Neurons and Glia

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Neurons and Glia

• Chapter 2
• Pg 32-57

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Obstacles to Study

• Cells are too small to see.
• To study brain tissue with a microscope, thin
  slices are needed but the brain is like jello.
• Formaldehyde used to fix or harden tissue early
  in 19th century.
• Brain tissue is all the same color
• Nissl stain revealed cell bodies
  cytoarchitecture
• Golgi stain revealed parts of the neuron.

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Golgi stain shows cell structure
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Brodmann Areas

• Different areas of the brain with different
  functions have different kinds of neurons.
• Brodmann mapped the areas based on the kinds of
  cells found
• Cytoarchitectonic method
• 52 functionally distinct areas identified by
  number.

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Ramon y Cajals Principles

• Neuron doctrine neurons are like other cells.
• Principle of dynamic polarization electrical
  signals flow in only one, predictable direction
  within the neuron.
• Principle of connectional specificity
• Neurons are not connected to each other, but are
  separated by a small gap (synaptic cleft).
• Neurons communicate with specific other neurons
  in organized networks not randomly.

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Neuronal Circuits

• Neurons send and receive messages.
• Neurons are linked in pathways called circuits
• The brain consists of a few basic patterns of
  circuits with many minor variations.
• Circuits can connect a few to 10,000 neurons.

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Parts of the Neuron

• Soma the cell body
• Neurites two kinds of extensions (processes)
  from the cell
• Axon
• Dendrites
• All parts of the cell are made up of protein
  molecules of different kinds.

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How Neurons Communicate

• An all-or-nothing electrical signal, called an
  action potential, travels down the axon.
• The amplitude (size) of the action potential
  stays constant because the signal is regenerated.
• The speed of the action potential is determined
  by the size of the axon.
• Action potentials are highly stereotyped (very
  similar) throughout the brain.
• At the end of the axon (terminal button),
  neurotransmitter is released, which may start an
  action potential in another neuron.

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The Synapse

• The synapse is the point of contact between
  neurons.
• Axon terminal button makes contact with some part
  of an adjacent neuron.
• Synaptic vesicles containing neurotransmitter
  open when there is an action potential.
• Neurotransmitter may enter the adjacent neuron
  unused neurotransmitter is reabsorbed (reuptake).

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Dendrites

• Dendrites function as the antennae of the neuron,
  receiving input from other neurons.
• Dendrites are covered with synapses.
• Each synapse has many receptors for
  neurotransmitters of various kinds.
• Dendritic spines specialized dendrites that
  isolate reactions at some synapses.

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Dendritic Spines
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How to Tell Axons from Dendrites

• Dendrites receive signals axons send them.
• There are hundreds of dendrites but usually just
  one axon.
• Axons can be very long (gt 1 m) while dendrites
  are lt 2 mm.
• Axons have the same diameter the entire length
  dendrites taper.
• Axons have terminals (synapses) and no ribosomes.
  Dendrites have spines (punching bags).
• Dont be fooled by the branches both have them.

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Ways of Classifying Neurons

• By the number of neurites (processes)
• Unipolar, bipolar, multipolar
• By the type of dendrites
• Pyramidal stellate (star-shaped)
• By their connections (function)
• Sensory, motor, relay interneurons, local
  interneurons (Golgi Type II neurons)
• By neurotransmitter by their chemistry

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Parts of the Soma (Cell Body)

• Nucleus stores genes of the cell (DNA)
• Organelles synthesize the proteins of the cell
• Cytosol fluid inside cell
• Plasmic membrane wall of the cell separating it
  from the fluid outside the cell.

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Organelles

• Mitochondria provide energy
• Microtubules give the cell structure
• Rough endoplasmic reticulum produces proteins
  needed to carry out cell functioning
• Ribosomes produce neurotransmitter proteins
• Smooth endoplasmic reticulum packages
  neurotransmitter in synaptic vesicles
• Golgi apparatus Part of the smooth endoplasmic
  reticulum that sorts proteins for delivery to the
  axon and dendrites

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Kinds of Cells

• Neurons (nerve cells) signaling units
• Glia (glial cells) supporting elements.
• Miscellaneous other cells
• Ependymal cells form the lining of the
  ventricles, also aid brain development
• Microglia remove debris left by dead or
  degenerating neurons and glia.
• Veins, arteries, and capillaries in the brain.

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Functions of Glia

• Separate and insulate groups of neurons
• Produce myelin for the axons of neurons
• Scavengers, removing debris after injury
• Buffer and maintain potassium ion concentrations
• Guide migration of neurons during development
• Create blood-brain barrier, nourish neurons

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Kinds of Glia

• Oligodendrocytes surround brain spinal cord
  neurons and give them support.
• In white matter, provides myelination
• In gray matter, surround cell bodies
• Schwann cells provide the myelin sheath for
  peripheral neurons (1 mm long).
• Astrocytes absorb potassium, perhaps nutritive
  because endfeet contact capillaries (blood
  vessels), form blood-brain barrier.