Extracellular recording

The firing patterns of either single clusters or neurons of neurons in living animals in response to physiological stimuli are obtained through extracellular recording. This method uses two best electrodes commonly of tungsten or stainless steel. First one the exploring focal electrode is placed as close as possible to a neuron and the second one indifferent electrode is placed at a convenient distance. The Neuron activity will cause currents to flow among the two electrodes. These currents are amplified and fed to a computer. By convention, if the exploring electrode is positive with respect to the indifferent electrode an upward deflection is recorded. The polarity, amplitude, shape and timing of the recorded waveform generated through neural activity will depend on the position of the electrodes. The nearer the exploring electrode is to the neuron the larger the measured signal.  Changing the distance among the two electrodes or altering their relative positions will modify all the above parameters. Each of this can make extracellular recordings hard to interpret.

The method can be used in brain slices or other in vitro preparations or in vivo for instance, in anesthetized animals. A particularly useful method inserts electrodes into the brain which are attached to a connector cemented into the skull. This is done under anesthetic. The animal is allowed to recover. As needed, the recording circuitry amplifier to computer is plugged into the connector. This allows electrophysiology in conscious, behaving animals, by using sophisticated experimental protocols over long time.