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William H. Calvin
University of Washington
Seattle WA 98195-1800 USA
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Dendritic Synapses and Reversal Potentials:
Theoretical Implications of the View from the Soma
WILLIAM H. CALVIN
Departments of Neurological Surgery and of Physiology and Biophysics,
University of Washington School of Medicine,
Seattle, Washington 98105
Received February 5,1969The dendritic location of synapses, and the usual intrasomatic location of the recording and stimulating Microelectrode, will cause the postsynaptic potential (PSP) to be relatively insensitive to changes in the somatic resting potential. When reversal of a PSP is attained, this same decoupling effect causes the measured reversal ("equilibrium") potential to be in error by a large factor. When the synapse is located one-half space constant away, the reversal potential will appear to be 65% further away from the resting potential than it is at the subsynaptic site. At one space constant, the error is 171%. Other experimental design considerations in CNS synaptic physiology are: (a) The height-voltage (P-V) curve for a composite PSP may be predicted, showing that nearby PSP will mask more distant contributions. (b) If a large distant EPSP and a smaller nearby IPSP are mixed together, giving the appearance of a somewhat smaller EPSP, a steep P-V curve will result which could be misinterpreted as implying a somatic EPSP. (c) Biphasic PSP may result from spatially distributed synapses or from IPSP contamination. It is concluded that these problems may be avoided by the use of single-bouton PSP, which eliminate the possibility of spatial summation, temporal dispersion, and IPSP contamination. The demonstration of the conductance increase model for EPSP, as opposed to electrical synapses and electrogenic pumps, depends strongly upon such design considerations.1 The author is grateful to Katherine Graubard, who collaborated on the IPSP contamination work contained in this paper, and to John N. Barrett, for much useful discussion and suggestions. The Friday Harbor Laboratories are to be thanked for their hospitality during the preparation of the manuscript. Supported by NIH Grant NB 04053.is work was supported by the National Institutes of Health, grants NB 04053, NB 05934, NB 01752, and STI GM 260.