Ganglionic transmission in a vasomotor pathway studied in vivo

Intracellular recordings were made in vivo from 40 spontaneously active cells in the third lumbar sympathetic ganglion of urethane-anaesthetized rats. In 38/40 cells ongoing action potentials showed strong cardiac rhythmicity (93.4 +/- 1.9% modulation) indicating high barosensitivity and probable muscle vasoconstrictor (MVC) function. Subthreshold excitatory postsynaptic potentials (EPSPs) showed the same pattern. The 38 barosensitive neurons fired action potentials at 2.9 +/- 0.3 Hz. All action potentials were triggered by EPSPs, most of which were unitary events. Calculations indicated that < 5% of action potentials were triggered by summation of otherwise subthreshold EPSPs. 'Dominant' synaptic inputs with a high safety factor were identified, confirming previous work. These were active in 24/38 cells and accounted for 32% of all action potentials; other ('secondary') inputs drove the remainder. Inputs (21 dominant, 19 secondary) attributed to single preganglionic neurons fired at 1.38 +/- 0.16 Hz. An average of two to three preganglionic neurons were estimated to drive each ganglion cell's action potentials. When cells were held hyperpolarized to block spiking, a range of spontaneous EPSP amplitudes was revealed. Threshold equivalent was defined as the membrane potential value that was exceeded by spontaneous EPSPs at the same frequency as the cell's original firing rate. In 10/12 cells examined, a continuum of EPSP amplitudes overlapped threshold equivalent. Small changes in cell excitability could therefore raise or lower the percentage of preganglionic inputs triggering action potentials. The results indicate that vasoconstrictor ganglion cells in vivo mostly behave not as 1:1 relays, but as continuously variable gates.