Synaptic Signaling in Sympathetic Vasoconstrictor Pathways and the Effects of Injury

Vasoconstrictor pathways through sympathetic ganglia appear to relay the pattern of signals originating in the central nervous system largely unmodified to the vasculature. Each preganglionic neuron distributes the centrally-derived signals to a number of postganglionic neurons (divergence) by way of single suprathreshold “strong” synapses. Strong synapses have a very high safety factor and differ from the multiple subthreshold or “weak” inputs in (a) releasing a much larger number of ACh quanta and (b) using characteristic Ca2+ channel subtypes to trigger their release. The most prominent subtype at strong synapses is resistant to all known Ca2+ channel antagonists. When postganglionic neurons are partially denervated, strong synapses are rapidly restored by sprouting of residual connections within the ganglia. This is the only function of weak synapses so far demonstrated. These singular properties emphasize the importance of strong synapses for guaranteeing synaptic transmission through sympathetic ganglia in vasoconstrictor pathways.