Electrotonic characteristics and membrane properties of neurons in the inferior mesenteric ganglion in guinea-pig

Intracellular recordings were made from neurons (n = 75) in the inferior mesenteric ganglion (IMG) in guinea-pig to study the electrotonic characteristics and membrane properties of IMG cells which receive an excitatory, cholinergic input from mechanoreceptors in the gastrointestinal tract. An excitatory, cholinergic innervation from the periphery served as an index to identify the sympathetic neurons involved in the reflex inhibition of muscle tone when the gut is distended. Functionally identified neurons in the IMG were categorized into 4 subclasses (I, II, III and IV). Subclasses I and II comprised neurons which fired phasically (rapidly adapting), with the neurons in subclass II showing a voltage relaxation in the electrotonic potentials elicited by depolarizing current-clamps. Subclasses III and IV comprised neurons which fired tonically (slow adapting), with the neurons in subclass III also showing relaxation of electrotonic potentials. Active and passive membrane properties were determined for neurons in each of the 4 subclasses of IMG cells. Measured values for the charging time-constant, the threshold current and the voltage threshold for firing (as well as calculated values for the input capacitance, specific membrane resistance, total surface area, cell diameter and cell space-constant) distinguished the neurons classed as phasic-firing from the neurons classed as tonic-firing. There were no statistical differences between the membrane properties of subclass I and II phasic neurons, or the membrane properties of subclass III and IV tonic neurons, to explain why the neurons in subclasses II and III showed a relaxation in electrotonic potentials during current-clamp. In the light of recent voltage-clamp data on the IMG cells [5,9], the actions of time conductances for potassium ions are discussed to account for the variations in the electrotonic behavior of these IMG cells.