Chapter 17 - Toward a Spinal Cord Ontology

Abstract

This chapter is a speculative introduction to the creation of a new nomenclatural hierarchy for the spinal cord—an ontology. The traditional regional subdivision of the spinal cord is based on the levels of cervical, thoracic, lumbar, sacral, and coccygeal vertebrae. The boundaries of these vertebral regions correlate roughly with the functional regions of the cord, but the discrepancies are significant. The new regional classification proposed in the chapter is based on the distinct areas of gene expression and AChE/ChAT staining that define the limb enlargements and the sympathetic and parasympathetic preganglionic groups as an alternative to the “vertebral” classification. This new scheme clearly clashes with the traditional cervical, thoracic, lumbar, sacral, coccygeal designations, but is true to the developmental processes. Unlike the traditional classification, it can be applied to a range of species that have different numbers of cervical, thoracic or lumbar vertebrae, and takes into account intraspecific variation, such as “prefixing” and “postfixing” of limb enlargements. The system is founded on the assumption that the basic plan in mammals and birds is to have a group of five major spinal cord segments supplying the forelimb, followed by a long group of segments with sympathetic preganglionic neurons, which is in turn followed by a group of five major spinal cord segments supplying the hindlimb. The hindlimb group is immediately followed by two or three segments containing the parasympathetic neurons. The parasympathetic group is succeeded by the part of the spinal cord, which supplies the tail m scles. It is suggested that the new subdivisions be called prebrachial, brachial, interramal, crural, postcrural, and caudal.