Evolution and ontogeny of neural circuits

• Published in: The Behavioral and brain sciences 1984-09, Vol.7 (3), p.321-331
• Main Author: Ebbesson, Sven O. E.
• Format: Article
• Language: English
• Subjects: Biological and medical sciences; commissures; development; Evolution; Fundamental and applied biological sciences. Psychology; Isolated neuron and nerve. Neuroglia; lateralization; learning; neocortex; neural circuits; Neural networks; olfactory system; Ontogeny; plasticity; somatosensory pathways; superior colliculus; Target Article; thalamus; Vertebrates: nervous system and sense organs; visual pathways
• ISSN: 0140-525X; 1469-1825
• DOI: 10.1017/S0140525X00018379

Recent studies on neural pathways in a broad spectrum of vertebrates suggest that, in addition to migration and an increase in the number of certain select neurons, a significant aspect of neural evolution is a “parcellation” (segregation-isolation) process that involves the loss of selected connections by the new aggregates. A similar process occurs during ontogenetic development. These findings suggest that in many neuronal systems axons do not invade unknown territories during evolutionary or ontogenetic development but follow in their ancestors' paths to their ancestral targets; if the connection is later lost, it reflects the specialization of the circuitry. The pattern of interspecific variability suggests (1) that overlap of circuits is a more common feature in primitive (generalized) than in specialized brain organizations and (2) that most projections, such as the retinal, thalamotelencephalic, corticotectal, and tectal efferent ones, were bilateral in the primitive condition. Specialization of these systems in some vertebrate groups has involved the selective loss of connections, resulting in greater isolation of functions. The parcellation process may also play an important role in cell diversification. The parcellation process as described here is thought to be one of several underlying mechanisms of evolutionary and ontogenetic differentiation.