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 probably many other regions of the brain. Such smooth muscle cells are in the walls of blood vessels and in the gasterointestinal tract, for example in sphincters. 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. Another location for impulse formation is sensory receptor neurons, when excited.
Sensory neurons develop depolarizing currents in response to their normal stimulus.
Type 3 and until you see the spike train. The slow depolarization to threshold that occurs between spikes is called the pacemaker potential. See that by changing the value of the current you can change the interval between the impulses. Discuss how this result illustrates, or is used by, sensory receptors to signal stimulus intensity. The long processes of most vertebrate neurons are myelinated.