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DEVELOPMENT OF THE NERVOUS SYSTEM:Stages of development, Neurulation

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Neurological Basis of Behavior (PSY - 610)
VU
Lesson19
DEVELOPMENT OF THE NERVOUS SYSTEM
Objective
To familiarize the students with:
·  Various stages of neuronal development.
·  Development of the brain: from the fertilization to the various developmental stages in-utero,
and postnatally.
·  Cell differtiation, determination migration, (inside-out), cell competition, Cell death, growth
Cones, Nerve growth Factor and its role, Influences in growth and development of the brain
Once induction of the ectodermal layer by the mesodermal tissue has taken place, the cells start
differentiating as their direction of growth is determined by the induction. The three cell layers would
develop in different direction after the signal has taken place. The initial division of cells is
differentiated into different organs (heart, kidney, brain, stomach) this change has been induced by the
notochord which ahs thus determined the fates of cells/layers
Stages of development
There are several distinct and measurable stages of development of the brain which takes place.
·
Attachment of mesodermal tissue to ectodermal tissue leads to the formation of the notochord.
The notochord is the strip at the center of the upper surface of the ectodermal layer.
·
The neural plate is formed from this tissue and this line in the center grows as the plate grows.
A bulb like head end forms at one end to eventually form the forebrain and eye field/eye cups
area around the 17th day of intrauterine life. If the bulbous end is cut, it would quickly replace
itself. The cells are still rapidly dividing
·
The neural plate starts becoming longer and broader-growth in both directions (as the cells are
still dividing and multiplying at an extremely rapid rate)
On the18th day there is thickening of the outside ends. This leads to rising of ends and
·
deepening of the center. The sides rise joining in the middle. Thus, the rising of the plates form
a groove, this is called the neural groove. The neural groove deepens as the sides rise higher
and higher (remember, there is rapid cell growth and multiplication taking place).
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Neurological Basis of Behavior (PSY - 610)
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Neurulation
·
Neural tube (rapid cell multiplication and division), the tube folds and becomes tubular this is
when the primary neurulation takes place. The brain and spinal cord are formed at this stage.
On the 21st day, the fusion of the tube takes place and little groups of cells break away to form
bunch on both sides. These are called somites and extend in both directions (somites: bodies of
cells in the middle/top) to form the neural crest. These develop into the peripheral Nervous
system and the ganglionic system. The inside of this tube, the neural canal, is empty at this
stage.
·
The neural canal remains empty as the ventricular zone, where the next phase of development
will take place. Secondary neurulation: During the process of secondary neurulation (forming
of the neural systems) the tail end part of spinal cord is formed
The closure of the tube is simultaneous and inside cells starts dividing and multiplying, growing within
the tube. The cells inside increase in number rapidly, as cell division takes place in the ventricular zone.
Internal part of the tube consists of single layers of cells which keep multiplying and increasing in
number and as they increase in number the cells start moving out (The interesting question is who tells
them where to go, and where do they go? Does the region get crowded??). This is the phase when Cell
Migration takes place
Cell migration: it is an inside-out process, cells move form the inside of the tube towards outside. The
growing cells then form three layers: internal, middle, cell free:
1. The Internal layer: The inner most layer has cells which are still dividing
2. The Middle layer: Consists of cell bodies which eventually form grey matter. These cells do
not divide after this, these are the same cells found in the adult brain.
3. Cell free zone: This contains the fibre processes of the first two layers and as yet empty of
cells. The migration is still taking place. Once cells reach their destination they grow dendrites
and axons to reach out to meet other cells. Cells sprout growth cones for the axons and
dendrites, which lead the cells to grow to and to develop synapse.
Neural crests are forming ganglia, inputs into spatial column and outwards, Optic stalk grows out of
diencephalon to form eye cup- eye cups induce formation of lens from overlapping ectodermal
tissue. The frontal the eyecups: retina forms as an invagination of the bulging optic vesicle:
ganglionic cells first, and then bipolars, and photorecpetors last: eye born directly from brain tissue!
Common substance in both
Formation of the brain:
Forebrain, mid brain, hind brain: On the 24th day intrauterine, the head end part of neural tube
forms three bulbs. These three bulbs would form the forebrain, midbrain, hind brain (The question
is how do the cells know and the areas know that they are going to be the in front or middle or
back?) The front most becomes the forebrain, and the end toward the tail becomes the hind brain.
The hind brain connects with the spinal cord. At this point all three bulbs are not differentiated.
These are exactly the same. (How do they become different?). In the same manner as in the first
stages, each area depends on the other to be stimulated and through this interaction they
differentiate. At the stage also the surrounding area induces differentiation. The environmental
influences are important at every stage.
What if the three bulbs cut and rotated, always the bulb in the front forms the forebrain and the
middle, the mid brain and the last part forms the hind brain. The position determines direction of
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Neurological Basis of Behavior (PSY - 610)
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development! The same cells develop into different regions if their locations are changed
(environmental change!). At this stage cell division is rapid, and the neurons and glial cells are
forming. The migrating cells unite to form groups of neurons. Nuclei are forming as a result of rapid
proliferation of nerve fibre tracts and connections.
·  Cell division is rapid:
·  Migrating cells unite to form groups of nuclei\nuclei form connections
·  Rapid proliferation of fibre tracts and connections
·  Formation of the ganglionic systems from the neural crests, inputs into the spinal column and
outwards
·  Optic stalk grows out of the diencephalons to form the eye cup ( the eye is formed from the
same tissue as the brain, eye has similarities with the brain)
·  Eye cups induce formation of the lens from overlapping ectodermal tissue
Concepts
Regulation: If cells keep growing, connections expanding then how does it stop- who controls the
development, differentiation, and migration etc.
Self regulating: This is a process regulated by itself. A) Muscles move without the sensory input or
stimulation b) nuclei develop even if isolated from the organs, if we denervate (cut the nerves).
Cells proliferate at more than 40 times the normal adult brain. What happen that cells size reduces?
Cell proliferation (increases in number), cell migration (cells travel to their destinations form inside
toward the outside), maturation (developing extensions) interconnections formed (forming connections
with other cells); cell death (cells die off) is taking place.
Cell proliferation: Cells are dividing, spreading and increasing in number. Specific parts of the brain
begin to differentiate. Small piece of ectodermal tissue removed defect replaced by proliferation of
neighboring cells, however if surgery is done later, then it would remain as a permanent deficit. The cell
growth is in extreme density in ventricular zone. Cell growth much more than required about 1 ½ times
more than adult brain then cell death takes place and has to follow some principles.
Maximal cell division is taking place at this stage and neurons are being formed at 20.000 neurons per
minute.
Growth spurt: This is time when maximal cells are being formed, connections being formed and
systems of brain areas organized. This is between the 10-18th week of gestation. This is the time when
the brain cells of the growing embryo are sensitive to radiation, chromosomal anomalies, viral infections
(measles etc.). The fetus is born with defects such as mental retardation and blindness. The sensitivity of
the fetus and the newborn to other effects are from the 30th week- 2nd year post partum. The effects of
malnutrition on cell size, brain cell connections and myelination are irreversible.
Cell Migration: Cells migrate towards periphery from the inner core of the ventricular zone. The
principle of inside ­out migration is followed here. There is formation of radial glial fibres on which
cells travel from the periventricular zone. These form the transport system on which neurons travel!
These cells move up to the different cortical regions which would eventually form the 6 layers of the
cortex, some remain.
Radial Glial: cells attach on both sides, neurons move up, some slow, some fast... those which are fast
arrive earlier and form connections with other neurons. Once they form connections they survive. The
sooner the connections are formed, the greater chances of surviving.
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Cellular maturation.
This has four stages
a) The development of outgrowth and elongation of axons
b) Dendritic process emerging out of the cell body
c) Biochemical properties appropriate to the location and function of the location to which the cell
would stay
d) Development of synaptic connection.
The axons grow out of neurons first and these have a growth cone (a specialized structure with filopedia
cytoplasmic extensions--feelers needed for movement which lie on the growing processes). This is
affected by the Growth Factors, the active factors being the Nerve Growth Factor (NGF) in the growing
nervous system and factors that maintain metabolism of neurons (tropic factors). This is planned for a
specific site and target.
Dendrites develop after neurons/axons. Immediately upon reaching their destination neurons attach to
the site after detaching from the transport radial glial, and send out projections. If the neurons cannot
travel, they cannot compete, they would not survive. If they cannot from connections, they cannot
compete, they would die. In order to survive, cells sprout more extensions and form more connections.
This increases the cell's ability to compete for connections. If the connections not formed, cells will not
be able to survive. Only those cells survive which have successfully formed connections and would now
be able to continue receiving the NGF
Cell Aggregation: Cells make their way to the area in which they will function as adults using cell
adhesion molecules (CAMs). These cell adhesion molecules are formed on the surface of neurons and
other cells. These also give the cell ability to recognize the molecules and surfaces. Cells form
alignment in precision with other cells in the region. The question is how it is done. There is an intricate
programming which is still under study.
Fate Mapping: researches such as Pasko Rakic sued the fate mapping procedures. This involves
injecting labelled substance in the growing brain at various embryonic ages and following the migration
of the neurons later to see where they end up. These studies have shown that a) Regional specialization
of areas and neurons appears early on in development, b) The deeper cortical layers generate first and
most superficial layers are formed last c) Inside-Out migration: The migration is at a fast pace, the cells
migrate to the inner layers first. Neurons of the outer layers formed later must migrate through the
earlier formed layers to eventually arrive at their destination, d) In the development and growth
principles i) large cells develop before small ii) motor cells develop before sensory neurons and iii)
neurons develop before the glial cells.
Thus, we have seen an intricate relationship of genetic and biological programming with environmental
stimulation in the development of the brain. Even though the paths of development are well laid out,
stimulation is important for neurons to continue moving, developing and surviving.
References
1. Kalat J.W (1998) Biological Psychology Brooks/ Cole Publishing
2. Carlson N.R. (2005) Foundations of Physiological Psychology Allyn and Bacon, Boston
3. Pinel, John P.J. (2003) Biopsychology (5th edition) Allyn and Bacon Singapore
4 Bloom F, Nelson and Lazerson (2001), Behavioral Neuroscience: Brain, Mind and Behaviors (3rd
edition) Worth Publishers New York
5. Bridgeman, B (1988) The Biology of Behaviour and Mind. John Wiley and Sons New York
6. Brown,T.S. And Wallace. (1980) P.M Physiological Psychology
Academic Press New York
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Table of Contents:
  1. INTRODUCTION:Descriptive, Experimental and/ or Natural Studies
  2. BRIEF HISTORICAL REVIEW:Roots of Behavioural Neurosciences
  3. SUB-SPECIALIZATIONS WITHIN THE BEHAVIORAL NEUROSCIENCES
  4. RESEARCH IN BEHAVIOURAL NEUROSCIENCES:Animal Subjects, Experimental Method
  5. EVOLUTIONARY AND GENETIC BASIS OF BEHAVIOUR:Species specific
  6. EVOLUTIONARY AND GENETIC BASIS OF BEHAVIOUR:Decent With Modification
  7. EVOLUTIONARY AND GENETIC BASIS OF BEHAVIOUR:Stereoscopic vision
  8. GENES AND EXPERIENCE:Fixed Pattern, Proteins, Genotype, Phenotypic
  9. GENES AND EXPERIENCE:Mendelian Genetics, DNA, Sex Influenced Traits
  10. GENES AND EXPERIENCE:Genetic Basis of behavior, In breeding
  11. GENES AND EXPERIENCE:Hybrid vigor, Chromosomal Abnormalities
  12. GENES AND EXPERIENCE:Behavioral Characteristics, Alcoholism
  13. RESEARCH METHODS AND TECHNIQUES OF ASSESSMENT OF BRAIN FUNCTION
  14. RESEARCH METHODS AND TECHNIQUES OF ASSESSMENT OF BRAIN FUNCTION:Activating brain
  15. RESEARCH METHODS AND TECHNIQUES OF ASSESSMENT OF BRAIN FUNCTION:Macro electrodes
  16. RESEARCH METHODS AND TECHNIQUES OF ASSESSMENT OF BRAIN FUNCTION:Water Mazes.
  17. DEVELOPMENT OF THE NERVOUS SYSTEM:Operation Head Start
  18. DEVELOPMENT OF THE NERVOUS SYSTEM:Teratology studies, Aristotle
  19. DEVELOPMENT OF THE NERVOUS SYSTEM:Stages of development, Neurulation
  20. DEVELOPMENT OF THE NERVOUS SYSTEM:Cell competition, Synaptic Rearrangement
  21. DEVELOPMENT OF THE NERVOUS SYSTEM:The issues still remain
  22. DEVELOPMENT OF THE NERVOUS SYSTEM:Post natal
  23. DEVELOPMENT OF THE NERVOUS SYSTEM:Oxygen level
  24. Basic Neuroanatomy:Brain and spinal cord, Glial cells, Oligodendrocytes
  25. Basic Neuroanatomy:Neuron Structure, Cell Soma, Cytoplasm, Nucleolus
  26. Basic Neuroanatomy:Control of molecules, Electrical charges, Proximal-distal
  27. Basic Neuroanatomy:Telencephalon, Mesencephalon. Myelencephalon
  28. Basic Neuroanatomy:Tegmentum, Substantia Nigra, MID BRAIN areas
  29. Basic Neuroanatomy:Diencephalon, Hypothalmus, Telencephalon, Frontal Lobe
  30. Basic Neurochemistry:Neurochemicals, Neuromodulator, Synaptic cleft
  31. Basic Neurochemistry:Changes in ionic gates, The direct method, Methods of Locating NT
  32. Basic Neurochemistry:Major Neurotransmitters, Mesolimbic, Metabolic degradation
  33. Basic Neurochemistry:Norepinephrine/ Noradrenaline, NA synthesis, Noadrenergic Pathways
  34. Basic Neurochemistry:NA and Feeding, NE and self stimulation: ICS
  35. Basic Neurochemistry:5HT and Behaviors, Serotonin and sleep, Other behaviours
  36. Basic Neurochemistry:ACH and Behaviors, Arousal, Drinking, Sham rage and attack
  37. Brain and Motivational States:Homeostasis, Temperature Regulation, Ectotherms
  38. Brain and Motivational States:Biological Rhythms, Circadian rhythms, Hunger/Feeding
  39. Brain and Motivational States:Gastric factors, Lipostatic theory, Neural Control of feeding
  40. Brain and Motivational States:Resting metabolic state, Individual differences
  41. Brain and Motivational States:Sleep and Dreams, Characteristics of sleep
  42. Higher Order Brain functions:Brain correlates, Language, Speech Comprehension
  43. Higher Order Brain functions:Aphasia and Dyslexia, Aphasias related to speech
  44. Higher Order Brain Functions:Principle of Mass Action, Long-term memory
  45. Higher Order Brain Functions:Brain correlates, Handedness, Frontal lobe