7 - Brain development and evolution

Summary

Introduction

The neocortex is a thin layer about 2 to 4mm thick that covers the surface of the cerebrum. Neuronal elements occupy 30% of the neocortex, the remainder being accounted for by glial, vascular, and pericytal elements. Only 10% of the neural somata belong to the Golgi type I cells, the source of cortical efferents. Interneurons (Golgi type II cells) account for the majority of cells in the cortex as well as for a majority (80%) of total cortical synapses. In humans, enlargement of the neuronal somata and its dendritic arbor is no longer detectable after the 6th year of life Scammon, 1932; Caviness et al., 1997).

The brain achieves its maximum volume by the middle of the second decade of life (for both males and females), reaching a plateau by the 12th year (Dekaban and Sadowsky, 1978). According to Wilmer (1940), brain weight comprises 21% of total body weight at 6 months (fetal period), 15% at term, and only 3% in the adult. The brain is dominated by the cerebrum, which occupies 90% of its volume, and 60% of the cerebrum is gray matter. Most of the gray matter is neocortex. It is 60 times the volume of the diencephalon, the second largest cerebral gray matter structure. A weight difference between the brains of males and females is usually the result of a greater volume of central white matter and a larger cerebellum in the male (Caviness et al., 1997).