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(A) During intracellular recordings, an action potential is initiated from a negative resting membrane potential (e.g., ~55 mV), reaches a peak membrane potential (solid arrow), and is followed by a repolarization of the membrane and usually an afterhyperpolarization. An inflection in the rising phase of the spike (open arrow) is often observed. This reflects the delay between the initial segment spike that initiates the action potential (occurring prior to the open arrow) and the somatodendritic action potential that it triggers (occurring after the open arrow). (B) A computer was used to differentiate the membrane voltage deflection occurring in the action potential in (A) with respect to time, resulting in a pattern that shows the rate of change of membrane voltage. Note that the inflection is exaggerated (open arrow), and the peak of the action potential crosses zero (solid arrow), because at the peak of a spike, the rate of change reaches zero before reversing to a negative direction. (C) A trace showing a typical action potential in a dopamine neuron recorded extracellularly. The extracellular action potential resembles the differentiated intracellular action potential in (B). This is because the extracellular electrode is actually measuring the current crossing the membrane during the action potential and is therefore, by definition, equivalent to the absolute value of the first derivative of the voltage trace in (A). The amplitude of the extracellular spike is indicated in volts, because the parameter measured is actually the voltage drop produced across the electrode tip by the current flux and is therefore much smaller than the actual membrane voltage change that occurs in (A).

Source. Adapted from Grace AA, Bunney BS: "Intracellular and Extracellular Electrophysiology of Nigral Dopaminergic Neurons, I: Identification and Characterization." Neuroscience 10:301-315, 1983. Copyright 1983, International Brain Research Organization. Used with permission.

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