The New York Review of Books

Jean-Pierre Changeux is France’s most famous neuroscientist. Though less well known in the United States, he has directed a famous laboratory at the Pasteur Institute for more than thirty years, taught as a professor at the Collège de France, and written a number of works exploring “the neurobiology of meaning.” Aside from his own books, Changeux has published two wide-ranging dialogues about mind and matter, one with the mathematician Alain Connes and the other with the late French philosopher Paul Ricoeur.

Changeux came of age at a fortunate time. Born in 1936, he began his studies when the advent both of the DNA age and of high-resolution images of the brain heralded a series of impressive breakthroughs. Changeux took part in one such advance in 1965 when, together with Jacques Monod and Jeffries Wyman, he established an important model of protein interactions in bacteria, which, when applied to the brain, became crucial for understanding the behavior of neurons. Since that time, Changeux has written a number of books exploring the functions of the brain.

The brain is of course tremendously complex: a bundle of some hundred billion neurons, or nerve cells, each sharing as many as ten thousand connections with other neurons. But at its most fundamental level, the neuron, the brain’s structure is not difficult to grasp. A large crown of little branches, known as “dendrites,” extends above the body of the cell and receives signals from other neurons, while a long trunk or “axon,” which conducts neural messages, projects below, occasionally shooting off to connect with other neurons. The structure of the neuron naturally lends itself to comparison with the branches, trunk, and roots of a tree, and indeed the technical term for the growth of dendrites is “arborization.” (See the illustration below.)

We’ve known since the early nineteenth century that neurons use electricity to send signals through the body. But a remarkable experiment by Hermann von Hermholtz in 1859 showed that the nervous system, rather than telegraphing messages between muscles and brain, functions far slower than copper wires. As Changeux writes,

Everyday experience leads us to suppose that thoughts pass through the mind with a rapidity that defies the laws of physics. It comes as a stunning surprise to discover that almost the exact opposite is true: the brain is slow—very slow— by comparison with the fundamental forces of the physical world.

Further research by the great Spanish anatomist Santiago Ramon y Cajal suggested why the telegraph analogy failed to hold: most neurons, instead of tying their ends together like spliced wires, leave a gap between the terminus of the neuron, which transmits signals, and the receptor of those signals in the adjacent neuron. How signals from neurons manage to cross this gap, later renamed the synaptic cleft (“synapse” deriving from the Greek for “to bind together”), became the major neurophysiological question of the early twentieth century.

Most leading biologists at that time assumed that neurons would use the electricity in the nervous …