500 trillion connections
A note for Lesson no. 2, "Your Brain Is a Network," in Lisa Feldman Barrett.
Some context from page 33 is:
...over five hundred trillion neuron-to-neuron connections.
Scientists disagree about the exact number of synapses in a human brain. The average (male) human cerebral cortex contains 164 trillion synapses but estimates range widely, as do estimates for the number of synapses per cortical neuron (see Silbereis et al., 2016, Box 1 and references therein). If we take a lower estimate of approximately 7,200 and multiply that by 128 billion, we would estimate that there are over 920 trillion synapses in the average brain. Another estimate is that an average neuron will have 17,500 synapses, resulting in an estimate of more than 2 quadrillion synapses in the human brain. These estimates rely on the questionable assumption that a typical neuron found in the cerebral cortex is similar to all other neurons throughout the rest of the human brain, and doesn't account for the fact that there are age and sex differences in the number of neurons (and therefore in the number of synapses).
In Seven and a Half Lessons About the Brain, I decided to go with the most conservative estimate I've come across — 500 trillion — found in a chapter by the neuroscientist David Linden. (This chapter introduces a delightful book of essays by leading neuroscientists, written in a relatively accessible way.)
- Tang, Yong, Jens R. Nyengaard, Didima MG De Groot, and Hans Jørgen G. Gundersen. 2001. "Total Regional and Global Number of Synapses in the Human Brain Neocortex." Synapse 41 (3): 258–273.
- Silbereis, John C., Sirisha Pochareddy, Ying Zhu, Mingfeng Li, and Nenad Sestan. 2016. "The Cellular and Molecular Landscapes of the Developing Human Central Nervous System." Neuron 89 (2): 248–268.
- Pakkenberg, Bente, Dorte Pelvig, Lisbeth Marner, Mads J. Bundgaard, Hans Jørgen G. Gundersen, Jens R. Nyengaard, and Lisbeth Regeur. 2003. "Aging and the Human Neocortex. Experimental Gerontology 38 (1-2): 95–99.
- Lennie, Peter. 2003. "The Cost of Cortical Computation." Current Biology 13 (6): 493–497.
- Pakkenberg, Bente, and Hans Jørgen G. Gundersen. 1997. Neocortical Neuron Number in Humans: Effect of Sex and Age. Journal of Comparative Neurology 384 (2): 312–320.
- Linden, David J. 2018. "Our Human Brain was Not Designed All at Once by a Genius Inventor on a Blank Sheet of Paper." In Think Tank: Forty Neuroscientists Explore the Biological Roots of Human Experience, edited by David J. Linden, 1–8. New Haven: Yale University Press.