The Next NCC Frontier Neuronal Interiors and the Cytoskeleton

Membrane-based neuronal input-output activities involve changes in synap-tic plasticity, ion conductance, neurotransmitter vesicle transport/secretion and gap-junction regulation - all controlled by intra-neuronal networks of filamentous protein polymers known as the cytoskeleton. If simple input-output activities fully described neural function, then fine-grained details might not matter. But simple input-output activities - in which neurons function as switches - are only a guess, and most likely a poor imitation of neurons' actual activities and capabilities.

To gauge how single neuron functions may exceed simple input-output activities, consider the single-cell organism paramecium. Such cells swim about gracefully, avoid obstacles and predators, find food and engage in sex with partner paramecia. They may also learn; if placed in capillary tubes they escape, and in subsequent attempts escape more quickly. As single cells with no synaptic connections, how do they do it? Pondering the seemingly intelligent

22 Personal communication from Roger Penrose.

activities of such single-cell organisms, famed neuroscientist C.S. Sherrington [206] conjectured:

"of nerve there is no trace, but the cytoskeleton might serve".

If the cytoskeleton is the nervous system of protozoa, what might it do for neurons?

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