A Myelin Map of Trunk Folds in the Elephant Trigeminal Nucleus

Abstract

Elephants have elaborate trunk skills and large, but poorly understood brains. Here we study trunk representations in elephant trigeminal nuclei, which form large protrusions on the ventral brainstem. Dense vascularization and intense cytochrome-oxidase reactivity distinguish several elongated putative trunk modules, which repeat in the anterior-posterior direction; our analysis focuses on the most anterior and largest of the units, the putative nucleus principalis trunk module. Module neuron density is low and non-neural cells outnumber neurons by ∼108:1. Dendritic trees are elongated along the axis of axon bundles (myelin stripes) transversing the trunk module. Furthermore, synchrotron X-ray phase contrast tomography suggests myelin-stripe-axons transverse the trunk module. We show a remarkable correspondence of trunk module myelin stripes and trunk folds. Myelin stripes show little relation to trigeminal neurons and stripe-axons appear to often go nowhere; these observations suggest the possibility that myelin stripes might serve to separate trunk-fold domains rather than to connect neurons. The myelin-stripes-to-folds mapping allowed to determine neural magnification factors, which changed from 1000:1 proximally to 5:1 in the trunk finger. Asian elephants have fewer (∼640,000) trunk-module neurons than Africans (∼740,000) and show enlarged representations of trunk parts involved in object wrapping. We conclude the elephant trigeminal trunk module is exquisitely organized into trunk-fold-related units.