Simultaneous Single Unit Recording in vitro with a Photoetched Laser Deinsulated Gold Multimicroelectrode Surface

Flat photoetched gold conductors 12 μm wide and 2 μm thick, situated on a glass plate and deinsulated at their tips with single 8 ns UV laser pulses, have been utilized to record single unit extra-cellular activity from brain ganglia of the snail Helix pomatia. A fixed array of 36 such conductors, terminating in six rows and six columns in a 0.5 mm x 1 mm area, is capable of monitoring simultaneous single unit activity from numerous neurons. Spike amplitudes of 300-500 μV are generally observed from the predominant 40 μm diameter cells of these ganglia. Giant neurons usually produce signal amplitudes of over 3 mV that are simultaneously seen by many electrodes. Signals can be monitored by merely resting a locally desheathed ganglion on the recording area under its own weight in a shallow pool of Ringer. A 10 μm diameter crater in a 3-4 μm thick insulation layer has an impedance of 2-4 MΩat 1 kHz. The capacitance of this metal electrolyte interface is about 0.5 pF/μm2, suggesting that the UV laser produces a partially colloidal gold surface. With this recessed tip design, simultaneous single unit recording from small neurons appears ensured if electrode impedances are below 4 MΩ, shunt impedances are above 30 MΩ, and glia cells are not allowed to reinsulate the end of the gold conductor.