2/10/15 – Lecture 8 – Neural Development, Developmental Neurogenesis, Brain Development, and Environment- Goal: to learn how a structure as a complex and incredible as our brain is formed- The Human Brain: has ~100 billion neurons and even more glial cells (10x the neurons) with specialized structures and functionso Developmental Neurogenesis: allows cells to be created/formed and differentiate during development Early Stages of Human Development- Zygote – fertilization to 4 days old, huge cell so it just divides for a bit- Blastula/Blastocyst – hollow ball of cells between 4 and 5 days- Gastrula – one side tucks in on itself and creates a 3 layered ball of cells, forms at 7-10 dayso 3rd layer (inner most layer) – Endoderm – forms internal organso 2nd layer (middle layer) – Mesoderm – forms muscle, blood and boneo 1st layer (outer layer) – Ectoderm – forms skin and nervous system Neural Plate: part of the back ectoderm that forms the neural tube by folding in on itself Neural Tube: hollow tube of cells that eventually forms the brain and spinal cord Neural Crest: these cells eventually migrate out and form the peripheral nerves- Incomplete folding can result in anencephaly (often causes still birth) or spina bifida (lack of brain formation) Formation of the Nervous System: MORE neurons and more synapses are formed during development than are actually needed (which are finished before birth, after birth, and which processes come first and last)- Cell Proliferation: formation of two new cells from one cello Begins shortly after conception and is mostly complete at 4-5 months post-conception (CANNOT form new neurons post-birth)o Forms >100 billion neuronal precursor cells, much more than are actually needed Primitive Neuroectodermal Tumor (PNET): rare but dangerous tumor found in children, forms because neural stem cells refuse to stop proliferating- 5 year survival rate is usually around 50%- Cell Migration: the movement of cells from one place to anothero Starts around 8-9 weeks after conception and is complete about 2-3 months before birth Tangential Migration: moving up/down or around- Neural crests cells to form Schwann cells and neurons in peripheral nerves Radial Migration: moving out from the center- Cerebral Cortical neurons to form the layers of the Cerebral Cortex- Cell Differentiation and Maturation: the process by which unspecialized stem cellsor precursor cells take on their final specialized stateo Chemical Differentiation: hormones and other chemical signals activate master regulatory genes (genes that produce a protein that then turns onneuron-specific proteins Neuron specific proteins: voltage gated ion channels, neurotransmitter receptors, neurotransmitter synthesis enzymes,etc (done by the time of birth)- Master-regulatory gene -> produces a gene that turns on proteins needed for different specified cellso Morphological Differentiation: cell taking on its adult form, dendrites andaxons being to form (continues into childhood) Grow dendrites: increasing the length and branching of dendrites(larger and more complex increases surface area for synapse formation) Extend axons: may need to extend millimeters to a target, needs 2 clues (follows the “smell”)- 1. Secretion of chemoattractants from the target cell thatguides it to them- 2. Cell adhesion molecules that then grasp the axon and adhere it to the target cell- Synaptogenesis: the formation of a functional synapse between an axon from a presynaptic neuron and the dendrites of a postsynaptic neurono Starts during prenatal development and continues into late childhood, the creation of proper synaptic structure by cellular activity in both neurons 1. Presynaptic neuron needs to synthesize and store neurotransmitter vesicles 2. Post synaptic neuron needs to form and insert neurotransmitter receptors into membrane- Number of synapses in the human brain exceeds 100 quadrillion (WAY more than we need)- Cell Death and Synaptic Pruning:o Neurotrophins: special hormone-like chemicals secreted by postsynaptic neurons (dendrites) in response to electrical activation by presynaptic neurons (axons) Not neurotransmitters but do promote neuronal growth and survival of presynaptic neurons and maintenance of their synapses Stronger connection allows for more neurotrophic support, weaker connections eventually die- Leaves the best neurons with strong, functional connections- Kills the weaker cells that aren’t firing the action potentials o You can also kill just the synapses rather than theentire cell- Myelination: the formation of myelin sheaths around neuronso Begins just after birth and continues into young adulthood (mid 20s), doesn’t progress evenly throughout the brain Hind brain (where basic functions are), Sensory input regions (need to take in all the information), Motor output regions (to move and root and survive), regions involving decision making and impulse control myelinates last Allows neurons to fire more rapidly so greatly to improve neural