Neurons themselves are complex and variable. Axons vary in their speed and reliability of transmission. Each neuron makes a treelike branching structure that reaches out to receive synaptic input from other neurons, as a tree’s branches reach out to sunlight. The branches, called dendrites, differ in their sensitivity to synaptic input, with the molecular composition as well as shape of a dendrite determining how it would respond to the electrical input it receives from synapses.

Nor are any of these parts of a living brain fixed entities. The brain’s components, including the neurons, axons, dendrites and synapses (and more), are constantly adapting to their electrical and chemical “experience,” as part of learning, to maintain the ability to give appropriately different responses to different inputs, and to keep the brain stable and prevent seizures. These adaptations depend on the dynamic molecular machinery in each neural structure. The states of all of these components are constantly being modulated by a wash of chemicals from brainstem neurons that determine such things as when we are awake or attentive and when we are asleep, and by hormones from the body that help drive our motivations. Each element differs in its susceptibility to these influences.

To reconstruct a mind, perhaps one would not need to replicate every molecular detail; given enough structure, the rest might be self-correcting. But an extraordinarily deep level of detail would be required, not only to characterize the connectome but also to understand how the neurons, dendrites, axons and synapses would dynamically operate, change and adapt themselves.