Fig. 2.

Synaptic summation is independent of input position. A, Diagram of the experiment. Two microiontophoresis pipettes were positioned on the dendritic tree of a pyramidal neuron in culture. Glutamate was ejected first from each pipette individually and then from both simultaneously. The algebraic sum of the individual potentials was then compared with the actual potential recorded with simultaneous stimuli. B, Averaged results from a representative cell (5 trials). The twolower lines are the responses from each of the pipettes, the dashed line indicates their algebraic sum, and thetop solid line is the simultaneous response measured. Note the overlap between the expected and actual summed depolarizations, indicating linear summation. Calibration: 2 mV, 25 msec. C, Schematic diagram of a neuron showing pipette locations studied, including apical (Ap), second or higher order branches (S), and basal dendrites (B). D, Histogram of the linearity of the summed responses, measured by the ratio of actual to expected peak amplitudes, for different input configurations. No significant deviations from linearity are observed (ANOVA, p < 0.05; number of experiments in parentheses).