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PSYC 410 (48)


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McGill University
PSYC 410
Richard Koestner

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  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 axon.  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 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 membranes  The dendrites of spinal motor neurons are very close to passive.  However, many other neuronal dendrites are not passive  A variety of neu
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