L-07 Synaptic Physiology

Front 1

Current through electrical synapse

Back 1

High conductance low resistance pathway of hyperpolarizing or depolarizing current through cells (via gap junctions)

Does not allow for flexible system but very fast.

Front 2

Current through chemical synapse

Back 2

Current can pass over one membrane but does not affect adjacent cell.

Front 3

Voltage change due to current in presynaptic and postsynaptic cell - electrical synapse

Back 3

Presynaptic - Voltage rises in exponential fashion with a time constant to a steady state voltage.

Postsynaptic - Same kinetic features of voltage deflection as presynaptic cell, smaller amplitude.

Front 4

Flow through electrically coupled cells

Back 4

Electricity, ions, Ca2+, ATP, G proteins can all flow through. Cells are biochemically coupled.

Front 5

Purpose of gap junctions in astrocytes

Back 5

Equilibrate K+ levels across large area of brain, especially in pathological conditions.

Front 6

Cortical pyramidal neuron feature

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Dendritic spines

70% of synapses in brain are glutaminergic, 80% of these are located on dendritic spines

Front 7

Site of inhibitory and stimulatory synapses on dendrite

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Stimulatory - on dendritic spines

Inhiibtory - on dendritic shaft

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PSD and Synapse strength

Back 8

Higher postsynaptic density = stronger synapse

Front 9

Sites of synapses along axons

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Synapses are formed around boutons

Can be terminal bouton where axon terminates

Mostly on passant boutons - along swellings on axon.

Front 10

Active zone

Back 10

Region specified for vesicle fusion

Front 11

Functions of postsynaptic density

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Anchors neurotransmitter receptors at synapse

Coordinating intracellular signaling by a) positioning intracellular kinases and b) direct interactions with actin cytoskeleton

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Protein forming majority of intracellular part of dendritic spine

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Actin cytoskeleton

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Release site

Back 13

Place where vesicles release contents in active zone.

Each active zone may have one or more than one release site.

Front 14

Most heavily studied synapse in brain

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In hippocampal formation

CA3 neurons synapsing on CA1 pyramidal neurons

Front 15

Unique feature of neuromuscular junction synapse

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Synapse is 20 microns long rather than 1 micron long synapse of CA3/1

Has many release sites on active zone so synapse is strong and reliable - high fidelity

Front 16

Calyx of Held

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Synapse in auditory pathway

Presynaptic axon - coming out of auditory-sensory system

Many hundreds of synapses onto postsynaptic cell body - helps sense high frequency of neurotransmission reliably.

Front 17

Synapses on Purkinje neurons

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Cerebellar granule cells

Climbing fibers

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Cerebellar granule cells - Purkinje neuron synapse

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Cerebellar granule cells - most common neuron in brain. 100,000 granule cells synapses on Purkinje neuron.

Each synapse is from one granule cell.

Front 19

Climbing fiber - Purkinje neuron synapse

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1:1 climbing fiber to Purkinje neuron, but the one climbing fiber makes 10,000s of synapses on Purkinje neuron.

Front 20

Evocation of postsynaptic response

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Presynaptic - passes threshhold. Sodium rushes in and depolarizes, potassium conductance repolarizes membrane.

Postsynaptic - responds after a delay by depolarizing. Excitatory PSP because makes cell more likely to fire an AP.

Front 21

Measuring EPSC and compare with EPSP

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Voltage clamp and measure current of post synaptic cell (EPSC)

EPSC starts at same time but rises more quickly than does EPSP

EPSC is downwards because it is in an inwards current.

Front 22

Reaching threshhold in post synaptic cell

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Larger EPSC induces EPSP beyond threshold, causing action potential in post synaptic cell.

Front 23

Synaptic vesicle size

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40 nm - extremely tightly regulated for given synapse or neurotransmitter because vesicle is stuffed with neurotransmitter, so any change in vesicle size changes amount of neurotransmitter in a quanta

Front 24

Size of synaptic cleft and area of post-synaptic density

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30 nm synaptic cleft - very small to minimize synaptic delay and dilution of neurotransmitter

0.1 um2 area of PSD

Front 25

Number of possible synapses in brain

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10^3 synapses per neuron

10^10 neuron per brain

Unlimited number of synapses practically

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Number of proteins on synaptic vesicle

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500 proteins

Front 27

Number of neurotransmitter molecules/vesicle

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2,000 glutamate

10,000 ACh

Front 28

Density of postsynaptic receptors

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100 (glutaminergic) - 10,000 (neuromuscular junction)/um2

Front 29

Number of molecular species in postsynaptic proteome

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500-1000

Front 30

Is Ca2+ required for release of neurotransmitter?

Back 30

Stimulate presynaptic axon and record from postsynaptic cell.

If remove Ca2+ from extracellular solution, postsynaptic potential is eliminated.

Add glutamate via ionophoresis to test whether postsynaptic cell is still able to respond to glutamate in absence of Ca2+ - it is.

Front 31

Is Ca2+ sufficient to evoke neurotransmitter release?

Back 31

Add photolabile Ca chelator (caged calcium) to presynaptic cell terminal.

Exposing photolabile Ca chelatory to UV light breaks bond and frees Ca2+ into cell.

Also add fluorescent Ca indicator to measure amount of Ca.

More Ca2+ produced induces larger postsynaptic responses.

Front 32

Is there synaptic delay when large amount of Ca2+ released in presynaptic axon?

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Yes

Front 33

Experiment - how many Ca2+ ions are required for neurotransmitter release

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Highly exponential relationship between presynaptic cell calcium concentration and neurotransmitter release - much more release with small increases in Ca.

Slope is 4 or 5 - 4-5 Ca2+ ions must be cooperating to initiate release.

Front 34

Synaptic delay

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Millisecond between beginning of action potential and EPSC

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Late phase of synaptic delay

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Ca2+ entry/activation of fusion - starts near peak of action potential and occurs 0.3 ms before EPSC.

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Can synaptic delay be due to diffusion of transmitters?

Back 36

No - diffusion of transmitters occurs too rapidly

Front 37

Does neurotransmitter release occur via graded amounts or simultaneous release of fixed amounts?

Back 37

Simultaneous released of fixed amounts. Vesicle quantizes amount of neurotransmitters released

Front 38

Miniature EPSPs

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Deprive muscle of Ca2+ to block action-potential mediated transmision.

miniature excitatory postsynaptic potentials still exist.

Fairly stereotypical, not at neuromuscular junction.

Kinetically, mEPSPs look like action potential without full activation.

Front 39

Clues that EPSPs underlies action potential

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All of certain amplitude or greater

Look kinetically like subthreshold potential.

Front 40

Large EPSP at NMJ

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EPSP usually results in action potential and contraction. But you can lose recording electrode.

So, block contraction and reduce amount of release/postsynaptic excitation.

Record miniEPSPs and observe amplitude of evoked EPSP.

Front 41

Peak amplitude of mini EPSP, unitary response and spontaneous EPSP.

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Peak amplitude of unitary response - 0.4 - is same as mini. and same as amplitude of spontaneous EPSP.

Front 42

What EPSPs are evoked when you increase Ca2+?

Back 42

Same EPSP amplitudes (multiplication) are evoked. Just work a little less to get there.

Front 43

What EPSPs are evoked when you increase Ca2+?

Back 43

Curve moved right

Front 44

What EPSPs are evoked when you decrease Ca+?

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Number of failure eveents skyorcket because less Ca2+ to prompt release. Higher number of evoked responses at amplitude 1 (smallest event)

Front 45

Vesicle fusion in unitary synapse

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i.e. CA3/CA1 synapse

Only one vesicle fuses

Pretty weak