A human embryo creates twice as many neurons

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A note for Lesson no. 3, "Little Brains Wire Themselves to Their World," in Seven and a Half Lessons About the Brain by Lisa Feldman Barrett.
Some context from page 51‌ is:

A human embryo creates twice as many neurons as an adult brain needs, and infant neurons are quite a bit bushier than neurons in an adult brain.

The cerebral cortex develops via a process called "developmental exuberance," meaning there is an overproduction of neurons and their connecting parts (dendrites, axons and synapses), followed by selection (some survive, others are pruned back) that eventually brings the overall number of connections down to adult levels.[1][2] A major period of normal cell death occurs prenatally when about 50% of neurons are lost; pruning of neural connections occurs postnatally (along with a normal loss of glial cells).[3][4]


  1. Innocenti, Giorgio. M. and David. J. Price. 2005. “Exuberance in the Development of Cortical Networks.” Nature Reviews Neuroscience 6 (12): 955–965.
  2. National Research Council. 2000. From Neurons To Neighborhoods: The Science of Early Childhood Development. Washington, DC: National Academies Press.
  3. Stiles, Joan and Terry L. Jernigan. 2010. “The Basics of Brain Development.” Neuropsychological Review 20 (4): 327–348.
  4. Stiles, Joan. 2006. The Fundamentals of Brain Development: Integrating Nature and Nurture. Cambridge MA: Harvard University Press.