Fig. 4.

Supralinear summation of synaptic inputs from two sources. A, Averaged results from a representative AP cell (8 trials). The preparation was in a saline with high Ca²⁺ concentration to avoid polysynaptic pathways. The two bottom traces are the responses from individual ventral and dorsal P cell stimulation. The somata of both P cells were contralateral to the AP cell soma. In this case, the EPSP from P_v is smaller than from P_d, although the relative sizes varied in different preparations. The gray trace indicates the algebraic sum of the two EPSPs. Thetop trace is the response to simultaneous stimulation of both P cells; its peak is 122% of the algebraic sum of individual stimulation. B, Peak EPSP from simultaneous stimulation of contralateral dorsal and ventral (▪, 21 cells) or ipsilateral dorsal and ventral (■, 6 cells) P cells expressed as % of algebraic sum at different membrane potentials (mean ± SEM; 5–10 trials). The arrow indicates the zero-current membrane potentialV_o. Inset, EPSP peak amplitude from simultaneous stimulation of dorsal and ventral P cells for the data points shown in the main figure, showing that the EPSP amplitudes decrease with hyperpolarization, as in Figure3B.