Chapter 7 Flashcards
development in the human embryo and fetus
- in embryonic state, we are very dependent on nutrition and mom’s mental state for brain development; pre- and post-natal conditions strongly influence brain development
- stages:
+ start with a fertilized egg (zygote)
+ human embryo develops three cell layers
+ ectoderm, outer layer, becomes nervous system
+ NEURAL TUBE forms from neural ridges
+ anterior of neural tube subdivides into three parts: forebrain, midbrain, hindbrain - developing human: embryo (first 10 weeks), fetus (after)
stages of nervous system development
- neurogenesis: mitosis produces neurons from nonneuronal cells
- cell migration: cells move to establish distinct populations
- differentiation: cells become distinctive neurons or glial cells
- synaptogenesis: establishment of synaptic connections
- neuronal cell death: selective death of many nerve cells
- synapse rearrangement: loss or development of synapses, to refine synaptic connections
neurogenesis
production of nerve cells
+ non-neural cells (stem cells) divide through mitosis and form the ventricular zone
+ cells leave the ventricular zone and become either neurons or glial cells
- differences between invertebrates and vertebrates:
+ invertebrates: cell fate is highly determined genetically, with a hardwired and precise program (each of C.elegans’s 302 neurons’ development can be followed); environment has minimum influence
+ vertebrates: cell fate is less predetermined; shaped by cell-cell interactions -> depends on environment for growth of neurons
stem cells
undifferentiated cells
cells before they become a specific type (e.g. neurons, glial, skin, hair, etc.)
- all cells start as stem cells
- cells differentiate based on location
+ growth factors are released to guide cell development
- stem cells are present throughout embryonic tissues and sometimes in adult ventricles (not for new neurons but for hippocampus and olfactory bulb)
- have medical applications (e.g. Alzheimer’s and Parkinson’s disease are caused by lots of cell death at basal ganglia -> put in stem cells to grow back)
cell migration
- cells move away from the ventricular layer to target destination
- radial glial cells act as guides for cells to migrate along
- cell adhesion molecules (CAMs) promote adhesion of parts of the nervous system to guide cells
cell differentiation
- cells reach their destinations -> express genes and make the proteins they need
+ size and shape of cell depends on destination - process allows cells to acquire their specific appearance and function
synaptogenesis
brain cells change early in life through:
+ process outgrowth: growth of axons and dendrites
+ synaptogenesis: formation of synapses
- synapses form rapidly on dendrites and dendritic spines
+ spines proliferate after birth; connections are affected by experience + environment (e.g. nutrition + care; exposure to behaviors like eye contact, hug, converse, etc. help create neural connections for language perception + production, trust, etc.)
+ nerve cell bodies increase in volume to support dendritic tree
- extensions emerge from growth cones at the tips of axons and dendrites
- filopodia: fine outgrowths of growth cones, adhere to CAMs in the environment and pull the growth cone in a particular direction
+ growth of axons and dendrites is dependent on environment + location
+ target cells release CAMs -> filopodia gets attracted to and grow towards them
+ different target cells release different amounts of CAMs to compete for filopodia connection
example of differentiation based on the neural environment: motoneurons production
- cells in notochord release a protein that directs some cells in the spinal cord to become motoneurons
- induction: the influence of one set of cells on the fate of nearby cells
cell death
apotosis
cells get worn out or damaged and die
- cells have death genes which are expressed only during apoptosis
- only happens during brain development period in embryonic state; different from adult cell death (caused by aging, substance abuse, or neurodegenerative diseases)
- capases: family of proteases (enzyme); cut up + digest proteins and DNA -> cell death
+ example: amount of spinal neurons in chick and in human drops almost 1/2 or 1/3 during cell death
+ reason behind extensive growth: allow cells to compete with themselves -> select stronger connections for better circuit development
influences and functions of cell death
- factors that influence cell death:
+ size of synaptic target: reduced -> fewer neurons survive
+ neurotrophic factors competition: w/o enough of these chemicals produced by target cells, neurons die
+ nerve growth factor (NGF): produced by targets, taken up by axons of innervating neurons; function: keep cells alive + growing, prevent dying
+ also: brain-derived neurotrophic factor (BDNF) and other neurotrophins
+ important for adults; support neurons and prevent cell aging -> keep neurons healthy
+ growth of BDNF is trigged by exercise (cardio)
neuron-neuron competition
- neurons compete for connections to target cells
- neurons that make adequate synapses survive and grow; those fail to form synaptic connection die
- neurons also compete for target-derived chemicals, neurotrophic factors, that help them survive and grow
- post-natal development of synapses:
+ peak growth = before 5 years old
+ prefrontal cortex = last to develop; allowed more space for growth
+ pruning: remodeling synaptic connection
synapse rearrangement (synaptic remodeling)
refines synaptic connections
- influences of synaptic survival:
+ neural activity: stimulates neurons and strengthen neural connections in entire circuitry (e.g. practice makes perfect)
+ neurotrophic factor
- for humans, synaptic remodeling is evident in thinning of the gray matter in the cortex as pruning of dendrites and axon terminals progresses
+ thinning process continues in a caudal-rostal direction during maturation -> prefrontal cortex matures last
example for experience being an important influence on brain development: visual system
- neuronal development is not predetermined by neural predispositions but influenced by environment (local: tropic hormones, nutrition vs. external: outside experiences)
- visual system development:
+ early visual experience is crucial to establishing connection for visual system (visual cortex and eye are connected by a network)
+ depriving animals of lights to both eyes (binocular deprivation) produces structural changes in visual cortical neurons
+ experiment: cover animals’ eyes when they are just born -> keep for long time -> LOSS OF DENDRITIC SPINES AND SYNAPSES -> neuronal loss -> blindness
+ visual experience determines which synapses will be maintained (synapse rearrangement in visual cortex)
visual system development example: cats
- synaptic development = most intense from day 8 to 37 -> sensitive period: time during which experience can have profound influence on behavior
+ block visual perception during sensitive period: no neuron growth + blindness, after: can still perceive light but processing is not great
+ graph: rising = synaptogenesis (growth), falling = synaptic remodeling/rearrangement - axons from each eye compete for synaptic targets during development of visual cortex
+ Hebbian synapses: grow stronger/weaker depending on ability to affect a postsynaptic cell
+ experiment: cover right eye and leave left open
+ L: neighboring retinal cells (perceive light stimulus) tend to fire synchronously + drive postsynaptic neuron firing -> summation -> create action potential -> cause neurotropic factors release -> synapses and neurons survive + strengthened
+ R: no visual stimulus -> cells fire at random, rarely cause postsynaptic neuron to fire; don’t trigger action potential -> no release of neurotropic factors -> synapses and neurons die -> blindness
example about importance of experience in brain development: vocal learning in birds + humans
BIRDS
- birds learn songs by copying tutors (e.g. dads, peers)
- depriving auditory experience during sensitive period (first 2 months) affects song learning:
+ isolate with hearing intact: birds produce abnormal songs
+ isolate and deafen: worse songs
HUMANS
- language learning: not difficult during 3 or 4 to 9 or 10 y.o thanks to lots of neuroplasticity from critical period
- auditory exposure allows for sound reception and replication
