The sensory trigeminal system of a snake in the possession of infrared receptors. II. The central projections of the trigeminal nerve.

Abstract

AbstractThe central projections of the sensory roots of the trigeminal nerve in Python reticulatus have been studied by means of the Fink Heimer technique following transsection of these roots between the trigeminal ganglia and the brainstem.The sensory trigeminal fibres are distributed to the two separate sensory trigeminal systems, described before, i.e., the common sensory and the lateral descending trigeminal system. The projections of the three peripheral trigeminal branches are restricted to their own territories of the various tracts and nuclei. Generally, the ophthalmic root projects most ventrally; the mandibular root most dorsally and the maxillary root intermediately. The ophthalmic branch descends until the level of the first cervical nerve root, the maxillary and mandibular branches proceed beyond the second cervical nerve roots.The distribution pattern of the sensory trigeminal fibres differs in two major aspects from that of other vertebrates. The first difference is a large overlap of the projection areas from the mandibular and maxillary roots. The probable occurrence of this overlap within other, mammalian, species is discussed. The second major difference is the projection into the lateral descending trigeminal nucleus. The morphology of the degenerating fibres and “boutons terminaux” in this lateral descending system is markedly different from that within the common sensory system.Preliminary notes are presented to show that the common sensory and the lateral descending systems project to different areas in the mesencephalon and diencephalon. It will also be shown that the nucleus of the lateral descending trigeminal tract is exclusively concerned in processing the infrared information relayed by the pit organs.The possibility of the presence of a separate trigeminal system in other vertebrates, equivalent to the lateral descending trigeminal system of the python, is discussed.