Individual variability in the size and organization of the human arcuate nucleus of the medulla

The arcuate nucleus (Arc) of the medulla is found in almost all human brains and in a small percentage of chimpanzee brains. It is absent in the brains of other mammalian species including mice, rats, cats and macaque monkeys. The Arc is classically considered a precerebellar nucleus, presumably motor in function. However, several studies have found aplasia of the Arc in babies who died of SIDS (Sudden Infant Death Syndrome) suggesting a very different role for the Arc in the control of respiration. Aplasia of the Arc, however, has also been reported in adults suggesting that it is not critical for survival. We have examined the Arc in closely-spaced Nissl-stained sections in human cases to acquire a better understanding of the degree of variability of its size and location in adults. We have also examined immunostained sections to develop a neurochemical profile of this nucleus. Our data suggest that the Arc consists of rostral and caudal subdivisions. There is some degree of left-right asymmetry in Arc position, size and shape at all rostro-caudal levels. Neurons in the Arc express calretinin (CR), neuronal nitric oxide synthase (nNOS) and nonphosphorylated neurofilament protein (NPNFP) but not calbindin (CB) or parvalbumin (PV). There is also immunostaining for GAD and GABA receptors suggesting inhibitory input to Arc neurons. We support the classical idea of the Arc as a precerebellar nucleus; its absence in SIDS cases may not be the cause of death but rather a correlate of other brainstem abnormalities.