A Study of the Inferior Mesenteric and Pelvic Ganglia of Guinea-pigs with Intracellular Electrodes

1. Ganglion cells in the inferior mesenteric ganglion (IMG) and the pelvic plexus of the guinea-pig were studied using intracellular micro-electrodes.2. Ganglion cells had resting membrane potentials of 55-65 mV. Threshold for initiation of an action potential ranged from 10 to 20 mV depolarization. Action potentials often exceeded 100 mV in amplitude and were followed by an after-hyperpolarization of up to 20 mV.3. Synaptic responses were recorded from cells in the IMG in response to stimulation of the right and left hypogastric nerves, ascending mesenteric, inferior splanchnic and colonic nerves. It has been established that more than forty preganglionic fibres converge on any one cell. Preganglionic fibres to the IMG were also observed in the pelvic nerves.4. In contrast to the IMG, ganglion cells in the pelvic plexus received up to ten preganglionic fibres.5. Ganglion cells responded to supramaximal preganglionic stimulation with up to four action potentials.6. In the IMG, action potentials in response to synaptic action were followed by a prolonged period of hyperpolarization (after-hyperpolarization) and a later phase of prolonged depolarization (after-depolarization). The time course of these after potentials depended on the pattern of firing of action potentials during the period of stimulation. In the presence of dihydro-beta-erythroidine, or if synaptic action was insufficient to evoke action potentials, only the after-depolarization was observed.7. Other cells were impaled whose properties differed from those described above. In one group of cells the resting membrane potentials were higher (up to 85 mV), input resistances lower and the threshold for initiation of an action potential was higher. The other group were inexcitable, had high resting membrane potentials (up to 85 mV), low input resistances and underwent a slow depolarization in response to repetitive stimulation of preganglionic fibres.8. This study indicates that marked convergence of presynaptic fibres occurs on to ganglion cells of the IMG. The ganglion cells in the pelvic plexus receive a relatively small number of fibres, many of which exert intense synaptic activity ensuring a direct connexion to the central nervous system.