Why is the mesencephalic nucleus of the trigeminal nerve situated inside the brain?

Primary sensory neurons are usually situated in ganglia outside the brain, while the mesencephalic nucleus of the trigeminal nerve (Me5) is situated inside the brain. However, it remains unknown why only Me5 situated inside the brain is. The neurons of Me5 are the cell bodies of primary afferent fibers concerned with the muscles of mastication and periodontal receptors of both maxillary and mandibular teeth. Interestingly, there was no Me5 till the evolution level of the agnatha, vertebrates which lack jaws, while Me5 appeared with the evolution of jawed vertebrates, the gnathostomes. Thus, I speculate that the appearance of jaws necessitated the emergence of a novel sensory system including newly-made primary sensory neurons to co-ordinate jaw movement and this need was met by the appearance of Me5. Although primary sensory neurons are usually generated from the neural crest or the neurogenic placodes, primary sensory neurons in Me5 are derived from neuroepithelium of the dorsal midline of the midbrain. Taken together, I propose the following hypothesis; (1) Me5 did not exist till the evolution level of agnatha, which lacks jaw. (2) When jawed vertebrates evolved, a new sensory system including new primary sensory neurons for mastication was needed. (3) At that point, there was no capacity for the neural crest and neurogenic placodes to make primary sensory neurons. (4) However, there remained capacity only for the neuroepithelium of the midbrain to make primary sensory neurons. (5) Thus, Me5 was newly made inside the CNS.

Automated summary

The paper discusses the reasons for the presence of the mesencephalic nucleus of the trigeminal nerve (Me5) inside the brain and the characteristics of its neurons. The author hypothesizes that the emergence of jaws required a new sensory system, and the appearance of Me5 met this need.