Download 17-01-05 1 Golgi - stained neurons Neuronal function

17-01-05
Golgi - stained neurons
Neuronal function is intimately related to structure and structures are
fantastically complex. Your brain is an organ but it is not a glorified liver.
Externally recognizable parts of a typical
neuron:
- Soma (cell body)
- Axon and axon collaterals (usually but not
always)
Dendrites (spiny or smooth)
Presynaptic release sites (terminals, boutons
and non-localized release sites)
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Someneuronalmorphologiesfoundinthehumannervoussystem
Cell body:
also called “soma” plural “somata”
main metabolic centre of neuron
main (but not only) site of protein synthesis
lots of mitochondria
lots of endoplasmic reticulum
size in vertebrates:
small: 8 µm e.g. granule cells in cerebellum
large: 50 µm layer V motor cortical neurons
largest: 200 µm Mauthner cell in fish brainstem
size in invertebrates
small: 8 µm procerebral neuron in slug procerebral
lobe
GIANT: >600 µm R2 neuron in Aplysia (sea slug)
abdominal ganglion
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Dendrites:
- arise from cell body
- primary sites of input from other neurons:
# of inputs to cell determined by size and complexity
of the dendritic “arbor”
- can release transmitter in some neurons
- contain ribosomes which means they can
potentially make proteins locally!
- vary in diameter… generally thick proximal, thinner
distally
-contain lots of actin
Dendrites:
- arise from cell body
- primary sites of input from other neurons:
# of inputs to cell determined by size and complexity
of the dendritic “arbor”
- can release transmitter in some neurons
- contain ribosomes which means they can
potentially make proteins locally!
- vary in diameter… generally thick proximal, thinner
distally
-contain lots of actin
Axons:
- arise from cell body (vertebrate neurons)
- specialized for relaying electrical signals
- vary in length … a few 10’s of microns, to many cm.
- contain microtubules and microtubule binding proteins
- relatively constant diameter in any neuron
- always have specialized areas that release
neurotransmitter -- terminal or en passant
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axon in red, dendrites in blue
An example of a local interneuron… axon branches only near the
soma
These structural features: axons and dendrites serve the
primary functions of neurons to generate, send, receive,
and integrate (process) electro-chemical signals that
encode information.
The physiolgical processes required for neural
communication will be the focus between now and the
midterm exam
Neurons communicate with each other via synapses
Where are they? What are they? How do they work?
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It’s time to start looking at neuronal structure at a higher level
of resolution – submicron scale (electron microscopy);
micron scale (confocal, multiphoton, widefield microscopy
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axon
dendrites
axon
Presynaptic
transmitter release site
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Transmitter released onto
receptors 15nm away across
a synaptic cleft
“Directed” synapses
axon
Axo-dendritic
Axo-somatic
Axo-axonic
dendro-dendritic
dendrite
“Non-directed”
hormonal/neuromodulatory
axon
Transmitter released onto
receptors 1µm to mm’s away
axonic (e.g. serotonergic brainstem neuron endings in cortex)
dendritic (e.g. dopaminergic ventral tegmentum neurons)
Dendrites of vertebrate central nervous system neurons have
dendrites
vertebrate
brain are
“spiny”
smallMany
branches,
oftenin
with
small “spiny”
protrusions
Nimchinsky, Sabatini, Svoboda, 2002 Ann Rev Physiol 64:313-353
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An example of a “structure-function” relationship: Dendritic
spines and compartmentalization of calcium influx.
Excess pumped out
Only a little Ca
escapes the spine
to the dendrite
Most is bound to Ca
buffers, e.g.
parvalbumin
Ca2+
Enters through ligand-gated or
voltage-gated channels
Spine can compartmentalize biochemical processes
Spine head can also produce amplification of local voltage
changes if voltage dependent channels are in spine head.
Axo-dendritic synapses
Enpassantsynapse
1 µm
Fiala and Harris,1999 In: Dendrites, Stuart, Spruston and Hauser eds.
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Circuits are made from connected neurons
Convergence…..defines by the number of inputs to a single
neuron. e.g. purkinje cell up to 100,000
Divergence …. number of targets innervated by one neuron
Neurons connect via synapses to create circuits:
Monosynaptic, disynaptic, polysynaptic, feedforward and
feedback circuit elements (draw on board….)
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