Locus Ceruleus Efferent Fibers Innervate Many Target Structures

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A striking feature of the LC is the extreme divergence of its projections, partly due to an intense collater-alization of the axons. Indeed, a few neurons (20,000 in humans and 1,500 in rats) provide extensive innervation of the brain from the olfactory tubercle to the spinal cord (Fig. 1).

Projections of the LC are organized in two ascending fiber systems: first, the dorsal NE bundle innervating the amygdala, olfactory tubercle, septum, bed nucleus of the stria terminals, hippocampus, entire neocortex, habenula, ventral and anterior thalamus, and hypothalamus, and second, the rostral limb of the dorsal periventricular pathway. Projections also include descending fibers to the cerebellum, medulla oblongata, and spinal cord. A certain topographical organization is observed because neurons from the ventral part of the LC preferentially project to the spinal cord, cerebellum, and hypothalamus, whereas neurons from the dorsal part project to the cortex and hippocampus. A functional organization was also observed concerning the projections to somatosensory structures. Studies employing double-fluorescent

Cleos Meynert

Figure 1 Noradrenergic pathways in the human brain. Sagittal human brain section containing the main projection sites of noradrenergic neurons. A6: Noradrenergic nucleus corresponding to the locus ceruleus. It widely projects to the anterior part of the brain and also to the cerebellum and spinal cord. A1-A2: Medullary noradrenergic nuclei, projecting essentially to the hippocampus, thalamus, and hypothalamus.

Figure 1 Noradrenergic pathways in the human brain. Sagittal human brain section containing the main projection sites of noradrenergic neurons. A6: Noradrenergic nucleus corresponding to the locus ceruleus. It widely projects to the anterior part of the brain and also to the cerebellum and spinal cord. A1-A2: Medullary noradrenergic nuclei, projecting essentially to the hippocampus, thalamus, and hypothalamus.

retrograde labeling strategies revealed that single LC neurons preferentially collateralize to different sites that process the same sensory information.

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