The Development and Plasticity of the Nervous System

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Chapter 3

• The Development and Plasticity of the Nervous
  System

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

• Layers 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.

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Structural Development Formation of Nervous
System

• Embryonic 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.

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3 Vesicle Stage
5 Vesicle Stage
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Structural Development Differentiation of the
Brain
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Structural Development Differentiation of
Embryonic Cells
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Structural Development The Developing Spinal
Cord

• Alar plate - gives rise to sensory neurons and
  interneurons of the spinal cords 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.

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Structural Development The Developing
Ventricular System

• Develops in the cavity inside the neural tube,
  contains cerebral spinal fluid
• Four ventricles
• Lateral ventricles
• Third ventricle
• Fourth ventricle

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Cellular Development Formation of Neurons and
Glial Cells

• A layer of ectodermal cells form on the inner
  surface of neural tube and divide to form
• Ventricular layer which then divides into
  daughter cells
• Daughter 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.

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Glial Cell Development

• Glial cells also 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.

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Cellular Development Formation of Neurons and
Glial Cells

• Migrating cells
• Guided by radial glial cells
• 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
  neurons have been found in the brains of
  schizophrenic patients.

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Cellular Development Formation of Neurons and
Glial Cells
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Cellular Development Formation of Neurons and
Glial Cells

• Neurogenesis 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.

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Neural Cell Differentiation

• Cell-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.
• The notochord influences new neurons to become a
  spinal motor neurons.

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Formation of Neural Connections

• Once a cell has differentiated, it must establish
  connections with other neurons.
• Neurons grow toward target cells
• Axon emerges from growth cone
• Filopodia - consist of spine-like extensions that
  appear to be searching

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The Movement of Filopodia and the Growth Cone
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Formation of Neural Connections Axonal Growth

• Guidepost cells serve as a map when the
  filopodia reach them, the growth cone adheres to
  that cell and the guidepost cells redirect axonal
  growth to target cells.
• Neurotrophins released by the target cell
• Attract the filopodia of developing neurons
• Repels others to ensure only appropriate axons
  move toward the target
• Target cell determines the neurotransmitter
  released from the presynaptic neuron

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The Importance of Neural Activity

• Neural 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

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Neural Cell Death

• Apoptosis - genetically programmed cell death
• Synaptic 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

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Disorders of Development Down 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

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Neural Degeneration

• Causes
• Tumors
• Seizure Disorders
• Cerebrovascular Accidents
• Degenerative Disorders
• Disorders Caused by Infectious Diseases
• Types of degeneration
• Anterograde
• Retrograde
• Transneuronal

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Neural 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 tumor
• Meningioma - Benign brain tumor

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Neural Degeneration Seizure Disorders

• Seizure - 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 occur
• Convulsion a violent sequence of uncontrollable
  muscular movements caused by a seizure
• Hippocrates was the first to note that seizures
  might have a physical cause

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Classification of Seizure Disorders
I. Generalized Seizures Tonic-clonic (grand mal) Absence (petit mal) Atonic
II. Partial Seizures Simple Localized motor seizure Motor seizure with progression of movements Sensory Psychic Autonomic 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
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Specific Lobe Seizures

• Frontal 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.

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Cerebrovascular AccidentsStroke

• 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

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Cerebrovascular AccidentsEffects of a Stroke

• Right 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

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Cerebrovascular AccidentsRisk Factors for Stroke

• High blood pressure
• Cigarette smoking or exposure to secondhand smoke
  
• High cholesterol
• Diabetes
• Being overweight or obese
• Physical inactivity
• Obstructive sleep apnea
• Cardiovascular disease
• Use of some birth control pills or hormone
  therapies that include estrogen
• Heavy or binge drinking
• Use of illicit drugs

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Cerebrovascular Accidents Traumatic Brain Injury

• Vehicle-related collisions
• Violence
• Sports injuries
• Falls
• Explosive blasts

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Traumatic Brain Injury
Head Games
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Degenerative Disorders

• Transmissible Spongiform Encephalopathy
• Parkinsons
• Huntingtons
• Alzheimers
• Amyotrophic Lateral
• Sclerosis (ALS)
• Multiple Sclerosis

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Degenerative DisordersMultiple Sclerosis

• Autoimmune demyelinating disease
• Myelin protein crosses into general circulation
  causing an immune system reaction
• Sclerotic plaques interrupt neuronal signals

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Disorders Caused by Infectious Diseases

• Viral Encephalitis
• Herpes
• Polio
• Rabies
• HIV
• Meningitis
• Bacteria
• Syphilis
• Lyme Disease
• Malaria

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NeuroplasticityRegeneration of Damaged Neurons

• Neural regeneration
• Occurs in embryonic and neonatal nervous system
• In adults usually does not occur in CNS
• Occurs in PNS
• Glycoproteins present in mature PNS promote cell
  regeneration
• Oligodendrocytes synthesize a glycoprotein that
  inhibits axonal growth in CNS
• Collateral Sprouting neurons compensate for
  loss of neural connections in CNS by sending new
  axonal endings to vacated receptor sites

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Chromatolysis
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Neuroplasticity Transplantation

• Animal research - Substantia nigra damage has
  been reduced by implanting fetal tissue from
  donors into the damaged area.
• Human research - Parkinsons 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

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Neuroplasticity Stem Cells
Embryonic 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.