Cranial Nerves: Mind Your Head

Cranial nerves are relevant because all that everyone smells, sees, feels in the face, says, hears, senses equilibrium, moves the head, swallows, breathes, and heart beats, is led and mediated by cranial nerves. Cranial nerves carry the information from sensory organs to the brain, and the instructions for the muscles of all these organs in the face, neck, heart, and abdomen. While I was working at the Institute of Psychiatry at King’s College in London in 2002, one of my best friends suffered a traffic accident while riding on his motorcycle, without a helmet. This accident increased my panic and care to everything that could lead to an accident, and made the “Mind the Gap” warnings in the London tube more obvious to me than ever. My friend was enjoying a fantastic plan that wonderful June Sunday morning, consisting of riding a motorcycle through the beautiful mountains close to Madrid and finish back to the city to drink vermouth. After the accident, he became anosmic. Several weeks later, I learned that my friend had broken the base of the skull and the lamina cribosa, affecting the first cranial nerve, the olfactory nerve. He never recovered the sense of smell because he did not care enough about his head. My lesson was mind the head. In spite of being so relevant for injury treatment and surgery repair or nerve regeneration, much is still unknown about cranial nerve function, development, evolution, and morphology. One of the most obvious reasons is because of high complexity of the cranial nerves, in turn explaining the extreme interspecific heterogeneity, and intraspecific variability. This variability, for example, directly leads to an extreme difficulty in regenerative therapy approaches and reconstructive surgery in humans. What do we know about the reasons why the cranial nerves show this organizational and functional profile? To answer this question, this Special Issue of The Anatomical Record revises and updates the most recent knowledge and points of view about a number of topics related to cranial nerves. The co-Guest Editors responsible for the organization, recruitment, and compilation of the papers are Alino Martínez-Marcos and José Ramón Sañudo. Prof. Martínez-Marcos is Dean of the School of Medicine at University of Castilla-LaMancha, and has focused his investigations on the comparative neuroanatomy of some sensory and motor systems in a number of different taxa, olfactory, and vomeronasal systems in snakes, opossums, and rodents, and neural basis of tongue-flicking behavior from the vomeronasal system to the hypoglossal nerve. At present, he is involved in investigating the involvement of the olfactory and vagal systems in neurodegenerative disorders such as Alzheimer’s and Parkinson’s diseases. Prof. Sañudo is Full Professor and Head of the Department of Anatomy and Embryology of the University Complutense of Madrid (Spain). He has investigated in Clinical Anatomy and Embryology the last 40 years, publishing over 80 papers related to the facial and vagal cranial nerves. Both colleagues have done a remarkable, impressive work compiling 23 cutting-edge pieces of knowledge around cranial nerves, recruiting some of the known experts in the field. These articles belong to two groups, according to the key aspects of cranial nerve investigation: Ontogeny/Phylogeny and Morphology/Clinical Significance. The articles in the first group (Volume 1 of this Special Issue) raise relevant questions on cranial nerve patterning and comparative anatomy (with one of its interesting focus on amphioxus and lampreys), and provide new insights into the mechanisms underlying the development of the eye, and the oculomotor and vestibular complexes. The articles in the second group (Volume 2 of this Special Issue) explore the features of the extraand intra-cranial courses and branching pattern of the olfactory, optic, trigeminal, facial, cochlear, vestibular, glossopharyngeal, vagus, and hypoglossal nerves. All the articles pay special attention to the surgery on these nerves and the difficulties found to repair or to access neighboring areas after injury or disease. Next, I will briefly describe the key topics of the first volume of Special Issue papers on cranial nerve investigation: Ontogeny/Phylogeny. However, the lessons we can learn from this first volume of the present Special Issue on Cranial Nerves of The Anatomical Record have profound implications for the matters raised in the second volume (Morphology/Clinical Significance). Many reasons have been argued to explain why the cranial nerves are like they are. One of the most powerful explanations has to do with the way our bodies are organized. And in turn, to know about how our bodies are, means to tell an evo-devo story. The vertebrate head is the consequence of one more twist to the cephalization problem in nature. It is generally agreed today that the appearance of paired sensory organs and other structures in the frontal part of the head led the transition from filtering animals without brain to animals with a clear predator behavior (a certainly old idea known as the “new head” (Gans