Primary afferents and second order projections of the trigeminal system in a frog (Rana ridibunda).

The trigeminal primary afferents and the projections from the sensory trigeminal nuclei were studied in a frog (Rana ridibunda) by means of tract-tracing techniques. Primary fibers from separate ophthalmic, maxillary and mandibular trigeminal branches form short ascending and long descending tracts. Ascending fibers innervate the principal trigeminal nucleus, a field just dorsal to the trigeminal motor nucleus, and the supratrigeminal nucleus. Descending fibers terminate separately in the descending trigeminal nucleus, depending on their branch of origin. Other primary targets are the rhombencephalic reticular nuclei, the ventral and caudal octaval nuclei, the nucleus of the solitary tract and the spinal cord. Ipsilateral intranuclear connections are accompanied by extratrigeminal projections that reach, among others, the thalamus, the mesencephalic tectum and the cerebellum. It is concluded that many amniotic features are shared by the anuran trigeminal system.