Quantal Current Fields around Individual Boutons in Sympathetic Ganglia

The release of a quantum of neurotransmitter from an active zone of a bouton is accompanied by the flow of extracellular current that creates a potential field about the site of transmitter action beneath the bouton. It is shown theoretically that the density of the field at the peak of the quantal current gives rise to an extracellular potential that declines to values of less than 5 μV at 1.3 μm distance in the circumferential direction around the neuron and equally rapidly in the radial direction away from the neuron. A loose-patch electrode placed over a bouton distorts the quantal field about the bouton and calculations show that under current-clamp conditions, potentials of over 40 μV can be recorded with an electrode of tip diameter 2 μm, provided the separation between the tip and the neuronʹs surface is about 0.1 μm. Quantal release recorded from visualized boutons on rat monopolar pelvic ganglion cells with loose-patch electrodes is in agreement with the properties of the quantal potential field given in the theoretical analysis.