Extracellular current flow and potential during quantal transmission from varicosities in a smooth muscle syncytium

A discrete model has been developed that describes the extracellular current that flows in a smooth muscle syncytium upon the secretion of a quantum of transmitter onto a smooth muscle cell in the syncytium. This allows a description to be given of the current (called the excitatory junctional current (EJC)) recorded by an electrode of given diameter placed on the surface of the muscle, during synaptic transmission from a varicosity situated anywhere in the muscle. The EJC is of maximum negative amplitude when the varicosity is at the surface of the muscle near the inside rim of the electrode and decreases as the varicosity moves to the centre of the electrode. It is of maximum positive amplitude when the varicosity is at the surface near the outside rim of the electrode and declines rapidly in amplitude as the varicosity is removed further from the outside rim. Smaller diameter electrodes give larger EJCs than larger diameter electrodes for most positions of the varicosity on the surface of the muscle. The EJC amplitude declines for varicosities beneath the electrode that are not on the surface of the muscle, but deep in the tissue. The rate of this decline is greater the smaller the diameter of the electrode. The time-course of the EJC is largely invariant under changes in the position of the varicosity with respect to the recording electrode. Changes in the polarity of the current flow during a single EJC can occur, however, if two varicosities secrete transmitter simultaneously, one inside the electrode and one outside, and the time-course of the currents due to the individual varicosities is either the same or slightly different. This theoretical work has been used to interpret a number of recent experimental studies of extracellular current flow during autonomic neuromuscular transmission.