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
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
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
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
can be used to determine how many vesicles release neurotransmitter during normal
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
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
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 membrane.
The length constant ( ) is an index of how far depolarization can spread down a dendrite or
The longer the length constant, the more likely it is that EPSPs generated at distant
synapses will depolarize the membrane at the axon hillock.
Electrically passive dendrite depends on two factors:
The resistance to current flowing longitudinally down the dendrite internal resistance
The resistance to current flowing across the membrane membrane resistance
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 dendrite
The value of will decrease as internal resistance because more current will flow across
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 membranes
The dendrites of spinal motor neurons are very close to passive.
However, many other neuronal dendrites are not passive
A variety of neurons have dendrites with significant numbers of voltage