Maturation of reticular dendrites: Loss of spines and development of bundles

Abstract

Dendrite systems of most neurons in the brain stem reticular core undergo two significant changes between birth and maturity. The shaggy spinelike excrescences (heteromorphic protospines) which cover each dendrite and soma at birth, are lost completely, resulting in smooth, spineless reticular dendrites in the adult nervous system. It is suggested that these spinelike structures and their associated presynaptic terminals are concerned with primitive needs of the internal milieu and are replaced by a secondary system of terminals which provide substrate for more complex core functions. In contrast to the familiar radiative aspect of reticular dendrites in newborn organisms, dendrite shafts are rearranged into bundles which surround the fascicles of axons running longitudinally through the reticular formation. The bundles of dendrites are marked by closeness of apposition of the shafts and continuous change in the dendritic elements comprising them. On the basis of previous work, it is suggested that these bundles are the repository for central programs mediating certain stereotyped repetitive output patterns such as respiration and the rest-activity cycle.