Noradrenergic LC neurons appear to be relatively unique in the brain, being unsurpassed in the divergence and ubiquity of their projections through the central nervous system. In this regard, they share certain characteristics with peripheral noradrenaline neurons of the sympathetic nervous system. As such they would be assumed to play a very general role in modulating the activity of large populations of neurons in multiple, functionally diverse systems. Like other periventricular and reticular neurons, they have the potential to receive afferent information from multiple sources via long dendrites, upon which the majority of their inputs from brainstem and forebrain may arrive. They appear closely related to the cholinergic neurons of the laterodorsal tegmental nucleus, their neighbors that are located medial and rostral to them within the periventricular gray and that have similarly oriented and positioned long dendrites that would allow reception of similar afferent input as the LC neurons and also possibly interaction with the LC neurons. As evidenced by input to the noradrenergic cell bodies in the compact portion of the nucleus, a moderate GABAergic innervation, that may derive in part from local neurons, could have a potent influence on the activity of the cells. Periventricular GABAergic cells could also serve as intermediaries to other afferent input, from a distance, terminating in the periventricular region or from local neurons such as the cholinergic cells of the laterodorsal tegmental nucleus.