TERMINAL NERVE COMPLEX

Cranial nerve 0 or the terminal nerve (nervus terminalis: TN) refers t o a nerve or in some cases possibly a complex of nerves that most like ly derive from the olfactory placodes and relate the most anterior for ebrain derivatives to nasal and olfactory structures. Such a nerve(s) has been observed in at least some stages of development in all major groups of vertebrates save myxinoids or hagfish. The phylogenetic vari ation of TN components is presented in some detail. The functions of T N components are not yet well identified, but some indications are tha t it is sensory but not chemosensory, controls blood vessels and gland s in the nose and has peptide-containing fibers that may control repro ductive development and behavior as well as other neural functions. Th is paper has three parts: (1) a historical review, which provides a co nceptual framework necessary for understanding current problems in the anatomy and physiology of the TN; (2) a section on recent studies in areas selected for their potential impact on understanding the TN, and (3) a brief summary, which provides some tentative answers to the que stions 'what is the TN and what does it do?' and suggests directions f or continued research on the TN.