A neglected 'accessory' vasomotor pathway: implications for blood pressure control.

1. Distinct from 'regular' sympathetic preganglionic neurons, there exists a population of 'accessory' preganglionic neurons. The latter are distinguishable by their unmyelinated axons and their different functional properties. They synapse on the same ganglion cells. 2. Ongoing sympathetic activity is driven by 'regular' preganglionic neurons. 3. 'Accessory' preganglionic neurons drive hexamethonium-resistant ganglionic transmission: part of this is muscarinic and part not (possibly peptidergic or nitrergic). 4. 'Accessory' preganglionic neurons supply cardiovascular (vasomotor, cardiac, adrenal), but apparently not other, sympathetic pathways. 5. 'Accessory' preganglionic neurons are activated by arterial chemoreceptors. 6. Brief activation of 'accessory' preganglionic neurons potentiates ongoing post-ganglionic activity for tens of minutes by an action at the ganglion. This is probably by enabling previously subthreshold excitatory post-synaptic potentials to trigger action potentials. 7. Evidence is presented that microinjections of GABA into the rostral ventrolateral medulla activate the 'accessory' pathway while inhibiting the 'regular' pathway. 8. A role for this 'accessory' pathway in the long-term control of blood pressure in health and disease is predicted, but still untested.