In response to:
The Mental Life of Plants and Worms, Among Others from the April 24, 2014 issue
To the Editors:
Oliver Sacks provides a captivating synthesis in his review of nine books spanning twelve decades that have examined mental features of diverse biological species [“The Mental Life of Plants and Worms, Among Others,” NYR, April 24]. He skillfully maintains a level of description that balances detail and generality, except on one issue. In a short paragraph he states:
Although neurons may differ in shape and size, they are essentially the same from the most primitive animal life to the most advanced. It is their number and organization that differ: we have a hundred billion nerve cells, while a jellyfish has a thousand. But their status as cells capable of rapid and repetitive firing is essentially the same.
However, a close examination of “essentially the same” reveals highly significant differences. Some commonalities certainly exist in the nervous systems of these species, but their enormous cognitive variations reflect distinct critical details in their neuronal mechanisms.
At the cellular level, even between mice and men, the vastly greater number of neurons in human brains is largely attained by significant miniaturization of neurons. In general, for rodents and primates with the same brain size, for example, common agouti rodents compared to owl monkeys, the latter achieve a threefold-higher density of neurons per milligram of brain tissue. Moreover, compared to most invertebrates, the insulation of neuronal axons with myelin sheaths was a transformative step in the evolution of vertebrates that permits greatly accelerated nerve conduction speeds. Finally, at the level of individual synapses, the key NMDA class of receptors for glutamate, the major excitatory neurotransmitter, evolved new functions in vertebrates to modulate the synaptic plasticity underlying cognitive responses.
Stuart J. Edelstein
École Normale Supérieure
Oliver Sacks replies:
I am grateful to Professor Edelstein for emphasizing that vertebrate neurons can be structurally and chemically very diverse. In the human brain, indeed, there are more than fifty types of neuron, all specialized for different functions—including some neurons found only in big-brained primates, cetaceans, and elephants. Professor Edelstein brings out, too, how miniaturization can allow more neurons to be compacted in brains, and how greatly the myelination of vertebrate nerves enhances their conduction speed.
But these stratagems have also evolved independently in several invertebrate groups. Indeed, no vertebrate brain can match the compactness of an ant’s, and no vertebrate nerves can match the almost supersonic speed (210 meters per second) of conduction that Kusano et al. have measured in the humble Kuruma shrimp.
Invertebrates have taken a different path, and it will be fascinating, as the neurophysiologist Daniel Hartline writes, “to see unfold the still largely obscure story of the molecular evolution of invertebrate myelin, for comparison with the remarkable vertebrate tale.”