The nerve cell is polarized in the resting state, so it conducts electric signals. Early studies during the 18th century studies were conducted regarding the electrical stimulation of the neurons. Scientists observe that when a dissected nerve is supplied with electricity, the muscle innervated by the nerve contracts.
In the early 20th century, electrical recording of the brain impulses was developed. A sensitive voltmeter was initially used to measure the electrical activity of neurons.
Thus, the blanks can be filled with stimulation; recording.