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Chapter 5

Development of the human brain

What is neurogenesis? The birth of new neurons or glia.
Where does neurogenesis occur? In the ventricular zone.
What is the ventricular zone? A layer of cells lining the inner surface of the neural tube.
How do the cells in the ventricular zone reproduce? The progenitor (reproducing) cells in the ventricular zone divide by mitosis, which produces two identical "daughter" cells.WHt
What are the rules which govern which cells will migrate in the ventricular zone? -Cleavage line perpendicular to the surface of the zone will produce two additional progenitor cells. -Cleavage line parallel to the surface of the zone will produce a daughter cell that is not attached to the zone and thus free to migrate.
When do migrating cells in the ventricular zone start being formed by the progenitor cells? Around 7 weeks.
How does cell migration occur? The migrating cells are guided by radial glia. They wrap around the radial glia and move along them.
What are radial glia? Specialized progenitor cells which grow out from the ventricular layer to the outer margins of the nervous system. (think of it as the spokes of a wheel)
What happens to the radial glia after migration is complete? Most radial glia pull back their branches, but some remain in place throughout adulthood
What is meant by "migrating cells form the cerebral cortex in an inside out fashion"? The inner layers are formed first. Cells destined for the outer cortical layers must somehow travel through the inner layers.
What is aggregation? The process by which the migrating cells congregate with other cells to form structures.
What is differentiation? The development of more specialized cell types from stem cells.
What are differentiation-inducing factors (DIFs) Chemicals that cause differentiation.
What are the two phases in neural tube differentiation? The first process differentiatiates the dorsal and ventral halves of the neural tubes. The second process differentiates the neural tube along its rostral-caudal
What happens to the neurons of the neural tube during dorsal-ventral differentiation? The neurons in the ventral half develop into motor neurons, and neurons in the dorsal half develop into sensory neurons.
What happens in rostral-caudal differentiation? The nervous system is divided into the spinal cord, hindbrain, midbrain and forebrain.
What DIFs control differentiation of the spinal cord and hind brain? Hox genes.
hHow do intrinsic and extrinsic factors influence organization of the cerebral cortex? Cells respond to iput from adjacent cells in the form of released DIFS by taking on the characteristics of their ew location. etc
What are neurites? Immature projections from the neuron that will mature to become axons and dendrites.
What are growth cones? A swelling at the tip of the neurite that helps the branch reach its synaptic target,
What do growth cones consist of? 1. A main body containing mitochondria, microtubules and other organelles 2. Filopodia, which are long fingerlike extensions from a growth cone 3. Lamellipodia, which are flat sheetlike extensions from the core located betwen the filopodia.
What is the special ability of the filopodia and lamellipodia? They are capable of movement and can stick to elements of the extracellular environment which pulls the growing neurites along behind them.
What happens to microtubules in the growth cone? Microtubules move forward as the cone extends, forming new segments of the neurite.
WHat is the role of the filopodia? They can signal the growth cone to move in a direction, and respond to both attracting and inhibiting chemicals released by guidepost cells along the way.
What are guidepost cells? Guidepost cells are glia that guide axon growth by releasing chemicals that either attract or repulse growth cones.
Where can guidepost cells be observed? At the optic chiasm.
What is fasciculation? The phenomenon that occurs when neurites that are growing in the same direction often become stuck together.
What causes fasciculation? Molecules on the surface of the neurites called cell adhesion molecules (CAMs) cause the neurites to stick together.
What happens as the growth cones approach their target? They begin to form either dendrites or axon collaterals.
What is synaptic specificity? The process by which a neurite identifies appropriate postsynaptic target cells from the many cells in the vicinity.
What determines the type of neurotransmitter that would be used at the synapse? Target cells determine the type of neurotransmitter released by the presynaptic cell.
How is a synapse formed? 1. The growth cone makes contact with a muscle fiber that already has receptors for a specific neurotransmitter. 2. Receptors become clustered at the synaptic sites over time, guided by a sequence of chemical release by both pre- and postsynapic cells
In the CNS, how do astrocytes guide synapse formation? They release several types of DIFs.
What is apoptosis? Programmed cell death.
What are neurotrophins? A chemical released by target cells that interrupt cellular suicide programs, thus contributing to the survival of presynaptic neurons.
Give one example of a neurotrophin. Nerve growth factor (NGF).
How does cell death occur? When activated by cell death genes, enzymes known as capases break up DNA and proteins causing cell death.
How do neurotrophins prevent cell death? They bind to receptors on a neuron, inhibiting caspase activity and allowing the cell to survive.
Why is cell death important? Too many surviving cells might result in problems e.g. high concentrations of neurotrophins and thus lower apoptosis rates are found in some cases of autism spectrum disorder.
How does myelination occur? It occurs in a rostral direction. 1. Spinal cord 2. Hindbrain, midbrain and forebrain. 3. Forebrain is myelinated from inferior to superior and from posterior to anterior. 4. Sensory parts of the cortex myelinated earlier than the motor parts.
When can the first human myelin be observed? 24 weeks after conception.
What is the last area to be myelinated? The prefrontal cortex. It is not completely myelinated until early adulthood.
How does myelination explain how reasoning is affected? The last part of the CNS to be myelinated is the prefrontal cortex, which is responsible for complex cognition like reasoning. Young adults show greater activation of their more completely myelinated frontal lobes, producing more reasonable responses.
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