PSYCH261 Chapter Notes - Chapter 5: Neural Development, Neural Tube, Intellectual Disability

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Development and Plasticity of the Brain:
Maturation of the Vertebrate Brain:
CNS forms when embryo is about 2 weeks old
Dorsal surface thickens curl to form neural tube that surrounds !uid-#lled
cavity tube sinks under surface of skin enlarges at forward end and
di%erentiates into the brain rest becomes spinal cord !uid-#lled cavity
becomes central canal of the spinal cord + 4 ventricles of the brain
contains CSF (cerebrospinal !uid)
Growth and Development:
1. Proliferation (production of new cells)
Early development cells lining the ventricles of the brain divide, some cells
remain where they are (as stem cells) others become primitive neurons +
glia that begin migrating to other locations
Similar among vertebrates (di%ers mainly in cell division)
Human brain proliferate longer than chimpanzees
2. Migration (move):
After cells have di%erentiated as neurons or glia
Some neurons migrate faster than others, some don’t reach destination until
adulthood
Immunoglobulins + chemokines (chemicals) guide neuron migration
De#cit in these chemicals decreased brain size, decreased axon growth,
mental retardation
3. Di%erentiation:
Neuron forms its axon and dendrites (primitive neuron looks like any other
cell di%erentiate)
Axon grows #rst (migrating neuron tows its axon like a tail)
After migrating neuron reaches its destination, its dendrites begin to form
4. Myelination:
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Glia produce insulating fatty sheaths that accelerate transmission in many
vertebrate axons
Myelin forms #rst in the spinal cord hindbrain, midbrain, forebrain
Myelination continues gradually for decades (unlike the rapid proliferation)
5. Synaptogenesis:
Formation of synapses
Process begins at birth, continues throughout life
Neurons form new synapses, discard old ones
Process slows in older people + formation of new dendritic branches slows
New Neurons later in life:
Olfactory receptors (exposed to outside world + toxic chemicals half-life of
only 90 days)
Stem cells (in nose, remain immature throughout life periodically divide
one cell remains immature while other replaces dying olfactory receptor
grows its axon back to appropriate site in the brain)
Similar population of stem cells in the interior of the brain
Stem cells (divide to form daughter cells migrate to olfactory bulb
transform into glia cells/neurons necessary for maintaining olfactory bulb)
Stem cells also di%erentiate into new neurons in the adult hippocampus of
birds and mammals
Hippocampus (memory formation blocking formation of it = impairs new
memories)
Radioactive isotope of carbon (skin cells correspond to year it was tested,
skeletal muscle cells correspond to 15 years ago and replaced slowly, heart
cells + cerebral cortex neurons correspond to year of person’s birth)
After damage, the axons do form in the cerebral cortex on the contralateral
side (new neurons are formed)
Path #nding by Axons:
Chemical path #nding by Axons:
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Paul Weiss (grafted extra leg to a salamander, wait for axons to grow into it,
axons reached the muscles, extra leg moved in synchrony with normal leg
next to it)
Nerves attached to muscles at random, sent variety of messages, each one
tuned to di%erent muscle, like a radio, each muscle received many signals
from di%erent stations but responded to only one
Speci#city of Axon Connections:
Roger Sperry (cut optic nerve of a newt, it grew back and connected with the
tectum, which is amphibians’ main visual area normal vision established,
cut optic nerve and rotated eye and let the axon grow back to tectum, axons
from dorsal portion of the retina that was in the ventral portion before grew
back in the area responsible for dorsal retina, and so on)
Each axon regenerated to the same place where it had originally been,
presumably by following a chemical trail
Chemical Gradients:
Growing axon follows a path of cell-surface molecules, attracted by some
chemicals and repelled by others
Ex. TOPdv (topography dorsoventral) protein some concentration higher
than others, axons of retina grow toward the tectum retinal axons with
greatest concentration of TOPdv connect to tectal cells with greatest
concentration and vice versa)
Competition among Axons as a general principle:
Neural Darwinism (development of nervous system start with more
neurons + synapses than we can keep, synapses form with approximate
accuracy, selection process keeps some and rejects others, most successful
axons and combinations survive and others fail)
Determinants of Neuronal Survival:
Levi (muscles do not determine how many axons form, they determine how
many survive)
Nerve Growth Factor (NGF) protein, promote survival and growth of the
axon neuron forms synapse onto muscle, muscle delivers NGF
Apoptosis (“suicide program” neuron destroys itself if it does not make
contact with an appropriate postsynaptic cell by a certain age)
Ex. Visual cortex is thicker than average in people born blind (absence of
visual experience visual cortex is unable to prune out the ine%ective
inappropriate synapses)
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Document Summary

Cns forms when embryo is about 2 weeks old. Growth and development: proliferation (production of new cells) Early development cells lining the ventricles of the brain divide, some cells remain where they are (as stem cells) others become primitive neurons + glia that begin migrating to other locations. Similar among vertebrates (differs mainly in cell division) Human brain proliferate longer than chimpanzees: migration (move): After cells have differentiated as neurons or glia. Some neurons migrate faster than others, some don"t reach destination until adulthood. Deficit in these chemicals decreased brain size, decreased axon growth, mental retardation: differentiation: Neuron forms its axon and dendrites (primitive neuron looks like any other cell differentiate) Axon grows first (migrating neuron tows its axon like a tail) After migrating neuron reaches its destination, its dendrites begin to form: myelination: Glia produce insulating fatty sheaths that accelerate transmission in many vertebrate axons. Myelin forms first in the spinal cord hindbrain, midbrain, forebrain.

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