Evolution of Motor Systems: The Reticulospinal Pathways

Abstract

Some major trends in the evolution of vertebrate motor systems are illustrated by examples taken from studies of the reticulospinal pathways. Using the tools of modern systematic zoology, we try to determine whether certain features of these complex motor systems represent shared primitive characters, arrived at early in evolution, or whether they represent shared derived characters, which have arisen more recently. Several hypotheses are proposed in a manner which makes them testable and which suggests directions for future studies of reticulospinal systems. Recent evidence illustrates that reticulospinal systems are equally complex in a wide variety of vertebrates, not only mammals. Have these complex systems evolved independently several times or only once in vertebrate phylogeny? For purposes of illustration, we take two medullary reticular nuclei, reticularis (r.) gigantocellularis and r. magnocellularis of mammals (or r. inferioris pars ventralis and r. ventrolateralis of reptiles), and analyze their distribution, first in various vertebrate taxa and then within various families of the repitilian suborder Lacertilia (lizards and snakes). The evidence is strong that both of these nuclei are primitive characters for amniotes and we hypothesize that they are primitive for all vertebrates. The second nucleus, Rmc/RVL, shows considerable variability in appearance within one major taxon where it has been carefully studied, the Reptilia. We suggest that it should be studied in a wider variety of species within other classes. Among several possible functions for this nucleus, one of the most prominent (in mammals, at least) and seemingly basic ( e.g. , widely useful) is a role in mediating pain at spinal levels. Why then would its presence among the reptiles be so erratic?