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REVIEWS
DENDRITIC INTEGRATION OF
EXCITATORY SYNAPTIC INPUT
Jeffrey C. Magee
A fundamental function of nerve cells is the transformation of incoming synaptic information into specific patterns of action potential output. An important component of this transformation is synaptic integration the combination of voltage deflections produced by a myriad of synaptic inputs into a singular change in membrane potential. There are three basic elements involved in integration: the amplitude of the unitary postsynaptic potential, the manner in which non-simultaneous unitary events add in time (temporal summation), and the addition of unitary events occurring simultaneously in separate regions of the dendritic arbor (spatial summation). This review discusses how passive and active dendritic properties, and the functional characteristics of the synapse, shape these three elements of synaptic integration.
CABLE FILTERING
Dendrites have been commonly modelled as cables and the flow of current between two points of a dendrite has been usually assumed to decay as a result of filtering along the process.
HEBBIAN PROCESSES
Plastic processes that require temporal coincidence between incoming synaptic activity and postsynaptic depolarization.
AXIAL RESISTANCE
The resistance to the flow of ionic current along an axon or a dendrite. Axial resistance decreases as a function of the radius of the process and increases as a function of its length.
Most neurons of the mammalian central nervous system receive thousands of excitatory and inhibitory synaptic inputs that are widely spread across their intricate dendritic arbors. Whereas synaptic input is broadly distributed, action potential output usually occurs in a more localized region of the proximal axon1,2. Because of this spatial arrangement, the distance between the various synaptic inputs and the final integration site can vary to a great degree. The combination of this large variation in synaptic distance and the CABLE FILTERING properties of dendrites can, in theory,
cause the amplitude and temporal characteristics of functionally similar inputs to be highly variable at the final integration site38.
The impact of any given synapse on the neuron firing will depend primarily on the location of the synapse along the dendrite, unless the filtering properties of the dendrites are compensated for by specific mechanisms. This location dependence could be so profound that even the basic elements of synaptic integration would depend on the pattern of spatial input. Furthermore,...