2022-02-02 09:49:34

Prelude

How to play EyeWire (03:56)

Announcements

  • Exam 1 next Thursday, 2/10
    • 40 questions
  • Complete 1 “component/section” in EyeWire, earn 2 extra credit points.
    • Take screen shot, email to Iris via Canvas
    • Due before Friday, 2/11

Today’s Topics

  • Cells of the nervous system
    • Glia
    • Neurons
  • How do these cells communicate?

Cells of the nervous system

We are human

  • ~ 37 trillion (10^9) (Roy & Conroy, 2018) cells
  • 10-100 trillion non-human cells (gut, skin/hair, bloodstream, etc.)

How many neurons and glia?

  • Old “lore”: ~100 billion neurons
  • New estimate (Azevedo et al., 2009)
    • ~86 +/- 8 billion neurons
    • ~85 +/- 9 billion glia
  • 100-500 trillion synapses, 1 billion/mm^3

Could you count to 170 billion?

  • How many years to count to 170 billion?
  • 60 s/min x 60 min/hr x 24 hrs/day x 365 days/ yr = 31,536,000 s/yr
  • 1.7e11/31,536,000 = 5,390 years

Mass, Neurons, Non-Neurons

Non-neuronal cells by brain mass

Neurons by brain mass

Summary

  • # glia+ cells scales with brain size/mass
  • # neurons doesn’t scale with brain size/mass
    • cerebellum small but # of neurons large

The Human Advantage

Glia (neuroglia)

  • “Glia” means glue
  • Functions
    • Structural support
    • Metabolic support
    • Brain development
    • Neural plasticity?

Astrocytes

  • “Star-shaped”
  • Physical and metabolic support
    • Blood/brain barrier
    • Regulate concentration of key ions (Ca++/K+) for neural communication
    • Regulate concentration of key neurotransmitters (e.g., glutamate)

Astrocytes

Astrocytes

Myelinating cells

  • Produce myelin or myelin sheath
    • White, fatty substance
    • Surrounds many neurons
    • The “white” in white matter
  • Provide electrical/chemical insulation
  • Make neuronal messages faster, less susceptible to noise

Types of myelin-producing cells

  • Oligodendrocytes
    • In brain and spinal cord (CNS)
    • 1:many neurons
  • Schwann cells
    • In PNS
    • 1:1 neuron
    • Facilitate neuro-regeneration
  • Mnemonics: COPS/SPOC

Oligodendrocytes

Schwann Cells

Microglia

  • Phagocytosis
  • Clean-up damaged, dead tissue
  • Prune synapses in normal development and disease
  • Disruptions in microglia pruning -> impaired functional brain connectivity and social behavior, (Zhan et al., 2014)

Microglia

Neurons

Fun facts about neurons

Macrostructure of neurons

Structure of neurons

Dendrites

  • Branch-like “extrusions” from cell body
  • Majority of input to neuron
  • Cluster close to cell body/soma
  • Usually receive info
  • Passive (do not regenerate electrical signal) vs. active (regenerate signal)
  • Spines

Dendrites

Dendritic Spines

Soma (cell body)

  • Varied shapes
  • Nucleus
    • Chromosomes
  • Organelles
    • Mitochonrdria
    • Smooth and Rough Endoplasmic reticulum (ER)

Soma

Axons

  • Another branch-like “extrusion” from soma
  • Extend farther than dendrites
  • Usually transmit info

Axons

  • Parts
    • Initial segment (closest to soma, unmyelinated)
    • Nodes of Ranvier (unmyelinated segments along axon)
    • Terminals, axon terminals, terminal buttons, synaptic terminals, synaptic boutons

Axons

Synaptic bouton (terminal button)

  • Synapse (~5-10K per neuron)
  • Presynaptic membrane (sending cell) and postsynaptic (receiving cell) membrane
  • Synaptic cleft – space between cells
  • Synaptic vesicles
    • Pouches of neurotransmitters
  • Autoreceptors (detect NTs); transporters (transport NTs across membrane)

Synaptic bouton (terminal button)

Classifying neurons

  • Functional role
    • Input (sensory), output (motor/secretory), interneurons
  • Anatomy
    • Unipolar
    • Bipolar
    • Multipolar

Classifying neurons

  • By specific anatomy
    • Pyramidal cells
    • Stellate cells
    • Purkinje cells
    • Granule cells

Neurons by type

Next time

  • How neurons communicate

References

Azevedo, F. A., Carvalho, L. R., Grinberg, L. T., Farfel, J. M., Ferretti, R. E., Leite, R. E., … others. (2009). Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled-up primate brain. Journal of Comparative Neurology, 513(5), 532–541.

Bazargani, N., & Attwell, D. (2016). Astrocyte calcium signaling: The third wave. Nature Neuroscience, 19(2), 182–189. https://doi.org/10.1038/nn.4201

bbscottvids. (2009, September). Neuronal migration. Youtube. Retrieved from https://www.youtube.com/watch?v=t-8bxeWqSV4

Bhardwaj, R. D., Curtis, M. A., Spalding, K. L., Buchholz, B. A., Fink, D., Björk-Eriksson, T., … Frisén, J. (2006). Neocortical neurogenesis in humans is restricted to development. Proceedings of the National Academy of Sciences, 103(33), 12564–12568. https://doi.org/10.1073/pnas.0605177103

Chung, W.-S., Welsh, C. A., Barres, B. A., & Stevens, B. (2015). Do glia drive synaptic and cognitive impairment in disease? Nature Neuroscience, 18(11), 1539–1545. https://doi.org/10.1038/nn.4142

Magrassi, L., Leto, K., & Rossi, F. (2013). Lifespan of neurons is uncoupled from organismal lifespan. Proceedings of the National Academy of Sciences, 110(11), 4374–4379. https://doi.org/10.1073/pnas.1217505110

Roy, A. L., & Conroy, R. S. (2018). Toward mapping the human body at a cellular resolution. Molecular Biology of the Cell, 29(15), 1779–1785. https://doi.org/10.1091/mbc.E18-04-0260

Zhan, Y., Paolicelli, R. C., Sforazzini, F., Weinhard, L., Bolasco, G., Pagani, F., … Gross, C. T. (2014). Deficient neuron-microglia signaling results in impaired functional brain connectivity and social behavior. Nature Neuroscience, 17(3), 400–406. https://doi.org/10.1038/nn.3641