Hentall I D
University of Illinois, College of Medicine, Rockford, IL 61107-1897.
J Neurosci Methods. 1991 Dec;40(2-3):181-91. doi: 10.1016/0165-0270(91)90067-a.
This paper describes how an extracellular microelectrode may be used to stimulate neurons with brief, rectangular pulses and afterwards directly record the resultant activity. Two obstacles are the stimulus artifact lingering in the electrical circuitry and transient tip potentials (TTPs) arising from ion depletion at the electrode-tissue interface. Electronic switching between the stimulus source and the recording amplifier eliminates direct stimulus artifact from the electrical circuitry, although high but acceptable switching artifact remains. TTPs revert with time constants that are prominent in the desired recording (0.1-1 ms) and can reach 50 mV when more than 1 microA passes through a typical electrolyte-filled micropipette (for example 2-4 M omega, filled with 3 M NaCl, and placed in 0.1 M NaCl). They are always negative when cations flow into the tip, they are accompanied by a rise in microelectrode impedance, and they increase as a function of the resting electrode impedance, the duration and amplitude of applied current, and the dilution of the external electrolyte. TTPs were substracted by differential recording and stimulation through matched micropipettes (one in the brain and one in contiguous electrolyte) and in addition were reduced by pressure ejection of electrolyte. Directly elicited spikes (single or multiple) were detected about 0.5 ms after delivery of a rectangular stimulus pulse in the cerebellar cortex of pentobarbital-anesthetized rats. Typically, 3-4 units could be excited by less than 3 microA cathodal currents at any recording site. All-or-nothing properties, thresholds, and refractoriness to a second pulse within 2-4 ms verified the neuronal nature of the recorded signals. Complex wave forms, probably generated synaptically, were also seen. The technique of coincident extracellular recording and stimulation can be used as a universal search stimulus during microelectrode penetrations through the brain and in determining threshold-distance relations for extracellular stimulation. Where cell penetrations are unstable, it might be usefully substituted for intracellular technique in testing a neuron's behavioral or physiological influences or in exploring a cell membrane's response to drugs (in terms of excitability rather than voltage and impedance).
