Little Known Facts, Controversies and Misconceptions About Cranial Motor Nuclei (and a New Classification for All Motoneurons, Cranial, and Spinal)

Cranial nerves represent a notoriously complex province of the neuroanatomical landscape of the vertebrates. Here, we offer a selection of the anatomic, genetic, and developmental features of their efferent component that are often misrepresented, ignored or controversial, as a complement to more exhaustive treatments of the subject. Our description reveals that efferent (or "motor") neurons in vertebrates represent a vague anatomic category (such as that of interneurons) rather than a true neuron type; That motor neurons fall into three bona fide types, segregated on the rostro-caudal axis of the central nervous system; That each of the three types is highly related to a type of preganglionic autonomic neuron; and that this genetic and topographical arrangement of three motor/preganglionic types correlates, not perfectly yet remarkably, with three broad physiological functions.