The video below, which I obtained from the website of Prof. Tom Yin , shows an oscilloscope trace of a recording from a single auditory nerve fibre in vivo. The video is somewhat of a "classic" (they don't make oscilloscopes like that any more) but auditory nerve fibers haven't changed, and the video remains instructive. The crackling that you here in the background, and the spikes that you see in the green trace on the oscilloscope screen, are nerve impulses fired by the auditory nerve fibre. They are recorded from a very fine recording electrode (a microwire or a glass pipette) inserted into the auditory nerve of an anestehtized experimental animal. (No animals were hurt during the making of this movie). The first thing to notice is that the nerve fibre fires spontaneously at random intervals "at rest", when no sounds are presented to the experimental animal. When a 400 Hz pure tone is played, the firing rate increases, and the firing pattern becomes a lot more regular, with inter-spike intervals of approximately 1/400 of a second (or 1/200 of a second) becoming much more prevalent, as the nerve fibre's firing patter becomes entrained to the period of the sound stimulus.