Figure 4. Development of evoked EPSCs.

A, ensemble averages of EPSCs generated by a stimulus of threshold intensity (upper traces) and saturating intensity (lower traces) in the chick (a) and the embryo (b). Averaging was conducted for 5–10 traces at each intensity. The decay phase of EPSCs was fitted by an exponential function, thin traces. A single exponential function was used for the threshold EPSCs, and the sum of two exponential functions for the saturating EPSCs. The stimulus intensity and the time constant of the exponential functions are indicated above and to the right of each trace, respectively. The relative amplitude of the second exponential function to the first was 0.04 for the chick and 0.08 for the embryo. B, relationship between amplitude and stimulus intensity (scaled) of EPSCs for the chick (open circles) and the embryo (filled circles). Stimulus intensity was normalized for each neuron with a reference to the saturating intensity, 11–54 V. Amplitudes of EPSCs were averaged over the normalized stimulus intensities within a bin width of 0.2 and have been plotted at the centre of each bin indicated on the abscissa. EPSCs at a stimulus intensity of > 1.0 were evoked by the supramaximal intensity for each neuron, 14–59 V. The threshold intensity was between 0.5 and 13 V. The number of EPSCs within a bin width of 0.2 is indicated in parentheses in B. Ten to ninety per cent rise time (C) and decay time constant (D) of EPSCs measured from the EPSCs generated by the threshold intensity. B-D are from 10 chick neurons and 10 embryonic neurons.