Medical Science: Updates and Prospects Vol. 8

Cranial nerve zero (CN0), also known as the nervus terminalis or terminal nerve, is an often-overlooked neural structure described in addition to the traditionally recognised twelve cranial nerves. Although it was identified more than a century ago, CN0 remains absent from many standard anatomy textbooks because of its minute size, fragile morphology, and difficulty of identification during routine dissection. This narrative review summarises the available literature on the historical background, gross anatomy, embryological origin, proposed functional role, and clinical relevance of CN0. Reported anatomical studies describe CN0 as a delicate bilateral network of fibres located medial to the olfactory tract, extending from the region of the lamina terminalis and gyrus rectus toward the nasal cavity. Experimental and comparative studies suggest that the nerve may be associated with chemosensory pathways, gonadotropin-releasing hormone migration, and neuroendocrine mechanisms related to reproductive physiology. However, its precise function in humans remains incompletely understood and should be interpreted cautiously. The potential clinical significance of CN0 lies in its possible relevance to olfaction, reproductive endocrinology, developmental disorders such as Kallmann syndrome, and its vulnerability during neurosurgical and skull-base procedures. A clearer understanding of this neglected nerve may improve anatomical teaching, stimulate further developmental and functional research, and increase awareness among anatomists, neurosurgeons, and clinicians dealing with olfactory and hypothalamic regions. Overall, CN0 should be regarded as a clinically relevant but still incompletely characterised component of the cranial nerve system. The chapter will be of utmost use to neurosurgeons and endocrinologists, as this is a little-known nerve.