reorganization of Sensory Sensory RepresentationsRepresentations(Phantom Limb) Amputation of a limb results in axonal sprouts forming not only in the cortex but also in the spinal cord, brainstem, & thalamus

TherapiesTherapiesBehavioral Behavioral InterventionsInterventionsConsists mainly of supervised practice of impaired behaviors

DrugsDrugsNew drugs aided recovery with animals: Mimodipine, gangliosides, & pregesterone

Brain GraftsBrain GraftsReplacing dead cells with healthy donor cells