The effects of physostigmine on synaptic transmission in the inferior mesenteric ganglion of guinea‐pigs

Synaptic potentials were recorded with intracellular electrodes from cells in the inferior mesenteric ganglion of the guinea-pig.2. Half-widths of the synaptic potentials recorded fell into two groups: type L cells had long synaptic potentials (11.6-15.2 msec) and low thresholds (14.6 mV mean), type S cells had short synaptic potentials (6.1-9.3 msec) and high thresholds (29.9 mV mean).3. Physostigmine (1.2 x 10(-6)M) caused a significant increase in the half-width of both types of synaptic potential.4. Physostigmine caused a significant increase in the half-width of spontaneous synaptic potentials and an increase in their amplitude.5. Repetitive preganglionic stimulation, in the presence of physostigmine, led to a marked and prolonged depolarization in all cells. In most cells repetitive spontaneous firing of action potentials was then observed. This effect was blocked by atropine (1.4 x 10(-7)M).6. The effect of atropine on the half-width in a physostigmine-treated cell was inconsistent: although synaptic potentials in some cells were slightly shortened their half-widths were always greater than the control.7. It is concluded that cholinesterase plays a role in limiting the time course of the synaptic potential, by limiting the duration of action of acetylcholine.