Just as shrinking transistors makes computers more powerful, brains with smaller components could in principle pack in more power and become faster. The human neuron, however—and in particular, its long “tail,” called an axon—may already be at (or close to) their physical limit.
Axons are the nervous system’s telegraph wires, enabling neurons to form networks. When a neuron fires, it sends an electrical signal down its axon, which then stimulates other neurons. The signal travels down the axon by opening ion channels embedded in the cellular membrane, letting ions pass through. When enough ions cross a channel, they change the voltage across the membrane, which in turn causes the nearby channels to open, propagating the signal in a domino effect.
In principle, our brains could evolve to have thinner axons, which would save space so that more neurons and more axons could pack in. Thinner axons would also consume less energy.
Nature, however, already seems to have made axons nearly as thin as they can be: any thinner, and the random opening of the channels would make axons too noisy, meaning that they would deliver too many signals when the neuron was not supposed to fire.
The problem is that ion channels are not precisely controllable. Instead, they open and close at random many times a second. Electrical signals only change the likelihood that they will open. In a typical axon the random opening of an ion channel does not have serious consequences, because the channel closes again before letting in too many ions (first animation).
If evolution made axons much thinner, however, the opening of a single ion channel would often create a spurious signal which then would travel down the axon. Too much of this noise would make the neuron unreliable.
— Davide Castelvecchi