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Neurons and Glia
Chapter 2
Pg 32-57
Obstacles to Study
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Cells are too small to see.
To study brain tissue with a microscope, thin
slices are needed but the brain is like jello.
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Formaldehyde used to “fix” or harden tissue early
in 19th century.
Brain tissue is all the same color:
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Nissl stain revealed cell bodies – cytoarchitecture
Golgi stain revealed parts of the neuron.
Golgi stain shows cell structure
Brodmann Areas
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Different areas of the brain with different
functions have different kinds of neurons.
Brodmann mapped the areas based on the
kinds of cells found:
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Cytoarchitectonic method
52 functionally distinct areas identified by
number.
Ramon y Cajal’s Principles
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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:
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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.
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:
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Axon
 Dendrites
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All parts of the cell are made up of protein
molecules of different kinds.
How Neurons Communicate
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An all-or-nothing electrical signal, called an action
potential, travels down the axon.
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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.
The Synapse
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The synapse is the point of contact between
neurons.
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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).
Dendrites
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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.
Dendritic Spines
How to Tell Axons from Dendrites
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Dendrites receive signals – axons send them.
There are hundreds of dendrites but usually just one
axon.
Axons can be very long (> 1 m) while dendrites are <
2 mm.
Axons have the same diameter the entire length –
dendrites taper.
Axons have terminals (synapses) and no ribosomes.
Dendrites have spines (punching bags).
Don’t be fooled by the branches – both have them.
Ways of Classifying Neurons
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By the number of neurites (processes):
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By the type of dendrites:
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Pyramidal & stellate (star-shaped)
By their connections (function)
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Unipolar, bipolar, multipolar
Sensory, motor, relay interneurons, local
interneurons (Golgi Type II neurons)
By neurotransmitter – by their chemistry
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
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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
Kinds of Cells
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Neurons (nerve cells) – signaling units
Glia (glial cells) – supporting elements.
Miscellaneous other cells:
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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.
Functions of Glia
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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
Kinds of Glia
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Oligodendrocytes – surround brain & spinal
cord neurons and give them support.
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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.