Measurements of the specific membrane properties and neuronal geometry of cat motoneurones were used to calculate the excitatory postsynaptic potentials (e.p.s.p.s) produced by unit (quantal) conductance changes occurring at various locations on the dendritic tree. Calculations demonstrate that conductance changes of 80-190 x 10-10 mho are required to produce e.p.s.p.s having the same rise time and peak amplitude as the quantal e.p.s.p.s recorded in motoneurones by Kuno & Miyahara. Because quantal conductance changes are so large, synaptic activity can significantly reduce the effective specific resistance of the motoneuronal membrane. A quantal conductance change occurring at a high impedance distal dendritic site is calculated to produce an e.p.s.p. of 15-20 mV peak amplitude at that site. Significant non linear summation will occur between the e.p.s.p.s produced by conductance changes occurring simultaneously on the same dendritic branch. Calculations which take into account both non linear summation and the loss of synaptic charge through dendritic membranes predict that for these motoneurones the time integral of soma recorded quantal e.p.s.p.s originating on distal dendrites should be at least 20% as great as the time integral of a quantal e.p.s.p. originating directly on the soma. Quantal conductance changes occurring on 76% of the dendritic tree should produce soma e.p.s.p. time integrals at least 50% as great as those produced by somatic synapses.
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