PHGY 313 Lecture Notes - Neuromuscular Junction, Excitatory Synapse, Internal Resistance

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8 Feb 2013
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Principle of Synaptic Integration
Most CNS neurons receive thousands of synaptic inputs that activate different combinations of
transmitter-gated ion channels and G-protein-coupled receptors. The postsynaptic neuron
integrates all these signals and gives rise to a simple form of output: action potentials.
Synaptic integration is the process by which multiple synaptic potentials combine
within one postsynaptic neuron.
The Integration of EPSPs
o The most elementary postsynaptic response is the opening of a single
transmitter-gated channel
o The postsynaptic membrane of one synapse may have from a few tens to
several thousands of transmitter-gated channels
How many of these are activated during synaptic transmission depends
mainly on how much neurotransmitter is released.
o Quantal Analysis of EPSPs
The elementary unit of neurotransmitter release is the contents of a signle
synaptic vesicles
Each contain about the same number of transmitter molecules
o The total amount of transmitter release is some multiple of
this number
o Consequently, the amplitude of the postsynaptic EPSP is
some multiple of the response to the contents of a single
vesicle
Postsynaptic EPSPs at a given synapse are
quantized.
At many synapses, exocytosis of vesicles occur at some very low rate in
the absence of presynaptic stimulation
This tiny response is a miniature postsynaptic potential
Each of these is generated by the transmitter contents of one
vesicle
Quantal analysis is a method of comparing the amplitudes of miniature
and evoked postsynaptic potentials
can be used to determine how many vesicles release
neurotransmitter during normal synaptic transmission
Analysis at the neuromuscular junction reveals that a single action
potential in the presynaptic terminal triggers the exocytosis of
about 200 synaptic vesicles, causing an EPSP of 40 mV or more.
At many CNS synapses, the contents of only a single vesicle are
released in response to a presynaptic action potential, causing an
EPSP of only a few tenths of a millivolt.
o EPSP Summation
The neuromuscular junction has evolved to be fail-safe
It needs to work every time and the best way to ensure this is to
generate an EPSP of a huge size
In the CNS most neurons perform more sophisticated computations
requiring that many EPSPs add together to produce a significant
postsynaptic depolarization
This is what is meant by integration of EPSPs
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