Properties of preganglionic and postganglionic neurones in vasoconstrictor pathways of rats and guinea pigs

The electrophysiological properties of pre- and postganglionic neurones and their synaptic inputs have been examined in both in vivo and in vitro preparations. Electrically, both neurone types have similar low resting conductance and compact dendritic trees. In preganglionic vasoconstrictor neurones, both slow and fast excitatory and fast inhibitory potentials are probably involved in baroreceptor reflexes, discharge being initiated after summation. In contrast, postganglionic vasoconstrictor neurones receive only one type of fast excitatory input. One of the converging preganglionic inputs has a very high safety factor and always fires the postganglionic neurone ensuring that the centrally-derived pattern of discharge reaches the neurovascular junctions. We do not know if the other subthreshold inputs summate during natural activity in vivo, as it is not known whether functionally distinct preganglionic inputs converge on vasoconstrictor neurones in ganglia.