ELECTROPHYSIOLOGICAL AND MORPHOLOGICAL CLASSIFICATION OF MYENTERIC NEURONS IN THE PROXIMAL COLON OF THE GUINEA-PIG

Intracellular recordings were made from myenteric neurons in the proxi mal colon of the guinea-pig. The electrical behaviour of the neurons i n response to intracellular depolarizing current pulses, and to intern odal strand stimulation, was recorded. The intracellular electrode con tained the intracellular marker biocytin which was injected into impal ed neurons for subsequent histochemistry. Proximal colon myenteric neu rons displayed electrophysiological properties similar to myenteric ne urons in the small intestine, and were classified as either AH- or S-n eurons. AH-neurons were characterized by the presence of a slow afterh yperpolarization following an action potential. Internodal strand stim ulation evoked slow excitatory synaptic potentials in five out of six AH-neurons tested, but did not evoke fast excitatory synaptic potentia ls in 26 AH-neurons tested. S-neurons lacked a slow afterhyperpolariza tion, but internodal strand stimulation evoked fast excitatory synapti c potentials in all 113 neurons and slow excitatory synaptic potential s in seven out of 17 tested. A subpopulation of AH-neurons displayed a rhythmic oscillation in membrane potential which could be triggered b y an action potential. S-neurons could be subdivided into those that f ired tonically and those that fired phasically in response to long dep olarizing current pulses. About 80% of the AH-neurons were immunoreact ive for calbindin, as were 10% of S-neurons. A further 17% of S-neuron s, but no AH neurons, were calretinin immunoreactive. Morphological an alysis of filled neurons revealed eight distinct classes. Neurons elec trophysiologically classified as AH typically had a large, oval soma a nd several long tapering processes. Processes of AH-neurons branched i nto many adjacent ganglia. Almost all S-neurons were uniaxonal and man y axons ended in an expansion bulb in the myenteric plexus. S-neurons typically had broad, lamellar processes, or short, spiny processes. Ro ughly equal proportions of S-neurons had oral or anal projections. How ever, almost all S-neurons that were immunoreactive for calbindin or c alretinin projected orally. The results indicate that myenteric neuron s in the proximal colon of the guinea-pig are electrophysiologically s imilar to myenteric neurons in the small intestine, but there are a gr eater number of morphological and chemical categories.