The techniques for examining resting potentials can be extended to study the action potential. Although nerve action potentials are normally initiated by mechanical, chemical, or photic stimuli to classes of specialized receptors or by a process known as synaptic transmission (see Chapter 6), it is possible to elicit action potentials artificially in nerve cells and study their underlying ionic mechanisms in considerable detail and in a controlled fashion.
Figure 3 shows another idealized nerve cell with its cell body and attached axon. One microelectrode has penetrated the cell membrane so that the tip of the electrode is inside the cell. This electrode will be used to monitor the potential difference between the outside and inside of the cell. When this electrode penetrates the cell, a resting potential of about —60 mV is recorded. The cell is also impaled with a second microelectrode that will be used to alter the membrane potential artificially. This second electrode, called the stimulating electrode, is connected to a suitable current generator (in the simplest case, this current generator can be considered a battery). Obviously, there are two ways that a battery can be connected to any circuit. The battery can be inserted so that either its positive pole or negative pole is connected to the electrode. A switch is placed in the circuit so that the battery can be connected to and disconnected from the circuit at will.
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