Motor Development

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AFP/Getty Images

• The Genetics, Evolution, and Development of the
  Human Brain

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Humans Have 23 Pairs of Chromosomes
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Major Principles of Genetics

• DNA sequences, or genes, provide instructions for
  producing proteins.
• Ribosomes "read" DNA sequences that have been
  transcribed onto RNA in order to construct
  proteins.
• Alternative versions of a gene, or alleles, may
  be dominant or recessive.

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Genes Direct Protein Synthesis
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The Roles of Heredity and Environment

• Comparisons of identical twins and fraternal
  twins.
• Adoption studies.
• Identical twins raised either apart or together
  are very similar, whether the correlation for a
  particular trait is high or low.

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Lifespan Development
Pascal Goetgheluck/Photo Researchers, Inc.
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Blastocyst
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Early Differentiation

• Within the first week following conception, the
  human embryo has divided into three germ layers
• Ectoderm
• Mesoderm
• Endoderm
• Inducing factors differentiate the ectoderm layer
  into skin and nervous tissue.
• The neural tube will be retained in the mature
  brain as the ventricle system and central canal
  of the spinal cord.

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The Closing of the Neural Tube
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Neural Tube Related Birth Defects
Anterior neural pore
failure to close anencephaly
Posterior neural pore
failure to close spina bifida
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Overview of Neural Development

• Neurogenesis
• Migration
• Differentiation
• Synaptogenesis

• Apoptosis
• Myelination
• Refinement of synapses

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Neurogenesis and Migration

• New neural cells are produced in the ventricular
  zone lining the neural tube.
• Some daughter cells remain in the ventricular
  zone and continue to divide.
• Other daughter cells migrate away from the
  ventricular zone along radial glia.

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Neurogenesis
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Migration
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Differentiation

• Daughter cells differentiate into neurons or
  glia.
• The neural tube differentiates in two directions
  
• along the rostral-caudal axis.
• between the dorsal and ventral halves.
• Many genes, inducing chemicals, and types of
  input responsible for differentiation have been
  identified.

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Synaptogenesis

• Growth cones respond to the chemical and physical
  properties of the extracellular environment in
  order to reach their destinations.
• Once axons and dendrites are in place, both pre-
  and postsynaptic structures influence synaptic
  development.
• The identity of a cell's major neurotransmitter
  substance appears to be influenced by the
  postsynaptic neuron.

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The Growth Cone
From Bridgman, P.C. and Dailey, M.E., The
organization of myosin and actin in rapid frozen
nerve growth cones, Journal of Cell Biology,
108, pp. 95109
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Growth Cones Respond to External Cues
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A Synapse Is Formed
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Apoptosis

• Significant numbers of new neurons die during the
  development process.
• Neurons appear to compete for nerve growth
  factors, and those that fail to obtain this
  stimulation die.
• Synapses follow a similar pattern of
  overproduction followed by "pruning."

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Visual Synapses Peak at 1 Year of Age
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Myelination

• Myelination occurs from the spinal cord rostrally
  towards the forebrain.
• Sensory systems are myelinated before motor
  systems.
• Although the majority of myelination occurs very
  early in childhood, human myelination is not
  complete until about the age of 30 years.

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Experience Affects Development

• In some cases, the ability to rearrange synapses
  is limited to a critical window, while in other
  systems, plasticity continues throughout the
  lifespan.
• Examples of systems with critical windows
• Vision
• Language
• Effects of an enriched environment
• Imprinting

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Humans Need Social Interaction
Cynthia Johnson/Getty Images
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Ending a Critical Period

• Although we dont know exactly why a critical
  period ends, several hypotheses appear promising.
• Axon growth may end in an area.
• Synapses may mature.
• The presence or absence of neurotrophins may
  influence plasticity.

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Disorders of Development

• As the neural tube closes early in development,
  mistakes may occur, leading to spina bifida and
  anencephaly.
• Genetic disorders affecting development include
  trisomy 21, or Down syndrome, fragile X and
  phenylketonuria.
• Environmental toxins, including alcohol, tobacco
  and other legal and illegal substances, can
  interfere with the normal course of development.

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Fetal Alcohol Syndrome May Produce Physical and
Intellectual Abnormalities
Courtesy Dr. Sterling K. Clarren