Chapter 3Chapter 3

The Development and Plasticity of the Nervous System

The Structural Development of The Structural Development of the Human Nervous Systemthe Human Nervous System

Brain development begins at the point of conception when the ovum is fertilized by a sperm resulting in the formation of a zygote.

After the zygote divides the resulting developing human is called:• Embryo – next 6 weeks• Fetus – at week 9 and for the remainder of the pregnancy

Eight cell zygote

Development of the Nervous Development of the Nervous System: The Human EmbryoSystem: The Human EmbryoLayers of cells in the embryo:

• Ectoderm forms the nervous systems as well as the epidermis and parts of the eyes and ears

• Mesoderm forms connective tissue, muscle, blood, blood vessels

• Endoderm forms the linings of the body

Throughout the embryonic and fetal period different cell types are created; the process is called differentiation.

Neural Crest Cell Neural Crest Cell MigrationMigration

Structural Development: Structural Development: Formation of Nervous Formation of Nervous SystemSystemEmbryonic layers thicken to

become:• Neural Plate is the name for the

thickened ectodermal layer.• Neural folds push up to form a space

called the neural groove.• Neural tube forms from the neural

groove in 23 days. The brain and spinal cord develop from it.

3 Vesicle Stage 5 Vesicle Stage

Structural Development: Structural Development: Differentiation of the BrainDifferentiation of the Brain

Structural Development: Structural Development: The Developing Spinal CordThe Developing Spinal CordAlar plate - gives rise to sensory

neurons and interneurons of the spinal cord’s dorsal horn.

Basal plate - forms ventral portion of the spinal cord where motor neurons originate and the interneurons of the ventral root form

Sympathetic and Parasympathetic nervous systems also derive from the basal plate.

Cellular Development: Cellular Development: Formation of Neurons and Glial Formation of Neurons and Glial CellsCellsA layer of ectodermal cells form on the inner surface of neural tube and divide to form:Ventricular layer which divides into daughter cellsDaughter cells migrate:

• between the intermediate and marginal layers to form the cortical plate which develops into the cortex.

• to the subventricular layer, becoming either glial cells or interneurons.

• daughter cells remaining in the ventricular layer develop into ependymal cells, which form the lining of the four ventricles and the central canal of the spinal cord.

Structural Development: Structural Development: The Developing Ventricular The Developing Ventricular SystemSystemDevelops in the cavity inside the

neural tube, contains cerebral spinal fluid

Four ventricles:• Lateral ventricles• Third ventricle• Fourth ventricle

Cellular Development: Cellular Development: Formation of Neurons and Glial Formation of Neurons and Glial CellsCellsNeurogenesis is the formation of

new neurons.• Few neurons are formed after birth• Exceptions are:

cerebellar cells, olfactory receptor neurons, hippocampal neurons, and some cortical neurons

• These exceptions allow for neuroplasticity.

Cellular Development: Cellular Development: Formation of Neurons and Glial Formation of Neurons and Glial CellsCellsMigrating cells

• Guided by glycoproteins

• Glycoproteins allow neurons to bind to other neurons or radial glial cells (a handhold).

• Failures of the adequate production of glycoproteins may lead to behavioral deficits.

• Cell migration dysfunction is implicated in schizophrenia where abnormal distributions of neurotrophins have been found in the brains of schizophrenic patients.

Neural Cell DifferentiationNeural Cell DifferentiationCell-autonomous differentiation is

controlled by genetic programming.• A Purkinje cell will develop into its

distinctive form even if grown in culture out of its environment.

Induction - other cells influence the final form.• Spinal motor neuron is influenced by

the notochord to become a spinal motor neuron.

Glial Cell DevelopmentGlial Cell DevelopmentGlial cells develop from the ventricular

layer.Glial cells develop more after birth.A major function of glial cells is the

myelination of neurons: Schwann cells in the peripheral nervous

system wrap themselves around nerve axons; a single Schwann cell makes up a single segment of an axon's myelin sheath

Oligodendrocytes in the central nervous system wrap themselves around numerous axons at once.

Formation of Neural Formation of Neural ConnectionsConnectionsOnce a cell has differentiated, it

must establish connections with other neurons. Neurons grow toward target cells

(other neurons, organs) Axon emerges from growth cone Filopodia - consist of spine-like

extensions that appear to be searching

The The Movement of Movement of Filopodia and Filopodia and the Growth the Growth ConeCone

Formation of Neural Formation of Neural Connections: Axonal GrowthConnections: Axonal Growth

Neurotrophins released by the target cell• Attract the filopedia of developing neurons• Repels others to ensure only appropriate

axons move toward the targetGuidepost cells serve as a map to neuron

growth; when cells reach them, the growth cone adheres to that cell and the guidepost cells redirect axonal growth to target cells.

Target cell determines the neurotransmitter released from the presynaptic neuron

The Importance of Neural The Importance of Neural ActivityActivityNeural activity is necessary for

establishing appropriate neural connections.

Axonal remodeling is the process of axons connecting to the correct place; selectively strengthens the synaptic connection

Neural activity “wires” the connections for communication within the nervous system

New synaptic connections after birth allow more refined analysis of stimuli and more varied behavioral responses

Neural Cell DeathNeural Cell DeathApoptosis - genetically programmed

cell deathSynaptic pruning Theories of cell death

• Neurons compete for connections to target cells and the unsuccessful ones die.

• Neurons that receive a sufficient amount of chemical from the target cells survive; neurons that receive less die.

Neural development recap

Disorders of Development: Disorders of Development: Down SyndromeDown Syndrome

Genetic condition that causes delays in physical and intellectual development

Most common genetic cause of learning disabilities in children

Down syndrome results when one of three types of abnormal cell division involving chromosome 21 occurs

Trisomy 21 Mosaic Down Syndrome Translocation Down Syndrome

Neuroplasticity: Neuroplasticity: Neural DegenerationNeural Degeneration Causes

Tumors Seizure Disorders Cerebrovascular Accidents Degenerative Disorders Disorders Caused by Infectious Diseases

• Types of degeneration Anterograde Retrograde Transneuronal

• Chromatolysis - process of breakdown where degeneration occurs

Neural Degeneration: TumorsNeural Degeneration: Tumors

Tumor - Mass of cells whose growth is uncontrolled and that serves no useful function

Metastasis - Process by which cells break off a tumor and grow elsewhere in body

• Tumors damage brain tissue two ways Compression Infiltration

Glioma - Cancerous brain tumorMeningioma - Benign brain tumor

Neural Degeneration: Neural Degeneration: Seizure DisordersSeizure DisordersSeizure - a period of sudden, excessive

activity of cerebral neurons Briefly alters consciousnesses, movement, or

actions If neurons that make up the motor system are

involved, convulsions can occurConvulsion – a violent sequence of

uncontrollable muscular movements caused by a seizure

• Hippocrates was the first to note that seizures might have a physical cause

Classification of Seizure Classification of Seizure DisordersDisordersI. Generalized Seizures

A.Tonic-clonic (grand mal)B.Absence (petit mal)C.Atonic

II. Partial SeizuresA.Simple

1. Localized motor seizure2. Motor seizure with progression of movements3. Sensory4. Psychic5. Autonomic

B.Complex – includes 1-5 as above

III. Partial seizures evolving to a generalized cortical seizure – starts as IIA or IIB than becomes a grand mal seizure

Specific Lobe SeizuresSpecific Lobe SeizuresFrontal lobe seizures may produce unusual

symptoms that can appear to be related to a psychiatric problem or a sleep disorder.

Temporal lobe seizures may include having odd feelings such as euphoria, fear, panic and déjà vu.

Occipital seizures are often mistaken for migraines because they share symptoms including visual disturbances, partial blindness, nausea and vomiting, and headache.

Parietal lobe seizures can involve both sensory and visual sensations.

Cerebrovascular Accidents:Cerebrovascular Accidents:StrokeStroke

Hemorrhagic stroke Caused by the rupture of a cerebral blood vessel Most common cause is high blood pressure

Ischemic stroke Caused by the obstruction of blood flow to the brain Thrombus – a blood clot that forms within a blood vessel, obstructing blood flow Embolus – a piece of matter that dislodges from its site of origin and travels through the system

until it reaches a vessel to small to let it pass thereby obstructing blood flow

Cerebrovascular Accidents:Cerebrovascular Accidents:Effects of a Effects of a StrokeStrokeRight Brain

Paralysis on the left side of the body    Vision problems    Quick, inquisitive behavioral style   

Left Brain Paralysis on the right side of the body    Speech/language problems    Slow, cautious behavioral style   

Hindbrain Can affect both sides of the body May leave someone in a ‘locked-in’ state

Cerebrovascular Accidents:Cerebrovascular Accidents:Risk Factors for Risk Factors for StrokeStrokeHigh blood pressureCigarette smoking or exposure to secondhand

smoke High cholesterolDiabetes Being overweight or obese Physical inactivity Obstructive sleep apneaCardiovascular diseaseUse of some birth control pills or hormone

therapies that include estrogen Heavy or binge drinking Use of illicit drugs

Cerebrovascular Accidents: Cerebrovascular Accidents: Traumatic Brain InjuryTraumatic Brain Injury Vehicle-related collisions Violence Sports injuries Falls Explosive blasts

Traumatic Brain InjuryTraumatic Brain Injury

Head Games

Degenerative DisordersDegenerative Disorders

Transmissible Spongiform Encephalopathy

Parkinson’sHuntington’sAlzheimer’sAmyotrophic Lateral Sclerosis (ALS)Multiple Sclerosis

Degenerative Disorders:Degenerative Disorders:Multiple SclerosisMultiple Sclerosis

Autoimmune demyelinating diseaseMyelin protein crosses into general

circulation causing an immune system reaction

Sclerotic plaques interrupt neuronal signals

Disorders Caused by Disorders Caused by Infectious DiseasesInfectious Diseases

Viral EncephalitisHerpesPolioRabies

HIVMeningitisBacteria

SyphilisLyme Disease

Malaria

Neuroplasticity:Neuroplasticity:Regeneration of Damaged Regeneration of Damaged NeuronsNeurons

Neural regeneration• Occurs in neonatal and embryonic nervous system• In adults usually does not occur in CNS• Occurs in PNS

Glycoproteins are present in mature PNS that promote cell regeneration

Oligodendrocytes synthesize a glycoprotein that inhibits axonal growth in CNS

Collateral Sprouting - neurons compensate for loss of neural connections by sending new axonal endings to vacated receptor sites in the CNS

ChromatolysisChromatolysis

Neuroplasticity: Neuroplasticity: TransplantationTransplantationAnimal research - Substantia nigra

damage has been reduced by implanting fetal tissue from donors into the damaged area.

Human research - Parkinson’s disease patients have partial recovery of motor ability from transplanted fetal tissue.

Ethics - a major debate over the use fetal stem cells exists, acceptance might be higher for adult stem cell use

Neuroplasticity: Stem Neuroplasticity: Stem CellsCellsEmbryonic stem cells are found in an embryo, fetus or the umbilical cord blood. Depending upon when they are harvested, embryonic stem cells can give rise to just about any cell in the human body.

Adult stem cells - found in infants, children and adults. They reside in developed tissues such as those of the heart, brain and kidney. They usually give rise to cells within their resident organs.