Morphology of electrophysiologically identified myenteric neurons in the guinea pig rectum

Dye-filled microelectrodes were used to investigate relations between morphology and electrophysiological behavior of neurons in the myenteric plexus of the guinea pig rectum. The neurons were divided into two general classes on the basis of morphology. The first class had smooth ovoid somas with multiple long processes that exited the ganglion in several different fiber tracts and fit the description of Dogiel type II neurons. Neurons of the second class had a single long process that exited the ganglion in a fiber tract and sometimes projected to the circular muscle. Neurons of the second class were subdivided into three groups. One of these had short club-shaped dendrites like Dogiel type I neurons. Another had short tapering filamentous processes. Neurons with a single long neurite that could not be classified made up the third group. AH/type 2 electrophysiological behavior was found in both general classes of neurons. S/type 1 electrophysiological behavior occurred only in neurons of the general class with a single long neurite. Type 3 electrical behavior, characterized by robust fast synaptic input in inexcitable neurons, was found only in the cells defined by a single long neurite. Single-spike neurons behaved electrophysiologically like AH/type 2 neurons without long-lasting hyperpolarizing afterpotentials. This behavior was limited to the class of neurons characterized morphologically by a single long neurite.