Trigeminal ganglion and sensory nerves suggest tactile specialization of
elephantsLeopold Purkart, John M. Tuff, Malav Shah, Lena V. Kaufmann,
Carlotta Altringer, Eduard Maier, Undine Schneeweiß, Elcin Tunckol, Lennart
Eigen, Susanne Holtze, Guido Fritsch, Thomas Hildebrandt & Michael Brecht
Current BiologyJanuary 20, 2022Summary
Sensory nerves are information bottlenecks giving rise to distinct sensory
worlds across animal species. Here, we investigate trigeminal ganglion and
sensory nerves of elephants. The elephant trigeminal ganglion is very
large. Its maxillary branch, which gives rise to the infraorbital nerve
innervating the trunk, has a larger diameter than the animal’s spinal cord,
i.e., trunk innervation is more substantive than connections of the brain
to the rest of the body. Hundreds of satellite cells surround each
trigeminal neuron, an indication of exceptional glial support to these
large projection neurons.
Fiber counts of Asian elephant infraorbital nerves of averaged 4,00,000
axons. The infraorbital nerve consists of axons that are ∼10 μm thick and
it has a large diameter of 17 mm, roughly 3 times as thick as the optic and
6 times as thick as the vestibulocochlear nerve. In most mammals (including
tactile specialists) optic nerve fibers greatly outnumber infraorbital
nerve fibers, but in elephants the infraorbital nerve fiber count is only
slightly lower than the optic nerve fiber count. Trunk innervation (nerves
and ganglia) weighs ∼1.5 kg in elephant cows. Our findings characterize the
elephant trigeminal ganglion as one of the largest known primary sensory
structures and point to a high degree of tactile specialization in
elephants.
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