An electrophysiological study of inferior mesenteric ganglion of the dog

1. Intracellular recordings were made in vitro from 430 sympathetic neurons in the inferior mesenteric ganglion (IMG) of the dog. 2. Ganglion cells had resting membrane potentials between -35 and 70 mV; input resistance (R(in)) was approximately 22 MΩ. Cell rheobase to depolarizing current was 0.3 nA, and the action potential elicited was 80-100 mV in amplitude followed by an afterhyperpolarization of up to 15 mV in size, which decayed to the resting membrane potential over a range of 50-500 ms. 3. Neurons were classified as either phasic (188 of 280) or tonic (92 of 280) firing cells, depending on their discharge pattern in response to depolarizing current. 4. Two hundred eight of 430 neurons showed continuous electrical activity in the form of spontaneous excitatory postsynaptic potentials, 2-5 mV in amplitude. Continuous electrical activity was unaffected by tetrodotoxin (3 x 10^-6 M) but abolished by hexamethonium (10^-4 M). 5. A small number of cells (21 of 430) adopted a repetitive firing pattern not associated with injury discharge. These cells may have been pacemaking neurons. 6. Stimulation of peripheral and central nerves resulted in multiple synaptic input to ganglion cells. There was marked convergence of excitatory fibers to any one cell. Evoked synaptic potentials were abolished by hexamethonium (10^-4 M). 7. Synaptic input from peripheral and central nerves vould not be correlated with location of postganglionic neurons in the IMG. The possibility of neuronal intercommunication and dissemination of central and peripheral commands is discussed.