1 / 43

Wiring the Brain: Development and Plasticity

Explore the intricate operations of the brain through neurogenesis, synaptogenesis, and pathway formation. Learn how the brain establishes correct pathways, refines connections based on experience, and undergoes critical periods of synaptic rearrangement. Understand the mechanisms behind synaptic segregation, synaptic convergence, and binocular vision development. Discover the lasting effects of synaptic plasticity and the factors influencing cortical synaptic plasticity. Gain insights into the importance of enriched environments and the regulation of critical periods in brain development.

rbosworth
Download Presentation

Wiring the Brain: Development and Plasticity

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Neuroscience: Exploring the Brain, 3e Chapter 23: Wiring the Brain

  2. Introduction • Operation of the brain • Precise interconnections among 100 billion neurons • Brain development • Begins as a tube • Neurogenesis, synaptogenesis, pathway formation, connections formed and modified • Wiring in brain • Establishing correct pathways and targets • Fine tuning based on experience

  3. The Genesis of Neurons • Example: Mammalian retinogeniculocortical pathway

  4. Activity-dependent SynapticRearrangement • Synaptic rearrangement • Change from one pattern to another • Consequence of neural activity/synaptic transmission before and after birth • Critical Period

  5. The Elimination of Cells and Synapses • Changes in Synaptic Capacity • Synapse elimination modeled in the neuromuscular junction

  6. The Lateral Geniculate Nucleus (LGN)

  7. Activity-dependent SynapticRearrangement • Synaptic segregation • Refinement of synaptic connections • Segregation of Retinal Inputs to the LGN • Retinal waves (in utero) (Carla Shatz) • Activity of the two eyes not correlated -> segregation in LGN • Process of synaptic stabilization • Hebbian modifications (Donald Hebb)

  8. Activity-dependent SynapticRearrangement • Segregation of Retinal Inputs to the LGN (Cont’d) • Plasticity at ‘Hebb’ synapses • “Winner-takes- all”

  9. Activity-dependent SynapticRearrangement • Segregation of LGN Inputs in the Striate Cortex • Visual cortex has ocular dominance columns (cat, monkey) - segregated input from each eye • Synaptic rearrangement is activity-dependent • Plastic during critical period • Effects of congenital cataracts (if not removed early)

  10. Activity-dependent SynapticRearrangement • Synaptic Convergence • Anatomical basis of binocular vision and binocular receptive fields • Monocular deprivation: • Ocular dominance shift • Plasticity of binocular connections • Synaptic competition

  11. Activity-dependent SynapticRearrangement • Critical period for plasticity of binocular connections

  12. Activity-dependent SynapticRearrangement • Effect of strabismus on cortical binocularity

  13. Activity-dependent SynapticRearrangement • Modulatory Influences • Increasing age • Before and after birth • Enabling factors • Basal forebrain & LC must be intact for plasticity

  14. Elementary Mechanisms of CorticalSynaptic Plasticity • Two rules for synaptic modification • Wire together fire together (Hebbian modifications) • Out of sync lose their link • Correlation: heard and validated

  15. Elementary Mechanisms of CorticalSynaptic Plasticity • Excitatory Synaptic Transmission in the Immature Visual System • Focus on 2 glutamate receptors (Rs): • AMPARs: glutamate-gated ion channels • NMDARs: Unique properties

  16. Elementary Mechanisms of Cortical Synaptic Plasticity • Excitatory Synaptic Transmission • NMDA receptors have two unique properties • Voltage-gated owing to Mg2+ • Conducts Ca2+ • Magnitude of Ca2+ flux signals level of pre- and postsynaptic activation

  17. Elementary Mechanisms of CorticalSynaptic Plasticity • Long-Term Synaptic Potentiation • Monitor synaptic strength before and after episodes of strong NMDA activation • Accounting for LTP • AMPA insertion (“AMPAfication”) • Splitting synapses (doubling)

  18. Elementary Mechanisms of CorticalSynaptic Plasticity • Lasting synaptic effects of strong NMDA receptor activation

  19. Elementary Mechanisms of CorticalSynaptic Plasticity • Long-Term Synaptic Depression (LTD) • Neurons fire out of sync • Synaptic plasticity mechanism opposite of LTP • Loss of synaptic AMPARs • Loss of synapses? (unknown) • Mechanism for consequences of monocular deprivation

  20. Elementary Mechanisms of CorticalSynaptic Plasticity • Brief monocular deprivation leads to reduced visual responsiveness • Depends on retinal activity, NMDA activation, postsynaptic calcium

  21. Why Critical Periods End • Why do critical periods end? • Plasticity diminishes: • When axon growth ceases • When synaptic transmission matures • When cortical activation is constrained • Intrinsic inhibitory circuitry late to mature • Understanding developmental regulation of plasticity may help recovery from CNS damage

  22. Enriched environment: More complex brain structure. • Increased dendritic branching and synaptic density • Increased transmitter levels, total protein • Better at solving maze problems

  23. Concluding Remarks • Generation of brain development circuitry • Placement of wires before birth • Refinement of synaptic infancy • Developmental critical periods • Visual system and other sensory and motor systems • Environment influences brain modification throughout life

  24. Barn Owl : Space Map in Inf. Colliculus This is a computational map!

  25. ‘Supervised’ learning / plasticity Prism goggles experiment: ‘eye instructs the ear’ A V After 8 weeks w/ Prism goggles A V

  26. End of Presentation

  27. The Genesis of Neurons • Cell Proliferation • Neural stem cells give rise to neurons and glia

  28. The Genesis of Neurons • Cell Proliferation (Cont’d) • Cleavage plane during cell division determines fate of daughter cells

  29. The Genesis of Neurons • Cell Migration • Pyramidal cells and astrocytes migrate vertically from ventricular zone by moving along thin radial glial fibers • Inhibitory interneurons and oligodendroglia generate from a different site and migrate laterally

  30. The Genesis of Neurons • Cell Migration • First cells to migrate take up residence in “subplate” layer which eventually disappears • Next cells to divide migrate to the cortical plate • The first to arrive become layer VI, followed V, IV, and so on: “inside out”

  31. The Genesis of Neurons • Cell Differentiation • Cell takes the appearance and characteristics of a neuron after reaching its destination but programming occurs much earlier

  32. The Genesis of Neurons • Differentiation of Cortical Areas • Adult cortical sheet is a “patchwork quilt • Cortical “protomap” in the ventricular zone replicated by radial glial guides • But some neurons migrate laterally • Thalamic input contributes to cortical differentiation

  33. The Genesis of Connections • The three phases of pathway formation: • (1) pathway, (2) target, (3) address

  34. The Genesis of Connections • The Growing Axon • Growth cone: Growing tip of a neurite

  35. The Genesis of Connections • Axon Guidance • Challenge in wiring the brain • Distances between connected structures • But in early stages nervous system is a few centimeters long • Pioneer axons stretch as nervous system expands • Guide neighbor axons to same targets • Pioneer neurons grow in the correct direction by “connecting the dots”

  36. The Genesis of Connections • Axon Guidance • Guidance Cues: Chemoattractant (e.g., netrin), Chemorepellent (e.g., slit)

  37. The Genesis of Connections • Axon Guidance • Establishing Topographic Maps • Choice point; Retinal axons innervate targets of LGN and superior colliculus • Sperry (1940s): Chemoaffinity hypothesis • CNS axons regenerate in amphibians, not in mammals • Factors guiding retinal axons to tectum • Ephrins/eph (repulsive signal)

  38. The Genesis of Connections • Axon Guidance • Establishing Topographic Maps

  39. The Genesis of Connections • Synapse Formation • Modeled in the neuromuscular junction

  40. The Genesis of Connections • Synapse Formation • Steps in the formation of a CNS synapse: • Dendritic filopodium contacts axon • Synaptic vesicles and active zone proteins recruited to presynaptic membrane • Receptors accumulate on postsynaptic membrane

  41. The Elimination of Cells and Synapses • The mechanisms of pathway formation • Large-scale reduction in neurons and synapses • Development of brain function • Balance between genesis & elimination of cells and synapses • Apoptosis: Programmed Cell Death • Importance of trophic factors, e.g., nerve growth factor

More Related