Noradrenergic Locus Coeruleus Influences on Posture and Vestibulospinal Reflexes

In addition to the lateral vestibular nucleus (LVN), which exerts a direct excitatory influence on tonic α- and static γ-motoneurones innervating the limb extensors, there are other structures, located in the dorsolateral pontine tegmentum, which exert a modulatory influence on posture (Pompeiano, Horn & d’Ascanio, 1991). These structures include a distinct cluster of neurones, located in the locus coeruleus (LC) and subcoeruleus (SC), which are not only noradrenergic (NA) but also norepinephrine (NE) sensitive, due to the existence of self-inhibitory synapses acting on α2-adrenoceptors through mechanisms of recurrent and/or lateral inhibition (cf. Barnes & Pompeiano, 1991). This paper will review various experimental studies designed to analyse the mode of action of the LC and SC. It will be shown that the LC-complex controls posture by utilizing either a direct coeruleospinal (CS) projection or an indirect projection passing through the dorsal pontine reticular formation (pRF) and the related medullary inhibitory reticulospinal (mRS) system (Figure 1).