Dr Jon Witton Flashcards
NMJ
Multiple pre-synaptic boutons arise from a single motor neurone - form synaptic connections with a motor end-plate
Folds = increase surface area, increase number of ion channels
nAChR = cis-cis loop receptor; IC cis loop which contains cysteine residues (disulphide bridges); contains consensus sites for S/T and tyrosine protein kinases
Need 2x ACh to bind
Cleft ~ 20nM
[ACh] in cleft = 0.3-1 mM
[ACh] in vesicle = ~260 nM
End-plate potentials (EPPs)
Biphasic
Rapid phase - due to Na+ influx through nAChR
Cleft ~ 20nM
Exponential decay
- nAChRs deactivate (close)
- ACh rapidly hydrolysed + cleared from the synaptic cleft = prevents receptor desensitisation + depolarising block
Decay determined by the closing/deactivation rate = alpha
Depolarising block - prolonged opening (twitching) followed by inactivation of nAChRs, then cannot produce an AP/contraction (fasciculation)
Quantal neurotransmission - NMJ
1 vesicle = 1 quantum - fixed volume/package of NTs therefore generate mEPPs of a similar size
mEPP = quanta - insensitive to TTX (VGSC blocker) = does not require APs; released spontaneously
Experiment:
- Decreased EC [Ca] to 0.45-.09mM = smaller electrochemical gradient (2500-5000-fold)
- Stimulate pre-synaptic neurone - smaller influx of Ca (decreased driving force),
- Decreased probability of ACh vesicle fusion = decreased number of vesicles released (1-3 quanta)
== scaled down!
- Produced a histogram of multiple of mEPP amplitude
- Multiple quanta sum to generate an EPP
Physiological Ca gradients
IC = 200nM
EC = 2 mM
= 10,000-fold gradient
Temporal summation
Post-synaptic potentials (PSP) summate in TIME at the axon initial segment = produce a larger PSP - bring Vm to threshold to fire an AP
Single EPSP = not enough to reach threshold (activate VGSCs); multiple EPSPs (co-occur in time, add up, activate VGSCs)
BUT = non-linear summation occurs - the driving force decreases for subsequent EPSPs (therefore smaller)
The 1st PSP reduces the driving force for the 2nd PSP
Also influenced by the increase in the neuronal conductance due to more channels opening (decreases resistance), therefore more current required!
Influenced by:
- Number of EPSPs
- Amplitude of the EPSPs
- Temporal proximity of the EPSPs (decay over time; closer together in time = larger summation)
- Co-occurence of IPSPs (subtractive)
Spatial summation
PSPs interact in TIME and SPACE
The axon synapses at different spatial locations of the post-synaptic neurone = the temporally co-occuring PSPs summate in space + reach threshold!!
PSPs get smaller as they propagate (due to neuronal resistance) - the degree to how much smaller they get is dependent on the membrane properties, such as the electrical resistance
Good - no non-linear summation occurring because the
local depolarisation via 1 PSP has little effect on the driving force generated at a different synaptic location
Passive + Active dendritic propagation
Passive - no activation of voltage-gated channels through summation of EPSPs
EPSPs propagates down membrane + attenuates due to neuronal resistance
AIS = membrane potential is sub-threshold to generate an AP
Active - EPSPs summate, reach threshold + activate VGSCs = dendritic spike
Propagates along dendrite - activates other VGSCs therefore little attenuation occurs!
AIS = membrane potential reaches threshold = AP fired!
(Initial AP, if had not activated VGSCs would still be sub-threshold at the soma)
Dendritic spikes
Dendrites - summation of EPSPs causes activation of VGSCs which actively propagate spikes = less attenuation + activation of AP at axon initial segment!
Can also get NMDA spikes = activation of NMDARs on dendrites
NMDA = coincidence detector - requires post-synaptic depolarisation to remove Mg2+ blockade and pre-synaptic NT release (glutamate) to bind
Causes Ca influx = cause large depolarisation <10mV
Types of glutamatergic synapses
1:1 bouton/active zone and PSD/dendritic spine
Several boutons arising from 1 neurone forming several boutons, synapsing on several active zones
ie. Motor end-plate = 1 motoneuron, lots of synapse = highly efficacious
Glomerular bouton = 1 pre-synaptic bouton arising from 1 axon; several active zones
ie. Mossy fibre (1) to granule cell synapse (several)
Calyx-type bouton = axon envelope soma; several boutons synapsing onto the soma
Increases efficacy - AP threshold met every time!!!
ie. Auditory system - important adaptation!
Glutamate family
Ionotropic: NMDARs, AMPARs, kainate receptors
** GluN2b found extra-synaptically!
Group I = Gq = 1 + 5 - mostly post-synaptically
Group II + III = Gi/o
2 + 4 = found exclusively pre-synaptically; autoreceptors
AMPARs + Quanta
Fast excitatory synaptic neurotransmission
1 vesicle ~ 2/3000
Cleft ~ [1-2 mM]
Can look at spontaneous mEPPs via adding TTX = not AP-induced quantal release!
CEREBELLUM
Quantal variance decreases during synaptic development = mature EPSCs are multi-quantal (like nAChRs @ NMJ) but vary in discrete steps (due to consistent quanta size)
Glutamatergic spillover transmission
Spillover transmission - receptors have higher-affinity (see less glutamate) and are slowly or non-desensitising
Occur CEREBELLUM:
Climbing fibres synapsing at Purkinje cells - spillover transmission activates AMPARs on Basket cell interneurones
EAATs
EAAT 1/2 = astrocytic specific
EAAT 3/4 = neuronal specific
3Na+ in, 1H+ in, 2K+ out, glutamate in = reliant on Na-K-ATPase
Down-regulated by interleukin
- Interleukin released by P2X7-activated macrophages
- Down-regulation seen in sporadic ALS
- Involved in excitotoxicity = MS, stroke etc.
GABA
Cis-cis loop receptor - like 5-HT3
Cis-cis loop = extracellular - loop formed by 13 conserved residues between 2x cysteine which form a disulphide bond
Pentameric
Pseduosymmetrical = g-b-a-b-a
Synaptic = a1-3
Common = 2x b2, 2x a1, 1x g2
GABA binds at electronegative cleft - a-b2 interface
Benzodiazapines bind: alpha/gamma
Gamma2 = role in clustering of synaptic GABAaRs
Extra-synaptic = a4-6, delta
- Higher affinity
- Slower desensitisation
Conduction = differs between dimers/trimers
- Trimers = higher conduction ~ 25-28 pS (dimers = 15 pS)
- Mature brain = mostly trimers therefore little variation
(Immature brain = some dimers)
ABG (synaptic) –> ABD (extra-synaptic)
= ~5-fold reduction in MBD
= significant reduction in kinetics
Modification
- IC loops = P sites by various kinases + palmitoylation sites (cysteine residues)
- Modifications affect number of receptors at the synapse
GABA drugs
Benzo PAM: interface between gamma2 + alpha1
= therefore insensitive to extra-synaptic (tonic shunting inhibition)
- Increase probability of opening
Barbituates
Low conc. = PAM
Higher conc. = agonist
V high con. = antagonist (physically occludes pore)
- Increase MBD
