Architectural organization of the african elephant diencephalon and brainstem

The current study examined the organization of the diencephalon and brainstem of the African elephant (Loxodonta africana) - a region of the elephant brain that has not been examined for at least 50 years. The current description, employing material amenable for use with modern neuroanatomical methods, shows that, for the most part, the elephant diencephalon and brainstem are what could be considered typically mammalian, with subtle differences in proportions and topology. The variations from these previous descriptions, where they occurred, were related to four specific aspects of neural information processing: (1) the motor systems, (2) the auditory and vocalization systems, (3) the orexinergic satiety/wakefulness centre of the hypothalamus and the locus coeruleus, and (4) the potential neurogenic lining of the brainstem. For the motor systems, three specific structures exhibited interesting differences in organization - the pars compacta of the substantia nigra, the facial motor nerve nucleus, and the inferior olivary nuclear complex, all related to the timing and learning of movements and likely related to the control of the trunk. The dopaminergic neurons of the substantia nigra appear to form distinct islands separated from each other by large fibre pathways, an appearance unique to the elephant. Each island may send topologically organized projections to the striatum forming a dopaminergic innervation mosaic that may relate to trunk movements. At least five regions of the combined vocalization production and auditory/seismic reception system were specialized, including the large and distinct nucleus ellipticus of the periaqueductal grey matter, the enlarged lateral superior olivary nucleus, the novel transverse infrageniculate nucleus of the dorsal thalamus, the enlarged dorsal column nuclei and the ventral posterior inferior nucleus of the dorsal thalamus. These specializations, related to production and reception of infrasound, allow the proposal of a novel concept regarding the reception and localization of infrasonic sources. The orexinergic system of the hypothalamus displayed a medial hypothalamic parvocellular cluster of neurons in addition to the magnocellular clusters typical of mammals located in the lateral hypothalamus, and a novel medial division of the locus coeruleus was observed in the pons. These systems are related to appetitive drive and promotion of wakefulness, two aspects of elephant behaviour that appear to be inextricably linked. Lastly, we observed an extensive potential neurogenic lining of the ventricles throughout the brainstem that is present in even quite old elephants, although the function of these cells remains elusive. These observations combined demonstrate that, while much of the elephant brainstem is typically mammalian, certain aspects of the anatomy related to specialized behaviour of elephants are present and instructive in understanding elephant behaviour.