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NEUROCYTOLOGY
Sept. 2, 2009
Jennifer LaVail, Ph.D.
(http://anatomy.ucsf.edu/Pages/lavaillab/index.html)
Objectives
NEUROCYTOLOGY
To build on your understanding of the structure and diversity of the cells
that make up the nervous system
1. Structure and function of neurons
2. Axoplasmic transport
3. Structure and function of synapses
4. Structure and function of glial cells
What kinds of cells are these?
What kinds of cells are these?
Neuron?
Epithelial cell?
Glial cell?
What makes a neuron special?
The neuron has the same
biochemistry and complement of
organelles as other cells of the
body.
But, neurons are
remarkable
for their polarity.
Structurally the two
most distinctive features
of the neuron are its
polarized shape and
localized secretory
function.
Polarized shape -
Action potential
Among all the cells in
our body, only neurons
have axons.
The axon requires unusual
modifications of the
cytoskeleton and specialized
intracellular transport
mechanisms.
Action potential
Microtubule polarity
Microtubule polarity
Fibroblasts have continuous µts.
Fibroblasts have unipolar µts.
Neurons have multiple µts.
Axons have unipolar µts.
Dendritic µts have mixed polarity.
Soma
The neuron cell body
is the primary location
of protein synthesis.
Most of the proteins
for axon maintenance
and synaptic function
have to be transported
to the axon.
Where is the cell
body?
What is special
about this neuron
cell body?
Electron micrograph of a neuronal cell body
Cytoplasm = pink; nucleus = yellow
Electron micrograph of Nissl bodies (stacks of RER)
“protein synthesis machines”
Neuron cell body also has polarity
axon hillock - region with no
ribosomes…beginning of the
axon.
DRG axon hillock
Neuronal polarity
Dendrites
Input region of neuron
Dendrites py ramidal-Purkinje
Form of energy varies, e.g.,
chemical, stretch, thermal,
neurochemical.
Integrates excitatory and inhibitory
synaptic inputs.
Principal structures of a neuron
processes
Cell Body
Dendrites
Axon hillock
Initial Segment
Axon
Synaptic terminals
Pyramidal neuron, rat cerebral cortex
Blowfly neuron
Motoneuron axons grow long distances to muscle endplateso
The structure of chemical synapses
Motor end plate 2
Neuromuscular junction in Lateral rectus muscle
Chemical Synapse
Voltage-sensitive
calcium channels
Synaptic cleft
15-100 nm
CNS synapses
sy napse
Synaptic components:
Synaptic vesicles
Presynaptic membrane
Synaptic cleft
Postsynaptic membrane
Two large synaptic terminals
containing mitochondria
contacting a dendrite
A synaptic terminal containing
dense cored vesicle indicating
the presence of catecholamine
transmitters
A large dendrite and spine contacted by many synaptic boutons
Distinguishing characteristics of axons and dendrites
Characteristic
Axon
Dendrite
Ontogeny
develops early
develops later
Length
several µm to >1 m
limited
Size/Shape
0.1 to 20 µm
uniform diameter
0.1 – 5 µm
tapered
Axon hillock
present
absent
RER
absent
present
Cytoplasm vol.
100 x greater
relatively little
Number per neuron
1
>1
Excitability
conducts AP
passive spread
Myelin
present on axons
>than 1 um
rare but glial wrap
Major Point: Mature axon has no RER, ribosomes, Golgi, so no way to
make proteins. Proteins have to be transported into and along axon.
giraffe
Neuron cell body
~100 µm
Embryonic neurons move, axons grow.
Motile embry onic neuron
Differentiating neuronal cells (actin, microtubules and DNA)
Torsten Wittman
The components of axonal transport
Axonal transport
cargo
motor
microtubules
Cargo comprised of protein, peptides, proteoglycans and amino acids.
Most are transported within organelles. (No synaptic vesicle transport).
Rates are about 200-400 mm/day or much slower, 1-2 mm/day.
Bidirectional movement depends on balance of cytoplasmic motors.
Real time assays of viral transport
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and a
Sorenson Video decompressor
are needed to see this picture.
Anterograde transport of HSV
Axonal transport of virus
HSV transport
The myelin sheath along a myelinated CNS axon is composed of
sections of myelin made from a number of individual cells
Remember, however, not all axons are myelinated.
Small diameter axons generally have no myelin covering.
Axon PNS
In the PNS axons less than 1 µm in diameter are unmyelinated.
RELATIVE SIZES
Structure
(mm)
Visual Cortex (thickness)
5
Cortical column (thickness)
0.25
Neuron cell body
Red blood cell
Neuromuscular junction
Sympathetic axon
CNS synaptic bouton
Dense core synaptic vesicle
Synaptic vesicle
Smooth endoplasmic reticulum
µm
250
15-100
7
2-4
0.2
0.5-2.0
nm
200
500-2000
100
50
5