Trigeminal and Related Spinal Projections

Bernd Fritzsch

doi:10.1201/9781003092810-4

The trigeminal sensory system, the largest sensory input of all sensory projections, extends from the rhombomere 2 to the spinal cord. Sensory input provides a unique distribution to several sensory receptors that extend through the lemniscal to the cortex. Development starts with a set of genes that drive three branches of the trigeminal ganglion reaching the ophthalmic, maxillary, and mandibular neurons, which depend on the bHLH gene, Neurog1. The diversity of glabrous and hairy sensory inputs is unique to each fiber but have a common innervation, the Merkel cells, that depends on the bHLH gene, Atoh1. The central trigeminal neurons depend, among several genes, on the bHLH genes Ascl1, Olig3, and Ptf1a, that initiate the neuronal differentiation of several genes (Phox2b, Lmx1b, Lbx1), to develop excitatory and inhibitory second order neurons. Hoxa2 is essential for the normal expression of rhombomere 2 that counteracts the expression of Fgf8 in r0. The mesencephalic trigeminal projection extends from the midbrain to the cortex; it bifurcates to innervate trigeminal sensory neurons with peripheral fibers, in particular the mandibular branch. The lemniscal system is continuing sensory information to reach the thalamus, the VPM. From the somatosensory cortex, it will provide three primary and secondary innervations that present a ‘homunculus’ in mammals. Two genes, Fgf8 and Wnt1, have been identified for the normal somatosenation. The sensory innervation can be adjusted by addition or deletion of sensory input, generate both manipulations and genetic alterations to drive the unique whiskers of rodents to reach the barrel field.

Trigeminal and Related Spinal Projections

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