The electrical activity of the dorsal unpaired median neurone: past directions

past directions Historically, many electrophysiological studies performed in vivo indicated that the somata of some insect neurones were electrically inexcitable (Hoyle and Burrows, 1973) or displayed only attenuated action potentials (Crossman et al., 1971) that corresponded, sometimes, to electrical activity propagated electrotonically from more or less distant regions of active membrane. However, a distinct population of neurones, with somata located along the dorsal midline of many ganglia of the insect ventral nerve cord, constituted a notable exception to this rule. These neurones, which made symmetrical left and right branches, were originally described by Plotnikova (1969) in locusts and were later named dorsal unpaired median (DUM) neurones (Hoyle et al., 1974). Like many other insect neurone somata located in the ganglion, DUM neurone cell bodies (45–60 μm in diameter) were readily accessible and easily penetrated with intracellular microelectrodes. The first few reports indicating that the action potentials of DUM neurones were generated intrasomatically came from studies performed on cell bodies located on the dorsal surface of the terminal abdominal ganglion (TAG) of the cockroach Periplaneta americana (Callec and Boistel, 1966; Kerkut et al., 1968; Jego et al., 1970; Crossman et al., 1971). These electrophysiological studies proved that the cell bodies were capable of generating spontaneous overshooting action potentials and that Na+ was responsible for the depolarizing phase of the action potentials (Jego et al., 1970). A substantial number of electrophysiological investigations over the last 20 years have demonstrated an unexpected complexity regarding the electrophysiological properties of DUM neurones. The progeny of the median neuroblast (see Goodman, 1982; Boyan and Ball, 1993; Condron and Zinn, 1994) displayed a broad spectrum of electrical properties related to their neural geometry. It was reported, particularly in cockroach and locust, that adult DUM neurones exhibited 1633 The Journal of Experimental Biology 203, 1633–1648 (2000) Printed in Great Britain © The Company of Biologists Limited 2000 JEB2703