Fig. 5.

Comparison of electrical and optical measurements of membrane voltage made simultaneously in CA1, illustrating that the delayed depolarization was a property of neural as well as other membranes. The microelectrode recording was made from a pyramidal neuron soma, and one of the optical signals was taken from the same region of st. pyramidale. Note that although the optical signals were clearly related to the microelectrode signal, the action potential peaks were less prominent relative to the slower depolarizations. This may reflect temporal dispersion of the action potentials as well as contributions of optical signals originating in glial membranes that will depolarize but not support action potentials. The delayed depolarization was most evident in optical recordings from st. radiatum (see also Figs. 2, 3), and discrete secondary action potentials were not always recorded from this region. The large steps in the microelectrode recording are the result of encoding this signal in an image pixel without conditioning amplification; this was done to ensure temporal registration.