Synaptic Transmission Flashcards

1
Q

Synapse definition and classification

A
  • specialized zone of contact at which one neuron communicates with another
  • 10^11-10^12 neurons in human brain
  • average neuron has 1000 synapses
  • 10^15-10^16 synapses in the brain alone
  • electrical synapses: junctions between neurons permitting direct, passive flow of electrical current
  • chemical synapses: junction between neurons that communicate via secretion of NT
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2
Q

Structure of electrical synapses

A
  • electrical synapses are gap junctions
  • gap junctions are sites of close apposition (3nm)
  • precisely aligned, paired hemichannels made of connexins
  • gap junction made up of 10^3 gap junction channels
  • each gap junction channel made up of 2 hemi channels
  • each hemmichannel made of 6 connexins
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3
Q

Properties of transmission at electrical synapses

A
  • pores of gap junctions are wide and non selective
    • diffusion of ions and other small compounds
  • fast latency (<0.1ms)
  • post synaptic potential changes have same sign but lower amplitude
  • usually bidirectional
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4
Q

Function of electrical synapses

A

1.Species: cray fish, teleost fish
Connected neuron: motor circuit
Function: fast flight response

  1. Species: Sea hare
    Connected neurons: motor neuron
    Function: ink release
  2. Species: Mammals
    Neuron: GABAergic interneurons and retinal interneurons
    Function: synchronization of activity
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5
Q

Regulation of electrical transmission

A
  • gap junctions frequently closed
  • opening regulated by:
    1. Connexin phosphorylation by kinases
    2. large differences in membrane potentials
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6
Q

Structural features of chemical synapses

A
  1. Presynaptic bouton
  2. Synaptic vesicles containing NT
  3. Active zone: specialization where NT exocytosed
  4. Synaptic cleft: extracellular space between neurons
  5. Postsynaptic specialization: contains receptors and signalling/scaffolding proteins
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7
Q

Structural diversity of chemical synapses

A
  • Asymmetrical (Gray Type I): mostly excitatory
  • Symmetrical (Gray Type II): mostly inhibitory
  • small SV with little electron density
    • amino acid NTs, Acetylcholine
  • Small electron dense SVs
    • monoamines
  • large dense-core SVs
    • peptide NTs
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8
Q

Diversity of chemical synapse location

A
  • axospinous: synapses onto dendritic spins
    - excitatory
    - structural plasticity + compartmentalization
  • axodendritic: synapses onto dendritic shafts
    - excitatory or inhibitory

-axosomatic: frequently inhibitory

-axo-axonic: inhibitory
-dendro-dendritic: inhibitory
Neuromuscular junction

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9
Q

How AP elicit the release of NT

A
  1. AP arrives
  2. Voltage gated Calcium channels open
  3. Ca2+ triggered exocytosis of NT
  4. NT binds to receptor
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10
Q

How NT receptor activation leads to AP

A

A. Ionotropic: ligand gated ion channels, non selective
-current is fast in onset, decays quickly

B. Metabotropic: GPCRs that initiate opening of ion channels (K+)
-Current is slow in onset, long-lasting

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11
Q

How transmission at chemical synapse is terminated

A
  1. Voltage gated Na+ channels inactivate
  2. K+ channels open -> depolarization
  3. Calcium channels close after depolarization
  4. Na+/K+-ATPase, PM Ca2+-ATPase reestablish ion gradients
  5. NT is removed from synaptic cleft
  6. Some ionotropic receptors desensitize
  7. Postsynaptic potential dissipates
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12
Q

Timecourse of postsynaptic currents and potentials

A
  • individual ligand-gated ion channels open for few ms; close as ligand unbinds or desensitizes
  • synapse: many channels open simultaneously, close at different times => fast rise time, slower decay time

-post-synaptic potential has a slower rise and decay time due to capacitive property of membrane

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13
Q

Direction and Amplitude of currents/potentials

A
  • flux of ions determined by electrochemical gradient
  • if Vm = Erev (reversal potential) no net charge of transfer across membrane
  • if VmErev, efflux of cations (influx of anions)
    • outward current

-the greater the difference between Erev and Vm, the greater the driving force, greater synaptic current
I=g*(Vm-Erev)
-g=conductance of synaptic ion channels

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14
Q

Excitatory postsynaptic currents and potentials

A
  • EPSCs and EPSPs if they facilitate post synaptic AP

- excitatory if Erev is more positive than AP threshold

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15
Q

Inhibitory postsynaptic currents and potentials

A
  • IPSCs and IPSPs inhibit generation of AP
  • inhibitory if Erev is more negative than AP threshold
  • Erev hyperpolarization
  • Vrest shunting inhibition
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16
Q

Spatial summation of EPSPs

A
  • EPSPs derived from activation of single synapses cannot elicit APs by themselves
  • AP threshold can be reached through spatial summation of simultaneously activated excitatory synapses
  • inhibitory input contributes also
17
Q

Effects of dendritic cable filtering

A

-synapses on distal dendrites are at a disadvantage of eliciting AP in axon hillock due to electronis decay as EPSP is propagated to initiation site

18
Q

Voltage gated conductance in dendrites canamplify EPSP

A
  • in many neurons, coincident activation of clustered excitatory synapses can lead to opening of dendritic voltage gated sodium or calcium channels
  • resulting dendritic spike amplifies EPSP