Generating an action potential
Normally, sensory neurons receive natural stimuli such as touch, triggering electrical responses. In this experiment, an electrical stimulation replaces the natural one.
Stimulation changes the voltage of the cell to a more positive value. If the stimulus is strong enough and raises the cell voltage above a threshold value, the neuron generates an action potential.
A line graph labeled action potential moves steadily then sharply up to a spike and then descends to the starting level all before the mid-point of the graph.