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development
change in a specific property over time
developmental trajectory
refers to the normal rate of change in a group
prenatal neurodevelopment steps
induction of neural plate
neuronal proliferation
neuronal migration + aggregation
axonal growth + synapse formation
neuronal death + synapse elimination
the beginning
begins when the sperm fertilizes the egg, making a zygote
blastocyst implants around 7-10 days, continues to develop
neural plate
~18 days after conception, embryo has 3 layers
neural plate is on the ectoderm
formation of the neural plate is induced by chemicals from the mesoderm
neural plate will become the nervous system
embryo layers
ectoderm (outside)
mesoderm (middle)
endoderm (inside)
in the neural plate…
cells are stem cells
important properties
nearly unlimited capacity for self-renewal
pluripotent (can develop into many cell types)
division produces a new stem cell + another cell
stem cells can give rise to other cells at a very high rate
over time, the neural plate…
folds to form neural groove
sides of neural groove fuse to form neural tube (~24 days)
tube center will become the ventricular system + spinal canal
growths on the anterior of the tube (~40 days) later become midbrain, hindbrain + forebrain
neuronal proliferation
progenitor cells divide thickness of tube increases with more cells
most division occurs in the ventricular zone (tube interior)
proliferation affected by chemical signals from the dorsal surface (roof plate) + ventral surface (floor plate) of tube
some cells along the ventricular zone may retain this capacity
migration
movement of cells to their target locations
inside-out process (outside layers migrate last)
at this stage, cells lack processes (no dendrite/axons)
migration may be tangential or radial
mechanisms of each may be distinct
radial
migration of the cells outward
tangential
migration of the cells across
migration and cortical layers
deeper layers are migrated to first
aggregation
neurons align with other neurons in the same area
cell adhesion molecules (CAMs) vital here
CAMs are present on the surface of cells
CAMs recognize other cells and adhere to them
Gap junctions prevalent during this period
axonal growth basics
axons grow outward to their target
very precise process
at the end of each axon is a growth cone
each cone has filopodia
‘search’, extend + retract
growth cone
as you move forward, your growing this tract outwards
how does the axon “know where to go”?
chemical sensitivity
Sperry’s Exp - Healthy Frog
well developed visual processing pathway
sever the optic nerve and it will regrow
when an insect is dangled in front of a normal frog, the frog strikes at it accurately with its tongue
Sperry’s Exp - with eye rotation
rotate the eye and the frog misdirects its tongue
mismatch in the projection
when the eye is rotated 180 degrees, the frog misdirects its strikes by 180 degrees
Sperry’s Exp - with cut nerve
unalike regions are close together
when the optic nerve is cut and the eye is rotated by 180 degrees, at first the frog is blind
but once the optic nerve has regenerated the frog misdirects its strikes by 180 degrees
this is because the axons of the optic nerve, although rotated, grow back to their original synaptic sites
axonal growth pathway
small group of pioneer axons moves first
growth cones responds to various chemical signals
attractants + repellants
released by neurons + other cells in the matrix
other axons will follow the pioneer axons later, forming axonal bundles (eg. tracts)
eg. thick snow on the ground, first person to walk through the snow will have a horrible time but they will leave tracks for other people to follow
fasciculation
bundles of tracts
chemoaffinity hypothesis
the axon (pre-synaptic) is guided toward its target cell (post-synaptic) because that cell releases special chemicals
cell A releases chemical X
axon B is sensitive to chemical X, but axon C is not
axon B grows toward cell A, but axon C does not
however evidence suggests that this signaling is not simply point-to-point (eg. A to B) but more complex
chemoaffinity in reality
the retina or tectum were lesioned
if an area loses its normal axonal input, it will recieve input from other axons instead (1)
if axons have ‘lost’ their normal target, they will project to another target instead (2)
cells can receive new projections that they otherwise wouldnt in specific circumstances
chemoaffinity theory is proven false
topographic gradient hypothesis
variation of chemoaffinity theory
axons are sensitive to the same factors but in different amounts
exposure to factors is determined by the relative position of the axons in the tissue
Cell A releases Chemical X
Axon B and Axon C are both sensitive to Chemical X
however, Axon B is more exposed to Chemical X
Axon B grows toward Cell A but Axon C does not
it is not just the chemical that matters but the concentration of that chemical
high concentrations of one chemical may result in growth that are very different than those patterns that would be observed in low concentration
synaptogenesis
synapse formation occurs after axonal growth
astrocytes (and other glial cells) are important here
important for synaptic development
associated with more synapses
form more synapses than necessary
many synapses created are later removed
this is a method of the axon communicating chemically with a compartment of another cell
select for the best and eliminate the rest
many synapses we make are not useful and need to be eliminated
at the neuromuscular junction
an axon may “lose” at some synapses, but “win” at others
the inputs of a mature neuron are fewer but more elaborate and more effective
keep the strongest
what does synaptogenesis mean?
synapse formation
what if synapses aren’t formed?
when two cells are connected via a synapse, they exchange chemical signals
this form of signaling is vital to cell survival
cells that do not form synapses will often die
when a cell forms connections with another, the cell receives survival signal from the partner
Cell A connects with Cell B, Cell B releases important factors keeping Cell A alive
what are the survival signals?
neurotrophins are transmitted via retrograde signaling (from Cell B —> Cell A)
there is a limited amount of NTs released, which leads to a competition among terminals (NT hypothesis)
you keep about 50% of the cells you make
very competitive process
apoptosis
form of programmed cell death
cleaner process, wherein the cell’s contents are packaged for convenient disposal
less inflammation
preferred, safer process
necrosis
form of cell death (eg. via nutritional insufficiency)
cells ‘break apart’ + spill their contents (leave behind trash)
more risk for inflammation
only used in extreme situations
very problematic
microglia play an important role in mitigating inflammation and ‘cleaning up the mess’
cell death
is normal and a good thing
you generate about 50% more neurons than needed
many neurons are lost during development
the neurons that survive, you keep for a long time
in most of the CNS, new neurons are not generated
in the mouse brain
similar order of events as the human brain, myelinate an axon after its found its home not before
from birth to adulthood…
volume of brain quadruples (x4)
growth not due to a gain in neurons (in fact many neurons are lost), but other processes
synaptogenesis (more synapses)
dendritic arborization (growth of dendrites)
myelination of axons
some brain areas develop faster than others
primary sensory cortices develop early (associated with vital survival functions for infants
prefrontal cortex develops last
synaptic density with age
the density of synapses declines after the first year of life
synapses develop rapidly then pruning happens later on
consequences of pruning
the prefrontal cortex is involved in planning, initiating and inhibition of behavior (and thereby impulse control)
these functions are most developed at age 25 (and often poorly developed beforehand)
if we were to take synaptic pruning as an indicator of maturity, the 25 years old thing is true (cognitive peak)
development of the prefrontal cortex with time + experience explain the striking behavior differences between adolescents and adults
develop maturity over time w/experience is the evolutionary argument
alterations in PFC developmental trajectory may delay or impair executive function
people with a delay in PFC will have poor impulse control
synapse elimination
glial cells play an important role in synapse formation, elimination and maintenance
increasingly, we are considering the role that glial cells might play in disorders of the nervous system
disorders can be from too much cortex
synaptic properties are…
modifiable with experiences!
get rid of the synapses that are not needed
it is not quantity of the synapses, but the quality
what they do and how strong they are
developmental periods
critical and sensitive period
thought to be periods of high neuroplasticity
we can identify potential developmental periods with deprivation and enrichment studies in animals
periods in humans suggested by correlational data
periods of time where our nervous system has a lot of change or adjustment
critical period
time interval where an experience MUST occur for proper development
sensitive period
time interval where an experience has a relatively greater effect on development
critical period example
in animals, early visual deprivation (eg via blindfolding when young) disrupts the development of visual pathways
effects of early visual deprivation cannot be reversed by later experiences (even if you remove the blindfold)
will never get normal visual system
later visual deprivation is much less consequential (as it is outside the critical period)
why do critical periods end?
many theories; several are focused on axons
myelination of axons occur after critical periods close
myelination of existing neurons creates a physical barrier to growth and sprouting of other axons
myelination can also release certain factors which inhibit axonal growth, such as Nogo
a developing nervous system is easier to change than a mature one
need the input during the critical because that is when it is easy
sensitive periods for language
input is more meaningful for development
language is much easier to learn when you are young vs trying to learn a language when you are an adult
language acquisition is easiest between 3-7 years old
much harder to acquire after 18 years
why is learning a language when you are older harder?
difficult to test
motivation for second language learning is different than first
context in which second language is learned varies
for kids its to communicate
for adults its mainly for interest or a job
language acquisition may involve different mechanisms in different ages
when you are younger you are immersed in that language and there is no backup language
when you are older you spend an hr a week but you have a native language to fall back on if you are struggling
adult neurogenesis
means generation of new cells in adulthood
for the most part, the CNS has limited regenerative capacity
neurons, once lost, are lost forever (and were losing them all the time)
you can only really make new neurons in large amounts during development - we are thus continuously running out of cells
there may be exceptions
where does adult neurogenesis occur?
either in hippocampus or lateral ventricles
not a lot of new cells
humans have had a handful of cases
requires post mortem tissue
takes about 48 weeks to develop a new cell
the neurogenesis debate
neurogenesis occurs in most mammalian species studied - but it is currently unclear whether the extent is significant enough to be meaningful in adult humans
assuming it does occur in humans, why does it matter?
learning and mood regulation
not super meaningful if youre just making a handful of cells
why does neurogenesis matter?
when young, new adultborn neurons have enhanced excitability and plasticity relative to older developmentally-generated cells
enhanced hippocampal neurogenesis is correlated with improved memory and reduced anxiety
young neurons may play a role in stress resiliency, allowing for greater resistance to stress-induced depression
Neurodevelopmental Disorders
NDDs
disorders wherein there is abnormal development of the nervous system, leading to abnormal cognition and behavior
NDDs often emerge early in life (eg autism, adhd, intellectual disabilities and language disabilities)
high heritability, strong role of genetic factors
NDDs are considered distinct from aquired disorders, which usually emerge in adult hood and are the result of brain changes (eg injuries) in adult hood
abnormal development —> abnormal cognition and behavior
result of processes that have been ongoing for a long time
comorbidity
two or more conditions at the same time
people with one NDD are at much higher risk for having another
odds of autism and ADHD (~20/10,000) are much higher than expected by chance
comorbidity may be due to similarities in genetic factors or environmental factors
eg gene X contributes to ASD and ADHD
if you have the variation of it, you could get both disorders
schizophrenia
SZ
cluster of symptoms both
positive (add something) and
most commonly hallucinations
negative (absence or reduction of something that is usually present)
reduction in speech or motivation
flat emotional face
it is unlikely to have every single symptom, but most of the time you have a few positive and a few negative
neural features of schizophrenia
cortical atrophy (temporal cortex, HPC and PFC)
less gray matter
abnormal cell organization (HPC)
hypofrontality
reduced ability frontal lobe to process info
less active during tasks
tends to appear later in life
males is earlier, females is later
soft signs
impairments that predict later emergence of a disorder
not remarkable or strong
alterations in DA transmission
major risk factor of schizophrenia
prenatal + postnatal risk factors; some are “choices” (eg drugs) whereas others are “random accidents” (eg illness)
major factor is cannabis
family history has the greatest correlation
born in the winter
maternal depression
cannabis during development
correlation
heavy cannabis use during adolescence is a concern as it may impede brain development during a vital sensitive period
cannabis use is associated with an increased risk for schizophrenia (~2x) and an earlier onset
earlier onset of cannabis use is associated with more significant impairments in cognitive functioning
adolescent cannabis + the brain
white matter integrity reduced + gray matter reduced (in HPC and OFC just like schizophrenia)
DA hypothesis of schizophrenia
higher levels of DA metabolites (HVA)
more D2 receptors
positive symptoms are similar to the effects of drugs that increase DA signalling
positive symptoms reduced by drugs that block DA signalling
schizophrenia is NOT a disorder of too much dopamine
higher DA activity in mesolimbic pathway
lower DA activity in mesocortical pathway
antipsychotic drugs
most antipsychotics block D2 receptors
conventional antipyschotics are relatively selective in this action, atypical antipsychotics block other targets
if you inhibit dopamine a side effect is developing Parkinson’s like symptoms
fix one dopamine imbalance but create another in its place
autism symptoms
poor social interaction
fails to respond to name, poor eye contact, resists cuddling, prefers playing/being alone
may not recognize/respond to social cues
repetitive behaviors
arranging objects, making sounds, hand flapping, head rolling and body rocking
inability to switch between behaviors easily
slow language development
>2 years, repeat with words/phrases, abnormal tone/rhythm
can be evident early on in childhood
autism spectrum
heterogeneous group of disorders defined by a set of symptoms, varying degrees of symptoms + cognitive ability
autism epidemiology
~1% of population
more common in boys (3:1)
increase in diagnosis is associated w/increased awareness, increased parental age + more sensitive diagnostic procedures
conspiracy theories are associated with the increase including microplastics and vaccines but no evidence has proven these to contribute with higher rates
ASD + synaptic density
synapse number is higher in childhood and remains higher throughout adulthood
greater cortical expansion in specific areas may predict ASD risk
when we see more cortex it isnt healthy, and we see this in ASD
ADHD symptoms
two main symptoms which may manifest differently
inattention
lack of attention to details or careless mistakes
does not seem to listen when spoken to directly
hyperactivity/impulsivity
excessive fidgeting
running, climbing, restlessness in appropriate situations
three forms (combined + 2 predominant forms)
ADHD
6-10% of the population
recent data suggests rates are increasing over the past few decades (this is mysterious)
conspiracy theory: big pharma, drugs are needed for ADHD so the increased diagnosis is financially motivated
neural features of ADHD
reduced total cerebral volume as well as PFC, BG, dACC + cerebellum volume
delay in cortical maturation, prominent in the PFC
lower white matter volumes
lower DA levels (reward deficiency theory)
changes in the PFC are thought to be central to the disorder and its treatment with drugs
psychostimulants for ADHD
most of these drugs work by increasing dopaminergic or noradrenergic transmission in different ways
people with ADHD normally have less dopamine, so the drugs increase the dopamine levels
amphetamine actions
dopamine and noradrenaline transporter inhibition
monoamine oxidase activity inhibition (altered metabolism)
methylphenidate actions
dopamine and noradrenaline transporter inhibition
non-stimulant use for ADHD
30% of people may not respond to stimulants
other people might be at risk for certain drug interactions with classic stimulants
non-stimulants for ADHD are also available (targets noradrenaline reuptake), guanfacine and clonidine (which target alpha2 receptors, activated by noradrenaline)
different side effects for these particular drugs
these target receptors not transporters
normalizing DA levels
when you look at the healthy nervous system where everything is present, with the addition of the same drug does not correct anything, if anything it creates an imbalance
take a brain with too little dopamine and you inhibit dopamine transport, that is a good thing
if you take a brain that already has enough dopamine and you inhibit dopamine transport that is not necessarily useful and can be harmful