What drives the tonic activity of presympathetic neurons in the rostral ventrolateral medulla?

1. The present review discusses the mechanisms that maintain the tonic acti vity of presympathetic cardiovascular neurons in the rostral part of the ve ntrolateral medulla. 2. Experimental evidence is reviewed that indicates that these neurons rece ive both tonic excitatory and tonic inhibitory synaptic inputs. The former appear to be mediated, at least in part, by glutamate receptors and the lat ter appear to be mediated by GABA receptors. 3. There is also evidence that these neurons have the capacity to generate action potentials in the absence of synaptic inputs. However, at present, t here is not clear evidence that such an intrinsic pacemaker-like mechanism contributes to the tonic activity of these neurons under normal resting con ditions. 4. These neurons are also chemosensitive and this may contribute to their t onic activation under conditions of hypoxia or hypercapnia.