NEURONS AND GLIA
Introduction
“Neurophilosophy”
Brain (neurons) is the origin of mental abilities
Glia and Neurons
Glia
Insulates, supports, and nourishes neurons
90% of brain cells are glial cells
Neurons
Process information
Sense environmental changes
Communicate changes to other neurons
Command body response
The Neuron Doctrine
The birth of neurohistology
Microscopy invention
Discovery of fixation method for cutting thin slices
Staining methods for selectively coloring parts of cells
The Nissl Stain
Developed by by the German
neurologist Franz Nissl
Stain the nuclei and surrounding
material (Nissl body)
Made it possible to distinguish neurons
vs. glia and to study the arrangements
of neurons in different parts of brain
(cytoarchitecture)
Fig2.1
The Neuron Doctrine
The Golgi Stain
Camillo Golgi discovered that by soaking brain
tissue in a silver chromate solution, a small
percentage of neurons became darkly colored in
their entirety
Soma (cell body or perikaryon) and neurites
(axons and dendrites)
Fig 2.3
The Neuron Doctrine
Cajal’s Contribution
Santiago Ramon y Cajal used Golgi stain method and worked
out the circuitry of many regions of the brain : Father of
neuroanatomy
Golgi versus Cajal
Reticular theory vs. cell theory
Neuron doctrine
Neurons communicate
by contact, not
continuity
Final proof had to wait
until EM got developed
in the 1950s
The Prototypical Neuron
The Soma
~20 um in diameter
Cytosol: Potassium rich
watery fluid inside the
cell
Organelles: Membraneenclosed structures
within the soma
Cytoplasm: Contents
within a cell membrane
(e.g., organelles,
excluding the nucleus)
Fig2.7
The Prototypical Neuron
The Axon
Begins with axon hillock, initial tapered
segment where action potentials are
generated
Rough ER does not extend into axon
Protein composition of axon membarane
is fundamentally different from that of
soma
No protein synthesis in the axon
May extend from less than a millimeter to
over a meter long
May branch out (generally at right angles)
to form axon collaterals that could return
to the same cell (recurrent collaterals)
Diameter ranges from less than 1 mm to 25
mm in humans - The speed of nerve
impulses depends on axonal diameter
The Prototypical Neuron
The Axon Terminal (terminal bouton)
A site where the axon comes in contact
with other neurons and passes
information on to them
Terminal arbor or boutons en passant
Synapse - To fasten together
Innervation - making synaptic contact
Differences between the cytoplasm of
axon terminal and that of axon
No microtubules in the terminal
Presence of synaptic vesicles (~50
nm in diameter)
Dense covering of proteins on the
inside surface of the synaptic
membrane
Large number of mitochondria (high
energy demand)
The Prototypical Neuron
Synapse
Pre- and Postsynaptic sides :
directionality of information
flow
Synaptic transmission
Synaptic cleft
Electrical-to-chemical-toelectrical transformation
Neurotransmitter
The Prototypical Neuron
Axoplasmic transport
Wallerian degeneration
Degeneration of axon when severed (axotomy) is due to
the lack of protein synthesis machinery within axon
Kandel Fig 55-18
Anterograde transport by kinesin
and retrograde transport by MAP-1C
(dynein)
The Prototypical Neuron
Slow Axoplasmic transport
Paul Weiss’s experiment
Tied off a sciatic nerve (axon) to find that
material accumulate on the proximal side
of the knot
When the knot was untied, the bulged out
accumulation continued down the axon
The speed of movement was measured to
be about 1 - 10 mm per day ; SLOW
AXOPLASMIC TRANSPORT
Only anterograde direction
Slow transport itself can be at two
different speeds
Slower (0.2-2.5mm per day) : fibrillar
elements of cytoskeleton (neurofilament
subunits, tubulins..)
Faster (about twice as fast as the slower) :
various cytosolic proteins (clathrin, actin,
actin-binding proteins, enzymes..)
The Prototypical Neuron
Fast Axoplasmic transport
Bernice Grafstein
Injected radioactive amino
acids into somata
Traced the synthesized
(hot) proteins along the
axon
Large membraneous organelles are transported via fast transport
Includes vesicles of the constitutive secretory pathways, synaptic
vesicles precursor membranes, mitochondria, smooth ER elements..
ATP dependent but not protein synthesis dependent (once
synthesized)
Soma-independent (isolated axon still can transport
The Prototypical Neuron
Dendrites
Greek for ‘tree’
Dendritic tree for all the dendrites of a
neuron
“Antennae” of neurons - covered with
thousands of synapses
Dendritic membrane (postsynaptic
membrane) contains many specialized
receptors for neurotransmitters
Dendritic spines
Some neurons have these structures for
receiving some types of inputs
Discovered by Cajal
Believed to isolate various chemical
reactions
Dynamic structures affected by the type
and amount of inputs and
developmental changes of environment
Fig 2.17
Fig 2.18
Mental Retardation and dendritic spines
Brain function depends on the highly precise synaptic connections,
which are formed during the fetal period and are refined during
infancy and early childhood
95% of population falls within two standard deviations from the mean
of IQ (around 70 when the mean is set to be 100). Some 2-3% of
humans with intelligence score below are considered to be mentally
retarded IF the cognitive impairment affects the person’s ability to
adapt their behavior to the setting in which they live
Can have many causes
Genetic disorders such as PKU or Down
syndrome
Accidents or infection during pregnancy
or early childhood
Poor nutrition during pregnancy
Environmental impoverishment such as
the lack of good nutrition, socialization,
Fig A
sensory stimulation during infancy
Some with clear physical correlates
(retarded growth, abnormal structures of
head, hands, and body), most with only
behavioral manifestations
Dendritic spine abnormality has been found
to be correlated with mental retardation
Classifying Neurons
Classification Based on the
Number of Neurites
Unipolar cell
Found in invertebrate
nervous system - single
process with different
segments serving as
receptive surfaces or
releasing terminals
Bipolar cell
Two neurites
Multipolar cell
Most neurons in the brain
are multipolar
Kendal fig 2-4
Classifying Neurons
Classification Based on
Dendritic and Somatic
Morphologies
Often unique to a particular
region of the brain
Cortex - Stellate cells
(star-shaped) and
pyramidal cells (pyramidshaped)
Spiny or aspinous
Classifying Neurons
Further Classification
Based on connections within the CNS
Primary sensory neurons
motor neurons
interneurons
Based on axonal length
Golgi Type I - projection neurons that extend their axons
to other parts of the brain (e.g. pyramidal
neurons in the cortex)
Golgi Type II - local circuit neurons that have short axons
that do not extend beyond the vicinity of
cell body (e.g. stellate cells in the cortex)
Based on neurotransmitter type
Cholinergic, glutamatergic, GABAergic…
Glia
‘Sleeping Giants’ ?
Function of Glia
Supports neuronal functions
Without glia brain cannot
function!
Astrocytes
Most numerous glia in the brain
Fill spaces between neurons
Imporatant regulator of the
chemical contents of
extracellular spaces (Not much
left after filling up)
Envelop synaptic junctions restrict the spreading of released
neurotransmitters
Possess their own
neurotransmitter receptors!!
Glia
Myelinating Glia
Oligodendroglia (in CNS)
and Schwann cells (in PNS)
Insulate axons by wrapping
axons around
Myelin sheath
One Oligodedroglia can
provide insulation to several
axons but each Schwann cell
does to a only a single axon
Node of Ranvier
Region where the axonal
membrane is exposed
Glia
Other Non-Neuronal Cells
Microglia as phagocytes (immune)
Ependymal cells provide lining of fluidfilled ventricles and directs cell
migration during brain development
Vasculature