chapter 8: How Does the Nervous System Develop and Adapt?

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Chapter 8: How Does the Nervous System Develop and Adapt?
Research focus 8-1: Linking Serotonin to SIDS
Sudden infant death syndrome (SIDS): Unexplained death while asleep of a seemingly healthy infant
less than 1 year old.
Evidence that SIDS is a nervous-system disorder:
1) Higher chance of Serotonin –transporter abnormality
Serotoninergic system stimulates a mechanism to respond to high CO2 lvls in blood and expel it.
SIDS victims more likely to have a gene variation that makes the serotonin (5-HT) transporter
more efficient (serotonin is cleared from synapse faster).
So, faster reuptake of serotonin makes serotonin less effective in regulating life-threatening
events (ie. CO2 buildup during sleep). Thus, babies will breathe in lethal levels of CO2
2) Lower occurrence of 5-HT1A receptors in SIDS victims’ brains
This reduced serotonergic system’s effectiveness in regulating behaviour
More likely to occur in boys than girls, a finding that is consistent with higher SIDS mortality in
boys
Genetic manipulation of 5-HT receptors in mice (Enrica Audero and Colleagues, 2008) showed:
70% of mice with higher 5-HT1A autoreceptors in brainstem died within 120 days of birth
Various abnormalities in control of temperature and heartbeat (also seen during SIDS
episodes in human babies)
Since, serotonin autoreceptors help turn off serotonergic cells, a reduction of 5-HT
transmission is seen in other parts of the brain.
3) Increase number of 5-HT cells during fetal development (caused by prenatal exposure to alcohol,
nicotine, etc) is a defect which leads to changes in 5-HT1A receptors. (Hannah Kinney, 2009)
Three Perspectives on Brain Development
1) Structure development can be studied and correlated with the emergence of behaviour
2) Behavioural development can be analyzed and predictions made about what underlying circuitry
must be emerging
3) Factors that influence both brain stricture and behavioural development (e.g. Language or injury) can
be studied.
Predicting Behaviour from Brain structure
Structural development of the nervous system can be correlated with the emergence of specific
behaviours (e.g. development of certain brain structures can be linked to emergence of motor skills
like grasping.)
As the brain matures, their functions emerge and develop, and can be observed in behaviours.
Neural structures that develop quickly show their functions sooner (e.g. visual system develops
sooner than those responsible for speech)
Because the brain continues to develop into adulthood, some abilities emerge/mature later
Eg) the ability to plan efficiently is a cognitive behaviour controlled by the frontal lobes and
develops later in life. In the lab, this is measured using the
tower of Hanoi test.
Tower of Hanoi test: A mathematical puzzle consisting of
three rods and severall different-sized disks. The task is to
match the goal in as few moves as possible. 2 rules: (1) move
only one disc at a time, (2) no disc may be placed on top of a
smaller disc.
10year olds can do simple configurations, but complex
ones (ref pic) can only be done by 15-17yrs.
Adults with frontal-lobe injuries fail this
Correlating Brain Structure and Behaviour
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Observe behaviour for the emergence of new abilities and then inferring that to underlying neural
maturation
e.g. 1) As language emerges in kids, there should be corresponding changes in neural structures that
control language. The neural structures that control speech must mature in order for speech to occur.
Extensive speech training is useless if these structures haven’t matured.
E.g. 2) As frontal-lobe structures mature from adolescence to early adulthood, we look for related
changes in behaviour. Can also be done in reverse: As new
abilities emergy, we can infer that they are controlled by late-
maturing neural structures and connections
Influences on Brain and Behaviour
Identify and study factors that influence both brain and
behavioural development
This perspective suggests the fully matured brain is not
enough, and the events that shape how a structure functions
and produces certain behaviours must be also known
Eg) events: senory experience, injuries, hormones, and
genes influence brain functions
If one of these factors influence behaviour, then structures in
the brain that are changed by that factor are responsible for
behavioural outcomes
Eg) If an observed behavioural abnormality is caused by a
certain abnormally functioning brain structure, then that
brain structure must normally play a role in controlling the
behaviour.
Neurobiology of Development
2000 years ago, Roman Philosopher Seneca proposed the
idea of preformation: a human embryo is a mini adult and
only has to grow bigger
This idea declined by Mid 1800s when people realized
embryos look nothing like the adults they become
Embryos of different species more closely resemble one
another than their respective parents (Fig. 8-2)
Top row shows similarity in the embryos between
salamanders, chickens, nad humans.
In early development, all vertebrate species have similar
looking primitive heads, a region with bumps/folds, and a
tail.
As the embryo develops, it acquireds distinctive
characteristics of its species. z
The embryonic nervous system of vertebrates are similar in
strucutre as their bodies. Fig 8-3 shows the 3-chambered
brain of a young vertebrate empryo: forebrain, midbrain and
hindbrain.
Gross Development of the Human Nervous System
Prenatal stages Zygote: fertilization to 2 weeks, Embryo: 2-8 weeks, Fetus: 9weeks – birth.
15 days after fertilization, the embryo looks like a fried egg. This structure is formed by several
sheets of cells with a raised are in the middle called the embryonic disc (essentially the primitive
body)
3 weeks (21days) after conception: neural plate (thickened region of the ectodermal layer that gives
rise to the neural tube) occupies oart of the outermost layer of the emhryonic cells
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The plate folds inwards, forming the neural groove (fig 8-5).
Neural groove curls to form neural tube (structure in the early stages of brain development from
which the brain and spinal cord develop)  similar to paper being curled into a cylinder.
The open region the n centre of the tube remains open and matures into the brain’s ventricles
and the spinal canal (fig. 8-6)
Human body and nervous system development
7 weeks (49 days): embryo begins to resemble a mini human
100days: brain looks distinctly human
7months: gyri and sulci begin to form
9months: fetal brain looks like adult human brain, but cellular structure is different
Sexual differentiation
7 weeks: Genitals begin to form but no sexual dimorphism, or structural difference between the
two sexes yet.
60 days: Male and female genitalia become distinguishable
Sexual differentiation is stimulated by the presences of the sex hormone testosterone in male
embryos and its absence in female embryos
Testosterone: sex hormone secreted by the testes and responsible for the distinguishing
characteristics of the male
Testosterone changes the genetic activity of certain cells (e.g. the ones that form genitals),
but neural cells also respond to it. So certain regions of the embryonic brain may also show
sexual dimorphism 60 days after conception
Prenatal exposure to gonadal (sex) hormones act to shape male and female brains differently
because these hormones activate different genes in the neurons of the two sexes.
Experience affects M/F brains differently; so genes and experience begin to shape the brain
early in life.
Origins of Neurons and Glia
Cells in the brain start as multipotential stem cells, develop into precursor cells, then produce blasts
that finally develop into specialized neurons or glia
Neural stem cells: Self- renewing, multipotential cell that gives rise to any of the different types of
neurons and glia in the nervous system.
Lines the subventricular zone (lining of the neural stem cells surrounding the ventricles in
adults) and are also located in spinal cord and retina.
Becomes progenitor cells.
It’s Self-renewing because continuously divides Throughout one’s lifetime, a stem cell divides
into two stem cells; one dies and the other divides
It’s Multipotential because they give rise to all specialized cell types in the CNS
Progenitor cells (precursor cells): Precursor cell derived from a stem cell; it migrates and
produces a neuron (neuroblast) or glial cell (glioblast), which are nondividing cells
Neuroblasts: Product of a progenitor cell that gives rise to any of the different types of neurons
Glioblasts: Product of a progenitor cell that gives rise to different types of glial cells
Sam Weis and colleagues (1996) found stem cells can produce neurons and glia cells in adulthood
and aging brains. Implies that dead neurons should be replaceable.
By making use of hormonal signals in the brain, scientists assume they can make stem cells carry
out this replacement process. Eg) Neuropeptide prolactin levels increase in pregnant mice, signalling
the brain to make more neurons.
Stem cells differentiate into certain cells via gene expression: occurs when a formerly dormant gene
becomes activated by a chemical signal, and starts producing a specific protein that makes certain
cells. Eg) certain proteins will make skin cells, while others make neurons.
The different chemical environments needed for cell differentiation to occur is caused by the
activity of neighbouring cells, or by chemicals (e.g. hormones) transported in the bloodstream.
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