Synaptic Integration: Lectures 15-20 Flashcards
Where are neurotransmitters stored?
Most (glutamate, GABA-glutamate, Noradrenaline, Acetylcholine) are stored in vesicles 40-50nm. Neuropeptides and somatostatins are contained in larger denser vesicles (>100nm)
Why are neurotransmitters stored in vesicles?
Management of concentration Protection from degradation Regulation
How are vesicles produced and recycled?
- Components of synaptic vesicle delivered to plasma membrane 2a. Endocytosis of synaptic vesicle to form new vesicles directly 2b. Endocytosis of components and delivery to endosome 3b. Budding of synaptic vesicle from endosome 4. Loading of neurotransmitter into synaptic vesicle 5. Secretion of neurotransmitter by exocytosis in response to an action potential. Insert diagram
How is neurotransmitter released?
Stimulation (depolarisation- either natural or induced) leads to vesicular fusion with plasma membrane Dependent on calcium.
Why is neurotransmitter release dependent on calcium?
The two are cooperative: Ca2+ influx –> 3 or 4 fold increase in release.
What is the influx of calcium triggered by?
Voltage gated calcium channels of different types: - L: long lasting - P/Q: transient - N: neither (Neural) - R: resistant (residual)
What is the ionic requirements for a release of neurotransmitter?
Elevation in intracellular calcium levels - Not related to Na+ or K+ -Single channel opening unlikely to cause release
Where are the calcium channels located?
Close to the binding site in order to create the largest rise in intracellular calcium levels possible.
Why are P/Q channels so important?
Responsible for all ACh release and the majority of Ca2+ mediated release. - no other channel can compensate for ACh release - R and N channels can compensate for Ca2+ mediated release.
How is the amount of vesicular release quantified?
Electrical signal generated from the post-synaptic membrane is proportional to amount of neurotransmitter binding. Increase neurotransmitter –> increase PSP
Mini: release of individual vesicle at the neuromuscular junction, roughly equivalent to 10,000 molecules of Ach. This produces a mini PSP.
- Further electrical stimulation –> depolarisation in multiple of mini
Define Quanta?
Quanta: release of individual synaptic vesicles at the neuromuscular junction
Quantal release in the thalamus evokkes an excitatory post synaptic current (doesnt always have to be excitatory)
What evidence is there at vesicular release is quantal?
Depolarisation is a multiple of minis
K+ channel blockers –> similar response
Mean number of quanta = mean amplitude of EPSP, depending on probability (size and shape of depol, state of calcium channels, baseline calcium level, number of docked / primed vesicles, phosphorylation of presynaptic proteins)
Binomial probability
Explain binomial probability
Number of vesicles (n) is equal to the number of trails
Each trail has (p) probability of success
- Trails are independent so do not affect one another
N * P = mean of distrubution
N*P*(1-p)= variance –> SQR –> SD
Q = quantal context (mean mini ampltidue
Therefore: N*P*Q or N*P*(1-p)*Q
What links does vesicular release have to other things?
Inflammation- increases the release probability of vesicles –> pain
Nerve ligation (phosphorylated GluR1 receptor) –> increased probability of release
Botox blocks release of vesicles to prevent contraction of facial muscles
Define a channel:
When open provides a continous pore through the bilayer, allowing the flow of many ions
Cycles between open and closed conformations
Gated by: ligands, voltage, membrane stretch (leakage)
Define a transporter:
Solute binding site
Avaliable on one side of the bilayer or the other
Carries a few solute molecules per cycle.
- Slower than channels
What types of transporters are there?
Uniport = 1 ion’s path through
Symport = 2 ions transported together
antiport= 1 ion in 1 ion out
Can be active (ATP requiring, against gradient) or passive (down the gradient)
What are some examples of passive transporters?
GLUT- : transports glucose into cells
Cl-/ HCO-3 anion exchanger : regulates pH
What are examples some examples of active transporters?
Ca2+ ATPase, Na+:K+ ATPase - both in plasma membrane
H+K+ ATPase: in parietal cells (acidic environments)
How do transporters cooperate?
Active transporters can aid passive (Na+:K+ ATPase aids glutamate transport)
3Na+ and 1 H+ in and 1K+ out for 1 glutamate in
Also true for GABA transporters (1+ net uptake)
2Na+ and 1Cl- in for every GABA in.
What is electrochemical potential
If there is a potential difference in charge / concentration ions can diffuse through open membranes
Equilibrium potentials can also be calculated using the Nernst Equation = Eion= RT/zF x ln([ion]out/[ion]in)
Give some examples of reversal potentials?
How is glutamate transported?
- Glutamate converted to glutamine in glial cells
- Transported to pre-synaptic cells
- Glutamine converted back to glutamate
- Loaded into vesciles and then transported across the synapse
Where are glutamte transporters located?
Glial cells
Pre and post-synaptic cells
Close to terminal as increased stimulation –> increased transportation
Where are GABA transporters localised?
Pre- and post synaptically
On glial cell (GABA –> GABA transaminase –> Glutamate)
What is the link between glial and glutamte transporters?
1 to 1 relationship at the climbing fibre - Purkinje Cell synapse
Prevents glutamte spillover
What are the effects of Glutamte spillover
Slow rising CF-EPSC
Prolonged IPSCs (in mice)
Abnormal motor behaviour
What is the role of synapses, transporters and neurotransmitters in ischaemia?
- Decreased cerebral blood flow –> decreased ATP
- Leads to increased intracellular ion (Na, Ca, Cl, H20) and extracellular K+ concentrations
- Leads to neurotransmitter release
- Increase Ca inside the cell
- Cell death and damage
What link does calcium have to cell death and damage
Influx driven by glutamte through NMDA and AMPA receptors.
Released from intracellular stores and enters through VGCC.
NMDA receptor antagonists prevent damage
Complete interuption of blood flow to brain for 5 minutes –> severe damage to core zone
Leads to disruption of ionic graident –> glutamate in a non-calcium dependent manner.
How complex is the human nervous system?
10^14 synapse
11^11 neurons
Why is synaptic integration important?
Neurons recieve multiple input and provide multiple outputs.
Integration allows for information processing and determines nervous system function
What affects synaptic integration?
Neuronal morophology and distribution - complexity, distance from soma, relative positioning
Synaptic properties- amplitude of current flow at synapse, AP firing
Membrane potentials
- time constant –> temporal summation, contact sites simultaneously activated
- length constant –> spatial summation –> PSPs from different neurons summate.
What affect does synaptic integration have on output?
Total effect on soma membrane potential is the summation of all synaptic potentials
What is the purpose of Purkinje cells?
Integration of neurons in the cerebellum
Recieves input from climbing fibre and parrallel fibre
Describe a climbing fibre:
1 per Purkinje Cell but with many synaptic connections
Large synaptic current can trigger complex spike
Describe a parallel fibre:
Each purkinje fibre recieves thousands of parallel fibre inputs, but only one contact site
Describe syanpses across a purkinje cell, why are there so many?
Size of synaptic input is unimportant because it can summate causing the threshold to be reached?
Why does length constant affect spatial synaptic integration?
Amplitude reduces with distance based upon the length constant.
This is determined by membrane resistance and axial resistant as I= SQRT( Rm/ Ra)
Decrease Rm –> decreased leakage
Lower axial resistance –> less resistance along dendrites
Does it matter where the inputs come from
Inputs arrive from same distance to soma –> depolarisation = input 1 + input 2
Inputs arrive from different distance –> Depolarisation doesnt equal input 1 + input 2
- Earlier membrane potential causes ion channel opening, changes in membrane resistance and changes in the length constant.
- Makes it harder for the second input to spread.
How does temporal summation work?
A train of action potentials arrive.
If the PSP and time constant short then there is no temporal summation due to the decay of the first input
If PSP and time constant longer this can lead to summation where one PSP superimposes on earlier.
Decay is linked to time constant which depends on membrane reistance and capacitance t= Rm x Rc
- Less leakage –> increased increase till decrease - More charge is stored and discharged
What is the difference between temporal integration and coincidence detection?
Short time coincidence - coincidence detection, as EPSPs must arrive ‘simultaneously’ to summate
Longer time constant- greater period allowed for summation, so frequency of AP’s important –> temporal integration
What is a temporal integrator?
Requires a long-time constant?
Most / all EPSPs contribute to final APs
Output is independent of input in terms of timing
Output / activity proportional to input activity
Describe coincidence detection:
Short time constant so the timing of inputs are crucial - only coincidenct inputs produce an action potential
What do excitatory and inhibitory inputs correspond to?
Excitatory
Inflow of positive charge - Na+ flows in
Inhibitroy
Positive outflow or negative inflow - K+ out or Cl- in
What happens when you combine an excitatory and inhibitory input?
PSP output will have a membrane potential somewhere between the two inputs
Describe linear summation:
Excitatory and inhibitory synapses arrive from the same distance to the soma on different dendrities
Leads to combined effect
Describe what happens when there is an inhibitory synapse between the excitatory synapse and the soma on the same dendrite:
NON-linear summation
- Counteracts the current flow initiated at excitatory synapse (outflow of the positive charge), due to a lower of membrane resistance - change in the length constant
- This affects the spread of the EPSP
- The two PSPs combine in an unpredictable manner
Describe uncoupling of dendrites by inhibitory synapses?
Blocks the propagation of synaptic potentials to the soma.
Excitatory input is only recieved from dendrities with no inhibition
What is silent postsynaptic inhibition?
Synaptic Reversal Potential = Resting membrane potential
- Inhibitory PSP’s will have no affect on membrane activity despite being completely active, until it combined with an excitatory input
- Causes an inhibitory shunt on excitatory input
What mechanism does silent postsynaptic inhibition work by?
Ohms Law: V= R x I
Where V= change in Vm, R= membrane resistance and I= synaptic current
Inhibitory inputs –> decrease in membrane resistance –> decreased change in Vm
What is a good example of silent post-synaptic inhibition?
GABA- main inhibitory neurotransmitter in mammalian CNS.
Works via Chlorine channels- chlorine’s reversal potential is close to resting membrane potenital
How do you study temporal and spatial integration?
Problem: Normal recordings even if only one neuron activatied –> multiple synapses
Computational modules:
- As good as underlying assumptions, model and diagram - need to check real data
Photolysis of caged neurotransmitters:
UV light releases glutamate which was previously kept inactive. Neuron filled with fluroscent dye to visual process. UV light applied to small specific areas - input –> output (patterns –> distinct responses)
What is homonsynaptic short-term synaptic plasticity
Variation in amplitude of synaptic potenitlas due to prior activity.
Occurs mainly during high frequency stimulation
Can either be facilitative- synapse become more effective
Or depressive: synapse become less effective
- Requires a long gap between action potentials to reset
What are some examples of homo-synaptic short-term synaptic plasticity?
Climbing fibres undergo synaptic depression
Parallel fibres undergo synaptic facilitation
Schaffer collateral undergoes synaptic facilitation then depression
How is homosynaptic short-term synaptic plasticity classified?
Paired-pulse ratio:
Second pulse larger than the first- ratio >1 = facilitation
Ratio <1 = depression
What is the mechanism for synaptic facilitation?
1st Action Potential causes Ca2+ influx –> release of transmitter and priming of other vesicles (increase in the number of primed vesicles)
2nd action potential –> greater release of neurotransmitter
A train of AP’s leads to spike broadening
- longer depolarisation, greater Ca2+ influx, increased neurotransmitter release, increased synaptic response
What is the mechanism for synaptic depression?
1st Action Potential –> release of neurotransmitter from docked vesicles –> less vesicles dock
2nd Action Potential –> decreased release of neurotransmitter
Leads to progressively weaker post-synaptic potentials
Is there any pattern between the probability of synapse firing and faciltation or depression.
High probability synapse is associated with synaptic depression
Low probability synapses are associated with synaptic facilitation as more neurotransmitter can be primed.
Define homosynaptic:
1 pre-synaptic neuron to 1 post-synaptic
Define Heterosynaptic:
More than one pre-synaptic input
How are heterosynapses post-synaptically modulated?
Give an example
Modulatory input alters postsynaptic membrane sesnsitivity to neurotransmitter
E.g. GABAa receptor modulation by phosphorlyation
- Activation of PKA –> phosphorylation of GABAA ==? enhancing or suprresion depending on site
- Can lead to a change in receptor number –> change in sensitivity.
- Insulin promotes receptor addition, BNDF promotes removal
How are heterosynapses pre-synaptic modulated
Modulatory input affects pre-synaptic transmitter release- either inhibitory or facilitation
Inhibitory input causes a removal of excitatory input –> reduced Ca2+ influx –> reduced neurotransmitter release –> reduced PSP
What is an example of heterosynaptic facilitation?
Gill siphon withdrawal reflex in Aplysia californica
Sensitisation
Tactile stimulation –> weak response
Tail shock –> tactile stimulation –> enhanced withdrawal response
What is the mechanism for heterosynaptic facilitation?
Serotonin (5-HT) activates receptor –> activation of adenylyl cyclase –> increase in cAMP –> activates PKA –> phosphorylation of VGKC –> reduced K+ outflow –> AP broadens –> more Ca2+ influx –> increase neurotransmitter release
What are the different mechanisms for synaptic modulation
Pre-synaptic
- altered vesicle release
- altered Ca2+ entry
- altered vesicle recycling
Post-synaptic
- Altered receptor function
- Altered receptor number
What is long term potentiation?
Why is it dependent on plasticity?
A short burst of high frequency stimulation (tetanus) causes an elevated EPSP afterwards.
- Causes a lasting effect in the slope of the EPSP, this can last for a long time
Why is long-term potentiation important?
Reveals that the hippocampus is important for learning and memory (first discovered there)
- If removed then no new memories can be saved/ generated. (patient HM)
What forms of long-term synaptic plasticity are there?
Long term-potentiation
Long term-depression
Can be NMDA receptor dependent or independent
- can lead to long or short term changes in synaptic strength
