How does the nervous system develop? Flashcards
what are the 3 approaches to development of brain and behaviour?
- emerging brain structure correlates with emerging behaviour
- Emerging behaviour correlates with changes in brain structure
- factors influence both behaviour and brain development
What is an example of emerging brain structure correlating with behaviour?
myelination of motor cortex + increased complexity of grasping in infants
what is an example of emerging behaviours correlating with changes in brain structure?
increased complexity of language + cortical thickening of language areas
What is an example of factors that influence both brain and behaviour development?
Exposure to alcohol prenatally and impaired brain and behaviour development
What are the 3 stages of prenatal development?
Germinal stage (conception-3wks)
Embryonic stage (3wks-9wks)
Foetal stage (9wks-birth)
What happens from conception-3wks?
0-2wks - zygote travels down fallopian tube to uterus
cells multiply rapidly and differentiate as they multiply
Ectoderm thickens and becomes the neutral plate by day 8 of gestation. The sphere of dividing cells flattens
The neural plate folds to form the neural groove, which then closes over to form and a fluid-filled cylinder - the neural tube
What is a neural tube defect?
Anencephaly - neural tube fails to completely close over
underdeveloped skull and brain - life-limiting condition
5-10000 births
what happens from wk4-birth?
Embryo: up to 10wks
Foetus: wk10-birth
By week 4 -main divisions of CNS are formed:
- forebrain
- midbrain
- hindbrain
- spinal cord
By mid pregnancy, cerebral hemispheres have expanded to cover the rest of the brain
By 6 months, cell proliferation —> infolding —> sulci and gyri on surface of cortex. Lobes can be differentiated
What are the 7 stages of nervous system development?
- cell birth - neurogenesis
- cell migration
- cell differentiation
- cell maturation
- synaptogenesis - formation of synapses
- cell death and synaptic pruning
- myelination
what happens during cell birth(neurogenesis)?
non-neuronal cells divide by mitosis to produce neurons
Single layer of cells along inner surface of neural tube –> ventricular zone (all neurons and glial cells come from there)
What happens during cell migration?
Newly formed cells move from the ventricular zone to the correct destination in the nervous system
Travel along radial glial cells which act as guides
Cells move to establish distinct nerve cell populations (e.g., layers of cerebral cortex)
What happens during cell differentiation?
Cells reach their destinations and start to express particular genes
Cell-cell interaction coordinates development – ensures right type of neuron for that part of the brain
Cells acquire distinctive characteristics – e.g., neurons – as the result of expressing particular genes
What happens during cell maturation?
dendrite and axon growth
What happens during synaptogenesis?
Formation of synapses - neurons extend their axons and dendrites and form many synapses with one another.
continues after birth
What happens during cell death and synaptic pruning?
‘use it or lose it’: synaptic connections that do not become part of a functional neural network are lost
synaptic pruning/rearrangement and maturity proceed at different rates in different brain areas
Cell death aka apoptosis – ‘surplus’ cells die
What happens during myelination?
- axons become wrapped in fatty sheath
speeds the conduction of electrical signals
Some prenatally; rapid shortly after birth; continues into adulthood
What factors influence brain and behaviour development?
Intrinsic and extrinsic factors
What are intrinsic factors?
originating within the developing organism, e.g. genes
What is an example of intrinsic factors?
Down syndrome
extra chromosome 21 (trisomy 21)
physical properties (e.g. facial features)
developmental delays and intellectual disability
behavioral dysfunction varies
abnormal dendritic spines in the cerebral cortex
What are extrinsic factors?
outside the developing organism, e.g. exposure to alcohol before birth; environment
what is an example of extrinsic factors - eg. changes in visual cortex depending on experience?
Binocular/monocular deprivation experiments
Sensitive period during which visual experience crucial for proper development of vision
Cats
1st 4 months – synaptic development in visual cortex
humans
- cataracts - blurry vision until 6 months (then corrected)
- impaired face perception at 9 years
- ‘lazy eye’ (amblyopia)
- impaired depth perception, unless corrected in childhood
what is an example of extrinsic factors - eg. enriched environments: rats?
simple lab cages vs complex environment
Increased dendritic branching with complex environment
ALSO
- More hippocampal neurons
- Thicker cerebral cortex
how do extrinsic factors eg. socio-economic status and brain development in humans affect brain development?
Family income is positively related to cortical surface area, particularly in:
Temporal, frontal and prefrontal regions
Relationships most prominent in regions supporting language, reading, executive functions and spatial skills
lower income: small differences in income associated with relatively large differences in surface area
what is an example of extrinsic factors - eg. injury and recovery?
Timing of the injury is key
-
Rats:
- at birth - sparser dendrites in adults
- At 10 days - expanded dendrites as adults
-
Humans:
- in the womb or soon after birth - most dramatic consequences
- first two years —> may produce no behavioral consequences (e.g. language)
- Adulthood —> severe behavioral consequences (e.g., language)
- Brain has capacity to compensate for injury & recover functions, depending on timing
what is an example of extrinsic factors - eg. fetal alcohol syndrome?
prenatal exposure to alcohol
distinct facial features
changes in neuroanatomy
most common non-genetic cause of learning difficulties
severity varies:
- binge-drinking particularly harmful
- first trimester
what is an example of extrinsic factors - eg. Zika virus?
transmitted through mosquito bites
mild symptoms - fever, rash, headache etc
Infection during pregnancy = microencephaly
- Abnormal brain development and loss of brain tissue
how do intrinsic and extrinsic factors interact?
extrinsic factors can influence the expression of specific genes: epigenetic influences
- E.g., genetically identical mouse embryos implanted into 2 different foster mothers, behaviour is affected – effects of prenatal environments and postnatal experiences on how genes are expressed
How are age demographics changing?
- 1900: 4% 65+
- 2030: 20% 65+
Who do dementias affect?
- 1-6% over 65s
- 10-20% over 80s
Projections over the next 35 years estimate that 10-20 million elderly people in the United States will have mild to severe cognitive impairments
what are 3 examples of typical changes as we age?
Age-related decline in:
perception
motor function
memory and executive function
why do typical changes occur as we age?
myelin loss
reduction in size of specific brain areas (e.g. hippocampus)
what are atypical changes as we age (disorders)?
Dementia: acquired & persistent intellectual impairment
- Memory & cognitive deficits
- Impairment in social & occupational functioning
two types of dementias:
1. non-degenerative
- Mostly caused by problems in cardiovascular system (eg. vascular dementia)
2. Degenerative
- Primary cause in nervous system (eg. Alzheimers, Parkinsons)
- Thought to have a degree of genetic transmission
what is Alzheimers disease?
65% of all dementias
begins as a loss of memory for recent events
progresses to being unable to answer basic Qs
atrophy of the cerebral cortex, particularly temporal and frontal areas
cellular changes in the hippocampus which impair synaptic function:
- increase of senile/amyloid plaques (protein (beta-amyloid) from dead neurons)
- increase in neurofibrillary tangles (protein Tau) – abnormal filaments in nerve cells
what is a genetic risk factor of alzheimers?
Apoe4 gene
what are some examples of protective factors of Alzheimers?
physical and mental activity; adequate sleep; higher education; bilingualism(?)
what is Parkinson’s disease and what are some key symptoms in motor behaviour?
loss of cells in substantia nigra (basal ganglia), and consequent loss of the neurotransmitter dopamine
key symptoms in motor behavior:
- tremor at rest
- loss of spontaneous movement
- rigidity
- posture disturbances
- cognitive slowing
What is an environmental risk factor of Parkinson’s?
environmental pollutants
How is Parkinson’s treated?
physical therapy; L-dopa (dopamine precursor); deep brain stimulation
What is neuroplasticity?
The potential of the nervous system to change across the life-span as an adaptation to the environment
Explain neuroplasticity in romanian orphans
- Barren environments; lack of caregiver interaction
Malnourished; cognitive development impaired
Smaller brains; reduced brain activityBUT, foster care —> rapid developmental gains, IF adopted early
Brain can recover from brief (< 6 months) extreme deprivation
