THE human brain might be the most complex object in the known universe, but a much simpler set of neurons is also proving to be a tough nut to crack.
A tiny wasp has brain cells so small, physics predicts they shouldn’t work at all. These miniature neurons might harbour subtle modifications, or they might work completely differently from all other known neurons – mechanically.
The greenhouse whitefly parasite (Encarsia formosa) is just half a millimetre in length. It parasitises the larvae of whiteflies and so it has long been used as a natural pest-controller.
To find out how its neurons have adapted to miniaturisation, Reinhold Hustert of the University of Göttingen in Germany examined the insect’s brain with an electron microscope. The axons – fibres that shuttle messages between neurons – were incredibly thin. Of 528 axons measured, a third were less than 0.1 micrometre in diameter, an order of magnitude narrower than human axons. The smallest were just 0.045 μm (Arthropod Structure & Development, doi.org/jfn).
That’s a surprise, because according to calculations by Simon Laughlin of the University of Cambridge and colleagues, axons thinner than 0.1 μm simply shouldn’t work. Axons carry messages in waves of electrical activity called action potentials, which are generated when a chemical signal causes a large number of channels in a cell’s outer membrane to open and allow positively charged ions into the axon. At any given moment some of those channels may open spontaneously, but the number involved isn’t enough to accidentally trigger an action potential, says Laughlin – unless the axon is very thin. An axon thinner than 0.1 μm will generate an action potential if just one channel opens spontaneously (Current Biology, doi.org/frfwpz).
“That makes the axon impossibly noisy,” Laughlin says. Any “legitimate” action potentials will be drowned out.
Hustert suggests that a neuron might get around this problem by firing bursts of action potentials to cut through the noise, but Laughlin is sceptical. “They’d be firing furiously all the time,” he says, and every action potential costs energy.
Instead, the neurons might not bother with conventional action potentials at all. “They could be sending signals mechanically,” Laughlin says. The tiny axons might each carry a long rigid rod stretching down the centre. Pulling the rod could create a physical rather than electrical trigger for the release of a chemical that passes the signal on to the neighbouring neuron.
In larger animals this would be far too slow, says Laughlin, but in the tiny body of the greenhouse whitefly parasite, a partly “clockwork” brain might be the best approach.