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
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
Each contain about the same number of transmitter molecules
o The total amount of transmitter release is some multiple of
o Consequently, the amplitude of the postsynaptic EPSP is
some multiple of the response to the contents of a single
Postsynaptic EPSPs at a given synapse are
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
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
This is what is meant by integration of EPSPs EPSP summation represents the simplest form of synaptic integration in
the CNS. There are two types: spatial and temporal
Spatial summation is adding together of EPSPs generated
simultaneously at many different synapses on a dendrite
Temporal summation is the adding together of EPSPs generated
at the same synapse if they occur in rapid succession (within 1 –
15 msec of one another).
The Contribution of Dendritic Properties to Synaptic Integration
o Even with the summation of several EPSPs out on a dendrite, the depolarization
still may not be enough to cause a neuron to fire an action potential
o The effectiveness of an excitatory synapse in triggering an action potential
depends on how far the synapse is from the spike-initiation zone and on the
properties of the dendritic membrane.
o Dendritic Cable Properties
Assume that dendrites function as cylindrical cables that are electrically
passive (lacking voltage-gated ion channels).
There are two paths that synaptic current can take
One is down the inside of the dendrite
The other is across the dendritic membrane
At some distance from the site of current influx, the EPSP amplitude may
approach zero because of the dissipation of the current across the
The length constant ( ) is an index of how far depolarization can spread
down a dendrite or axon.
The longer the length constant, the more likely it is that EPSPs
generated at distant synapses will depolarize the membrane at the
Electrically passive dendrite depends on two factors:
The resistance to current flowing longitudinally down the dendrite
The resistance to current flowing across the membrane
Most current will take the path of least resistance
The value of will increase as membrane resistance increases
because more depolarizing current will flow down the inside of the
The value of will decrease as internal resistance because more
current will flow across the membrane.
The internal resistance depends only on the diameter of the dendrite and
the electrical properties of the cytoplasm
It is relatively constant in a mature neuron
The membrane resistance, in contrast, depends on the number of
open ion channels
o Excitable Dendrites
Some dendrites in the brain have nearly passive and inexcitable
The dendrites of spinal motor neurons are very close to passive. However, many other neuronal dendrites are not passive
A variety of neu