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Post-Synaptic Events
Graded vs Action Potentials
Excitatory vs. Inhibitory PSPs
Pre- vs. Post-Synaptic Inhibition
Divergence vs. Convergence
One synapse is generally not sufficient to
generate a threshold stimulus at the axonal
hillock.
Most neurons receive thousands of synaptic endings each of which contributes a postsynaptic potential
(EPSP or IPSP) of less than 1 mV.
Each of these PSPs are conducted to the axonal
hillock with decrement (decay).
An action potential will only be generated if the
sum of the IPSPs and EPSPs reaching the axonal
hillock at any time exceed the threshold voltage.
Dendrite of
postsynaptic
neuron
Axon terminals
of presynaptic
neuron
Axon
Glial cell
processes
Dendrite of
postsynaptic
neuron
Spatial
summation
excitatory
inhibitory
A
D
E
B
C
electrode
threshold
A
A+B A+B+C
D
D+E
A+B+C+E
Postsynaptic Inhibition
Presynaptic Inhibition
Divergent
circuit
Allows for
amplification
of a signal:
single pyramidal cell in the motor
cortex of the brain
15-20 internuncial cells in CNS
each stimulates several hundred motoneurons
each of these stimulates 100-300 muscle fibers
Divergence results in 50,000-fold amplification
Convergent circuit
Important
for
summing,
correlating
and sorting
information
in the CNS