2017-02-12 11:54:15

Visualizing the microanatomy of the brain (4:29)

Today's Topics

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

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

Glia (neuroglia)

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

Astrocytes

  • "Star-shaped"
  • Physical and metabolic support
    • Blood/brain barrier
    • Ion (Ca++/K+) buffering
    • Neurotransmitter (e.g., glutamate) buffering

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

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
  • 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

Neural communication

  • Electrical
    • Fast(er)
    • Within neurons
  • Chemical
    • Slow(er)
    • Between neurons

How are messages generated?

  • Electrical potential (== voltage)
    • Think of potential energy
    • Voltage ~ pressure
    • Energy that will be released if something changes

Potential energy

Types of neural electrical potentials

Resting potential

  • Measurement
    • Electrode on inside
    • Electrode on outside
    • Inside - Outside = potential

Resting potential

Resting potential

  • Neuron (and other cells) have potential energy
    • Inside neuron is -60-70 mV, with respect to outside
    • About 1/20th typical AAA battery
  • Like charges repel, opposites attract, so
    • Positively charged particles pulled in
    • Negatively charged particles pushed out

Where does the resting potential come from?

  • Ions
  • Ion channels
  • Separation between charges
  • A balance of forces

We are the champIONs, my friend

  • Potassium, K+
  • Sodium, Na+
  • Chloride, Cl-
  • Organic anions, A-

Party On

  • Annie (A-) was having a party.
    • Used to date Nate (Na+), but now sees Karl (K+)
  • Hired bouncers called
    • "The Channels"
    • Let Karl and friends in or out, keep Nate out
  • Annie's friends (A-) and Karl's (K+) mostly inside
  • Nate and friends (Na+) mostly outside
  • Claudia (Cl-) tagging along

Party On

Next time

  • Neural communication

Azevedo, Frederico AC, Ludmila RB Carvalho, Lea T Grinberg, José Marcelo Farfel, Renata EL Ferretti, Renata EP Leite, Roberto Lent, Suzana Herculano-Houzel, and 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). Wiley Online Library: 532–41.

Bazargani, Narges, and David Attwell. 2016. “Astrocyte Calcium Signaling: The Third Wave.” Nature Neuroscience 19 (2): 182–89. doi:10.1038/nn.4201.

Bhardwaj, Ratan D., Maurice A. Curtis, Kirsty L. Spalding, Bruce A. Buchholz, David Fink, Thomas Björk-Eriksson, Claes Nordborg, et al. 2006. “Neocortical Neurogenesis in Humans Is Restricted to Development.” Proceedings of the National Academy of Sciences 103 (33): 12564–8. doi:10.1073/pnas.0605177103.

Chung, Won-Suk, Christina A. Welsh, Ben A. Barres, and Beth Stevens. 2015. “Do Glia Drive Synaptic and Cognitive Impairment in Disease?” Nature Neuroscience 18 (11): 1539–45. doi:10.1038/nn.4142.

Magrassi, L., K. Leto, and F. Rossi. 2013. “Lifespan of Neurons Is Uncoupled from Organismal Lifespan.” Proceedings of the National Academy of Sciences 110 (11): 4374–9. doi:10.1073/pnas.1217505110.

Zhan, Yang, Rosa C. Paolicelli, Francesco Sforazzini, Laetitia Weinhard, Giulia Bolasco, Francesca Pagani, Alexei L. Vyssotski, et al. 2014. “Deficient Neuron-Microglia Signaling Results in Impaired Functional Brain Connectivity and Social Behavior.” Nature Neuroscience 17 (3): 400–406. doi:10.1038/nn.3641.