INFLUENCE OF THE MUCOSA ON THE EXCITABILITY OF MYENTERIC NEURONS

Intracellular microelectrodes were used to examine the active and pass ive membrane properties of neurons in the myenteric plexus of the guin ea-pig small intestine. Neurons of two types were examined: S neurons, which have prominent fast excitatory postsynaptic potentials and in w hich action potentials are not followed by long-lasting afterhyperpola rizations, and AH neurons, which have long-lasting afterhyperpolarizat ions following soma action potentials. In preparations in which the my enteric ganglia and longitudinal muscle, but no mucosa, were present, most S neurons (59/64) responded to intracellular depolarizing current with brief bursts of action potentials. Regardless of the strength of a depolarizing current of 500-ms duration, these neurons never fired action potentials beyond the first 250 ms. S neurons in this state wer e called rapidly accommodating. In contrast, within 600 mu m circumfer ential to the intact mucosa, 26/58 S neurons fired action potentials f or most or all of the period of a 500-ms insightful depolarizing pulse . S neurons in this state were called slowly accommodating. Depolariza tion of S neurons in the rapidly accommodating state caused a rapidly developing reduction in membrane resistance (outward rectification; on set about 7 ms). This rectification was absent from S neurons in the s lowly accommodating state. Tetraethylammonium blocked the early rectif ication and the changed neuronal state from rapidly accommodating to s lowly accommodating. Application of tetrodotoxin to neurons in the slo wly accommodating state revealed the early rectification, indicating t hat its absence from these neurons before tetrodotoxin was applied had been due to ongoing activity in axons providing synaptic input to the neurons. After the mucosa was disconnected from the other layers and laid back in its original position, ail S neurons close to the mucosa were in the rapidly accommodating state (17/17). Slow excitatory posts ynaptic potentials, evoked by electrical stimulation of nerve Tracts, converted 17 of 43 S neurons from rapidly accommodating to slowly acco mmodating and eliminated the early outward rectification in these neur ons. These results indicate that the action potential firing propertie s of S neurons can be changed by external influences, including the ac tivity of synaptic inputs that release a slowly acting transmitter. Sp ontaneous antidromic action potentials were recorded in 8/62 AH neuron s within 600 mu m circumferential to the intact mucosa. II is conclude d that, when the mucosa is intact, a background firing of sensory neur ons occurs which leads to a state change in many S neurons innervated by the active sensory neurons. We conclude that this state change is c aused by the block of a voltage-sensitive outward rectification. Copyr ight. (C) 1996 IBRO.