Neuronal Cell Biology Flashcards
neuronal doctrine
1) neurone is structural and funcitonal unit of NS
2) neurones are individual cells not continuous to other neurones (some are, so wrong)
3) 3 parts - dendrites, soma, axon
4) conduction from dendrites to soma (polarity)
initial segment (axon hillock)
AP origin
dendritic spines
blebs, post-synaptic elements, synapses with axon, lots activity in env means lots spines so plasticity, receive input from axon but signals can travel both ways
longest human neurone
just over 1m
so instead of moving things, you synthesise locally and derive products from other cells
retrograde
terminals up axon to soma
5mm a day
transport absorbed material or degraded membrane
anterograde
soma down axon to terminals
300-400mm a day / 5-10m
what is used for axonal transport
with neurofilaments, microtubules, motor proteins with ATP
Amyotrophic lateral sclerosis (ALS)
Stephen Hawking
is transport goes wrong, loss function in motor neurones, so degenerate because not active
local protein synthesis evidence
if inactivate neuronal enzyme then should be in axon according to proximo-distal gradient if everything is moving
expect greater activity where made and little long distance away but this wasn’t the case, both ends had equal rates of recovery of enzyme so mRNA of enzyme moving
some proteins are in synaptosomes but depleted in cell body so not made in cell body
transfect isolated dendrites - inject mRNA should translate if have all correct things for it
axonal transport
3’ untranslated regions on mRNA signal to carry it by motor proteins to post-synaptic density
lots mRNAs in dendrites
neuronal activity regulated translation and protein targeting
nanotubes
connect cells so neurones share using motor proteins
can move mitochondria from healthy to damaged neurones
glial cells
most abundant cell in CNS
surround neurones for support
role in synapse - can release NT
e.g. astrocyte, microglia, oligodendrocyte
astrocyte
mop up NTs, keep correct ionic env
microglia
scavengers
GFAP
glial fibrillary acidic protein
marker for glia
intermediate filament protein in glia
oligodendrocyte
forms myelin sheath
in CNS wrap over 50 neurones
what are synapses made up of
tripartite
so 2 neurones and glial cells
gap junctions
neurones electrically connected, don’t need NT
exosomes
share through extracellular vesicles
myelin is only in..
vertebrates because invertebrates make neurones bigger for faster transmission
myelin function
saltatory conduction (ions out at node only)
increases conduction velocity - 10x faster
insulated neurones
DONT MENTION SKIPPING
schwann vs oligodendrocytes
schwann cell (in PNS) wraps round 1 neurone oligo wraps 50, more flexible (in CNS)
myelin structure
wraps and spiral from inside out for compaction (done by proteins)
immunoglobin superfamily:
MBP
PLP, PMP-22, Po
MAG
myelin basic protein (in CNS)
huge +ve charge so attract everything together and drive compaction of myelin
PLP in CNS, PMP-22 and Po in PNS, same job, link lipid layers to wrap myelin
myelin associated glycoprotein (in CNS and PNS, more in CNS)
tucking under to start spiralling inside (initiation)
EAE (experimental autoimmune encephalomyelitis)
mouse model for multiple sclerosis
affects T-helper lymphocytes,invade PNS/CNS so demyelinate
demyelination
cause over 50 neuropathies like MS etc.
adrenoleukodystrophy is target for gene therapy
can remyelinate with stem cells
axonal guidance
process by which neurons send out axons to reach their correct targets
3 levels (pathway, target, address) identify what neurone is, make neurite with growth cone, receive signals so migrate to roughly right place (target) then makes selection (address)
cerebellum axon guidance
millions of same units synapses with big purkinje cell so makes connections with very defined neurones
granule cells in cerebellum make a million synapses into 1 purkinje cell (purkinje recieve impulses from granule)
mossy fibres only make 1 synapse
neurite and growth cone
is not yet axon and synapse because not electrically active yet
growth cone
actin+microtubule extension of developing neurite, seeking synaptic target
has instructions for finding route
sensing and movement
can carry on if cut cell body off
treadmilling
10-20um in 7 mins (not quick)
adhesion
membrane addition
actin polymerisation
adhesion (axon movement - treadmilling)
filopodia with microtubules stressing behind it (inside) so extend into it
filamentous actin on tip of filapodia
ECM with adhesive things
receptors (which bind substance) bind ECM inside (connected to filapodia)
2 roles - growth (attach to substrate) and guidance (navigation)
receptors on surface of growth cone touching adhesive properties of ECM that are guidance and target signals
membrane addition
F-actin can treadmil
move membrane from part of filopodia to the front
actin polymerisation
(after adhesion and membrane addition) THEN bind to next receptor to next adherence molecules
senses adherence molecules on surface, transmits signal to actin and moves it
actin treadmils, membrane moves, filapodia moves, is quick
ladder pathways
pathways in Drosophila embryo where neuroblasts divide and make neurones, sending projections up/down/across
midline is middle of ladder
BP102 Abs show ladder
rungs of ladder segmentally repeated
mutations lacking guidance molecules in Drosophila
no ladder
- roundabout - few longitudinal fascicles
- commissureless - nothing in middle (ones that cross are called commissures so lacking)
fascicles
bundle of structure of nerve
examples of pathways of Drosophila guidance molecules
and mutations
Ap in wt goes up and stays ipsilateral (on the side) on same side
pCC very ipsilateral
SP1 project contraleteral (cross to opposite side) then up
axon guidance tells to cross/not cross middle/edge
robo mutation everything crosses midline
comm mutation nothing crosses midline
axon navigation
- neurone identified by signalling/TFs - knows has to get to target A
- receives signals on the way like a relay, attractant molecules like netrin as well as adhesion molecules
- can be inhibitory molecules, same ones but act diff to diff neurones
target recognition
when get to target, stick to address with adhesion
guidance molecules examples
N-cadherin in EM (close neural tube), MAG (tucks myelin), DCC (receptor for slit), EPH (organise projections through eye)
Robo receptor
Robo receptor (roundabout)
on membrane (transmembrane receptor), binds to stuff, guidance by repulsion
mutation means lose repulsion from midline so go round and round
(slit ligand for robo in Drosophila affects guidance in embryo)
repulsion
growth cone grow towards neurone, contact, collapse back and repel but leaves contact but massively goes back - dynamic instability
(axon guidance repulsion)
netrin
attractive cue attracts neurones towards midline
cells (mostly glial) sit on midline and secrete netrin
slit
mRNA in same cells as netrin
so attract some neurones, repel other
slit mutant is like robo mutant
comm
Controls axon guidance across the CNS midline by preventing the delivery of Robo to the growth cone
comm on - binds to robo and directs it to lysosome for destruction, no robo so no receptor for slit so cross midline because attracted to netrin and robo can’t repel, but once crossed comm is downregulated so can’t go back
comm off - robo on so don’t cross
comm mutation
comm always off
no crossing
always ipsilateral side
robo mutant
always cross so roundabout
robo in drosophila
has 3 robos responsible for each tract (3 lines going up)
which pathway depends on which robo (middle, inside, outside)
guidance is different in vertebrates (mammals)
mutations
no comm
contralateral sensory neurone cell body sends projection to floor plate
sonic hedgehog attractant in midline
every projection crosses floor plate (like midline in fruitfly)
robo and slit important
3 robos encode cell surface molecules
robo 3 changes isoforms (3.1 3.2)
just robo 3 is null so never crosses midline, if knockout all robo it’s same as wildtype
synaptic refinement purpose
adhesion molecules and code is not enough to wire NS (target)
need activity to refine synaptic connections (address)
Sperry and Cajal
Sperry did epilipsy, cut hemispheres, evidence for Cajal’s neuronal interconnections from chemical recognition
structure of optic pathway
optic nerve to lateral geniculate nucleus (tectum), nerves cross optic chiasma, 2nd order neurones from tectum, connections to visual cortex in back of brain
projections from retina to tectum - anterior of retina to posterior of tectum and posterior retina to anterior tectum
frog evidence (Sperry)
frog optic nerve cut out and switched round so image other way round
showed that not learn from exp but is genetic
because changing optic nerve placement still went to same pathway so changed how you see and didn’t adjust
ephrin
ligand
secreted by tectum, project through optic nerve, direct to right area of tectum
chemospecificity hypothesis
neurons make connections with their targets based on interactions with specific molecular markers and, therefore, that the initial wiring diagram of an organism is (indirectly) determined by its genotype.
problems with chemospecificity hypothesis
experience and neuronal activity plays some role
don’t know code - would need huge no. combinations if each connection required diff molecule
doesn’t explain learning
BUT DSCAM gene in drosophila could explain hypothesis (but still problems with it)
DSCAM
lots exons so alternatively spliced, so enormous and could explain chemospecificity
but splicing only in some insects
mutation - not much happened in neural tubes in mammals only retina (amacrine cells in retina lose repulsion so stick together), not much splicing
vision - eyepatch kitten experiment
radioactive tracer in eye
2 weeks - tracer is uniform in layer 4C (3rd order synaptic connections)
3 weeks - change
5.5 weeks - segmenting
13 weeks - ocular dominance columns (stripes of neurones)
if both eyes blind - uniform distribution so not natural columns, need experience
so retinal activity required for precise synaptic connectivity in visual system (to make columns)
patch over 1 eye - inputs dominate from open eye
ocular dominance columns
stripes of neurons in the visual cortex of certain mammals that respond preferentially to input from one eye or the other
plasticity mouse evidence
deprived/bored –> neurones degenerate, pruned, less active, less receptors
so activity required for neuronal growth and underlies learning
aplysia
large sea slug
shows how learning changes neurones and receptors change density
axons that fire together..
wire together
2 fires simultaneously will consolidate and synapses become stronger
neurotrophins
secreted protein required for neuronal survival
signal to cells to survive differentiate and grow
acts as reward for neurone being active (plasticity) retrograde release from postsynaptic when pre signals to post
so if 2 neurones then more reward (consolidate strength)
venom exp
into fertilised chicken eggs, drive formation of neurones so nerve growth factor in venom
neurotrophin experiment
Ab against BDNF (brain derived neurotrophic factor) caused uniform distribution of tracer like in both eyes blind exp. so blocks reward circuit
