PHYSIOLOGICAL AND MORPHOLOGICAL PROPERTIES OF NEURONS IN SPHINCTER OFODDI REGION OF THE GUINEA-PIG

Intracellular recordings and dye injections were used to investigate n eurons located in ganglia of the sphincter of Oddi (SO) region in guin ea pigs. Four types of neurons were encountered based on physiological properties. The two most abundant types, tonic and phasic, had simila r membrane characteristics and morphologies but yet could be different iated by their spiking characteristics. Tonic cells spiked throughout a depolarizing current pulse and were sometimes spontaneously active. Phasic cells fired only a single action potential at the onset of a cu rrent pulse regardless of stimulus amplitude or duration. Both tonic a nd phasic cells had Dogiel type I morphologies. They typically had a s ingle long process and several very short processes emanating from the soma. NADPH diaphorase activity was demonstrated in cells with simila r morphologies, indicating that nitric oxide may be an intrinsic trans mitter in some of these cells. Cells with a prolonged afterhyperpolari zation (AH cells), similar to the type 1/AH cells of the gut, were rar ely encountered. This finding was consistent with the observation that very few calbindin D-immunoreactive neurons exist in this region. Act ion potentials could not be generated in the fourth type of neuron, ca lled nonspiking neurons, even though they did receive synaptic input. Most tonic and phasic cells received at least one nicotinic fast excit atory postsynaptic potential (EPSP). In addition, both slow EPSPs and inhibitory postsynaptic potentials were observed. Most AH cells receiv ed only slow excitatory synaptic input.