Spike train Many nerve and muscle cells have spontaneous activity - even with no active inputs they fire repeatedly. The heart pacemaker region does this, as do neurons in the respiratory center, and many other regions of the brain. Any nerve or muscle cell can be made to be spontaneously active by sufficient depolarizing current, which forces the cell to depolarize to threshold and fire impulses. Naturally spontaneous cells have structures in their plasma membrane which cause this depolarization. Another location for impulse formation is sensory receptor neurons, when excited.
Sensory neurons develop depolarizing currents in response to their normal stimulus.
Type enter until you see the spike train. The SLOW depolarization to threshold, that occurs between impulses, is called the pacemaker potential. See that by changing the value of the current you can change the interval between the impulses. Describe how your result illustrates, or is used by, sensory receptors to signal stimulus intensity.