Lecture 8 Flashcards
Neurobiology of development (2)
- Phylogenetic development
- Ontogenetic development
- B&B focuses on ontogenetic
Phylogenetic development
Evolutionary development of species from lower to higher species
Ontogenetic development
Development of an individual from the time of fertilisation to the organism’s mature form
Vertebrates
Animals characterised by brain and spinal cord
Embryos
Embryos of different species more closely resemble one another than their respective parents.
Embryos are not simply miniature versions of adults.
Gross development of the human nervous system (4 steps)
- Zygote: conception of 2 weeks
- Embryo: 2-8 weeks
- Foetus: 9 weeks to birth
- Baby: birth
Origin of brain cells (4)
- Neural stem cells
- Progenitor/precursor cells
- Neuroblasts or glioblasts
- Specialised cells
- NB: neural stem cells are multipotent - they give rise to the many specialised cell types in CNS
How does a stem cell know what to become?
Development of brain cells is influenced by chemical signals.
Gene expression is influenced by:
- Neurotrophic factors (chemicals), which may express specific genes and signal a cell to develop in a particular way
- Gene methylation, which may suppress specific genes
NB: environmental influences may alter gene expression
Stages of brain development
- Cell birth (neurogenesis)
- Cell migration
- Cell differentiation
- Cell maturation (dendrite and axon growth)
- Synaptogenesis (formation of synapses)
- Cell death and synaptic pruning
- Myelogenesis (formation of myelin)
Neurogenesis
Formation of neurons from stem sells in the neural tube is complete ~25 weeks after conception.
- Except in hippocampus
- NB: brain tumours in adults arise from glial cells (glioblastoma), brain tumours in children may also be neuronal (neuroblastoma)
Neural migration
Migration: 8-29 weeks
How do cells know where to go? - Neural migration
Radial glial cells pathways
Paths run from subventricular zone to cortical surface
Cortical layers develop from the inside out: first layer VI, then V.
- NB: some neurons go off road and follow chemical signals.
Cell differentiation
Brain relies on a general purpose neuron that matures into a specific cell type in a specific location when exposed to local environmental chemicals.
- Essentially complete at birth
Neural maturation
Dendrite development
- Dendritic arborisation begins prenatally and continues up to 2 years after birth.
Axon development
- Neurotropic chemicals attract or repel approaching growth cones
Synaptogenesis
Human brain has 100 trillion synapses
- Simple synaptic contacts 5 months after conception.
- Extensive synaptic development in deepest cortical layers 7 months after conception.
NB: not all synapses in e.g., visual cortex are visually related (synesthesia)
Cell death
Apoptosis = programmed cell death
- Superfluous neurons die through apoptosis because they depend on neurotrophic factors produced by target cells
Synaptic pruning
Synapses on remaining neurons persist into adulthood only when incorporated into a specific functional network, other synapses are eliminated
- NB: synaptic pruning induced by environmental cues may lead to variation in neural connectivity and perception.
Myelogenesis
Cerebral myelination is a rough index of cerebral maturation.
Increased connectivity in the default network is related to higher intelligence.
- Myelination of the cerebral cortex continues into adulthood
- White matter increases
- Grey matter decreases (NB: grey matter in language regions increases.
Piaget’s stages of cognitive development
- Stage I: sensorimotor
- Stage II: preoperational
- Stage III: concrete operational
- Stage IV: formal operational
Stage I: Sensorimotor
- Birth to 18-24 months
- Experience the world through senses and actions (looking, touching, mouthing)
- Object permanence, stranger anxiety
Stage II: preoperational
- 2-6 years
- Represents things with words and images but lacks logical reasoning
- Egocentricism, language development
Stage III: concrete operational
- 7-11 years
- Thinks logically about concrete events; grasps concrete analogies and performs arithmetical operations
- Conservation, mathematical transformation
Stage IV: formal operational
- 12+ years
- Reasons abstractly
- Potential for mature moral reasoning
Brain hormones
NB: behavioural differences between males and females are not solely the result of environmental experiences
- Androgens
- Estrogens
Induce changes in the brain
- Number of neurons
- Branching of dendrites
- Growth of synapses
Androgens
Mainly testosterone
- Male sex characteristics
- Secondary sex characteristics
- Changes in the brain
Estrogens
Estrone, estradiol, estriol
- Menstruation cycle, pregnancy
Chemoaffinity hypothesis
Specific molecules in different cells in various midbrain regions give each cell a distinctive chemical identity.
Incoming axons seek out a specific chemical and long in a general midbrain region
NB: precise positioning of neural placement is induced by postnatal experience.
