Trigeminal projections in snakes possessing infrared sensitivity.

The first order of projections of the trigeminal nerve were studied in four species of snakes, three of which are infrared sensitive, utilizing silver degeneration and cobalt iontophoresis techniques. The trigeminal nerve projected ipsilaterally to a main sensory nucleus, descending spinal nucleus, ventral trigeminal field and solitary nucleus with a contralateral projection to the base of the descending spinal nucleus in all four species. A major additional projection to the lateral nucleus of the trigeminal was present in all three infra-red sensitive species but absent in the other. Iontophoresis of cobalt chloride into a trigeminal branch composed entirely of infrared sensitive fibers and into a trigeminal branch containing no infrared fibers indicated that the lateral nucleus of the trigeminal nerve was the sole first target of the infrared system.