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Synapse definition and classification

-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


Structure of electrical synapses

-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


Properties of transmission at electrical synapses

-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


Function of electrical synapses

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

2. Species: Sea hare
Connected neurons: motor neuron
Function: ink release

3. Species: Mammals
Neuron: GABAergic interneurons and retinal interneurons
Function: synchronization of activity


Regulation of electrical transmission

-gap junctions frequently closed
-opening regulated by:
1. Connexin phosphorylation by kinases
2.large differences in membrane potentials


Structural features of chemical synapses

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


Structural diversity of chemical synapses

-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

-large dense-core SVs
-peptide NTs


Diversity of chemical synapse location

-axospinous: synapses onto dendritic spins
-structural plasticity + compartmentalization

-axodendritic: synapses onto dendritic shafts
-excitatory or inhibitory

-axosomatic: frequently inhibitory

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


How AP elicit the release of NT

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


How NT receptor activation leads to AP

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


How transmission at chemical synapse is terminated

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


Timecourse of postsynaptic currents and potentials

-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


Direction and Amplitude of currents/potentials

-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
-g=conductance of synaptic ion channels


Excitatory postsynaptic currents and potentials

-EPSCs and EPSPs if they facilitate post synaptic AP
-excitatory if Erev is more positive than AP threshold


Inhibitory postsynaptic currents and potentials

-IPSCs and IPSPs inhibit generation of AP
-inhibitory if Erev is more negative than AP threshold

-Erev hyperpolarization

-Vrest shunting inhibition


Spatial summation of EPSPs

-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


Effects of dendritic cable filtering

-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


Voltage gated conductance in dendrites canamplify EPSP

-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