The Neural Pathways That Produce Arousal

Abstract

As anatomical and physiological evidence accumulated in the wake of the publication by Moruzzi and Magoun (1949), it was eventually found that cortical arousal produced by the ARAS originates in certain nuclei of the rostral reticular formation and is mediated by two ascending pathways, one dorsal and one ventral (Jones, 2000). A major source of the arousal system resides in cells of two nuclei, the pedunculopontine tegmental nucleus (PPT) and the laterodorsal tegmental nucleus (LDT). The cells of these nuclei are located at the pontinemesencephalic junction, where they essentially surround and intermingle with fibers of the superior cerebellar peduncle. Recall that in the experiment of 1949, Moruzzi and Magoun used the superior cerebellar peduncle as the target of the stimulating electrode. This peduncle is also called the brachium conjunctivum, and the nuclei that are adjacent are often referred to as the peribrachial nuclei.^* It has been shown that these two nuclei contain acetylcholine-synthesizing neurons; we know, therefore, that the major cells of origin of the ascending arousal system are cholinergic (Jones and Beaudet, 1987; Jones and Webster, 1988). Figure 5.1 shows a frontal section through the catbrain and the location of the cholinergic cells of the peribrachial nuclei.