Psychology 2220A/B Chapter Notes - Chapter 9: Growth Cone, Occipital Lobe, List Of Association Football Teams To Have Won Four Or More Trophies In One Season

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The brain is a plastic (changeable), living organ that continuously changes in response
to its genetic programs and environment
Three general ideas are emphasized:
1. The amazing nature of neurodevelopment
2. The important role of experience in neurodevelopment
3. The dire consequences of neurodevelopmental errors
Because there is so little variation in most people’s early experience, the critical role of
experience in human cerebral and psychological development is not always obvious
PHASES OF NEURODEVELOPMENT
In the beginning, there is a zygote, a single cell formed by the amalgamation of an
ovum and a sperm
The zygote divides to form two daughter cells
Three things other than cell multiplication occur:
First, cells must differentiate some must become muscle cells, some must become
multipolar neurons, and so on
Second, cells must make their way to appropriate sites and align themselves with
the cells around them to form particular structures
Third, cells must establish appropriate functional relations with other cells
Developing neurons accomplish this in five phases:
Induction of the Neural Plate
Three weeks after conception, the tissue that is destined to develop into the human
nervous system becomes recognizable as the neural plate
Neural plate is a small patch of ectodermal tissue on the dorsal surface of the
developing embryo
The ectoderm is the outermost of the three layers of embryonic cells: ectoderm,
mesoderm, and endoderm
The development of the neural plate seems to be induced by chemical signals from
an area of the underlying mesoderm layer - an area that is consequently referred
to as an organizer
Tissue taken from the dorsal mesoderm of one embryo (the donor) and implanted
beneath the ventral ectoderm of another embryo (the host) induces the
development of an extra neural plate on the ventral surface of the host
The earliest cells of the human embryo are totipotent - that is, they have the ability
to develop into any type of cell in the body if transplanted to the appropriate site
As the embryo develops, the destiny of various cells becomes more specified
When the neural plate develops its cells lose some of their potential to become
different kinds of cells
Each cell of the early neural plate still has the potential to develop into most types
of mature nervous system cell, but it cannot normally develop into other kinds of
cells are said to be multipotent
The cells of the neural plate are often referred to an embryonic stem cells
Stem cells are cells that meet two specific criteria
They have a seemingly unlimited capacity for self-renewal if maintained in an
appropriate cell culture
They have the ability to develop into different types of mature cells
Chapter 9: Development of the Nervous System
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Because these cells still have the capacity for unlimited self-renewal and are still
multipotent, these cells are termed glial stem cells and neural stem cells
respectively
This capacity results from the fact that when a stem cell divides, two different
daughter cells are created: one that eventually develops into some type of body
cell and one that develops into another stem cell
Eventually errors accumulate, which can disrupt the process, that is why stem cell
cultures do not last forever
The neural plate folds to form the neural groove, and then the lips of the neural
groove fuse to form the neural tube
The inside of the neural tube eventually becomes the cerebral ventricles and spinal
cord
Neural Proliferation
Most cell division in the neural tube occurs in the ventricular zone - the region
adjacent to the ventricle
In each species, the cells in different parts of the neural tube proliferate in a
particular sequence that is responsible for the pattern of swelling and folding that
gives the brain of each member of that species the characteristic shape
The complex pattern of proliferation is in part controlled by chemical signals from
two organizer areas in the neural tube: the floor plate, which runs along the midline
of the anterior surface of the tube, and the roof plate, which runs along the midline
of the dorsal surface of the tube
Migration and Aggregation
Migration
Once cells have been created through cell division in the ventricular zone of the
neural tube, they migrate to the appropriate target location
During this period of migration, the cells are still in an immature form, lacking
the processes that characterize mature neurons
Two major factors govern migration in the developing neural tube: time and
location
Cell migration in the developing neural tube:
Radial Migration - proceeds form the ventricular zone in a straight line
outward toward the outer wall of the tube
Tangential Migration - occurs at a right angle to radial migration, that is,
parallel to the tubes walls
Most cells engage in both radial and tangential migration to get form their point
of origin in the ventricular zone to their target destination
There are two methods by which developing cells migrate, one is somal
translocation
In somal translocation, an extension grows from the developing cell in the
general direction of the migration; the extension seems to explore the
immediate environment for attractive and repulsive cues as it grows
Then, the cell body itself moves into and along the extending process, and
trailing processes are retracted
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Glial-mediated migration, once the period of neural proliferation is well
underway and the walls of the neural tube are thickening, a temporary network
of glial cells called radial glial cells, appears in the developing neural tube
At this point, most cells engaging in radial migration do so by moving along the
radial glial network
Orderly waves of migrating cells, progressing from deeper to more superficial
layers, referred to as an inside-out pattern
Cortical migration patterns are more complex than was first thought: many
cortical cells engage in long tangential migrations to reach their final
destinations, and the patterns of proliferation and migration are different for
different areas of the cortex
The neural crest is a structure that is situated just dorsal to the neural tube
It is formed from cells that break off form the neural tube as it is being formed
Neural crest cells develop into the neurons and glial cells of the peripheral
nervous system, and thus many of them must migrate over considerable
distances
Numerous chemicals that guide migrating neurons by either attracting and
repelling them
Some of these guidance molecules are released by glial cells
Aggregation
Once developing neurons have migrated they must align themselves with other
developing neurons that have migrated to the same area to form the structures
of the nervous system called aggregation
Migration and aggregation are though to be mediated by cell-adhesion
molecules (CAMs), which are located on the surface of neurons and other cells
Cell-adhesion molecules have the ability to recognize on other cells and adhere
to them
Elimination of just one type of CAM in a knockout mouse has been shown to
have a devastating effect on brain development
This finding suggests that abnormalities of CAM function may be causal factors
in some neurological disorders
Evidence that gap junctions play a role in migration and aggregation
Axon Growth and Synapse Formation
Axon Growth
Once neurons have migrated to their appropriate positions and aggregated into
neural structures axons and dendrites begin to grow from them
At each growing tip of an axon or dendrite is an amoebalike structure called
growth cone, which extends and retracts fingerlike cytoplasmic extensions
called filopodia, as if searching for the correct route
Chemoaffinity hypothesis of axonal development hypothesized that each
postsynaptic surface in the nervous system releases a specific chemical label
and that each growing axon is attracted by the label to its postsynaptic target
during both neural development and regeneration
The chemoaffinity hypothesis fails to account for the discovery that some
growing axons follow the same circuitous route to reach their target in every
member of a species, rather than growing directly to it
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