Terminal nerve complex.

Cranial nerve 0 or the terminal nerve (nervus terminalis: TN) refers to a nerve or in some cases possibly a complex of nerves that most likely derive from the olfactory placodes and relate the most anterior forebrain 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 variation of TN components is presented in some detail. The functions of TN components are not yet well identified, but some indications are that it is sensory but not chemosensory, controls blood vessels and glands in the nose and has peptide-containing fibers that may control reproductive development and behavior as well as other neural functions. This paper has three parts: (1) a historical review, which provides a conceptual 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 questions 'what is the TN and what does it do?' and suggests directions for continued research on the TN.