Nanotube-based neurotechnologyMonday, 29 December, 2008
A recent study in Nature Nanotechnology shows augmentation of neurons by growing them on a base of carbon nanotubes. The author’s hypothesis is that this is due to the carbon nanotubes conducting electrical impulses from the axon back to the dendrites, thus acting as a shortcut for the normal process of back propagation of action potentials.
Back propagation (propagation in the direction opposite to the arrow in the above diagram) occurs when the action potential flows not only along the axon, but also back to the dendrites. Back propagation of the action potential is required for synaptic potentiation, as it allows for extra excitation of the dendrites at those occasions when the axon has fired, thus leading to better coincidence detection via Hebbian learning (Magee and Johnston, 1997). Neurons strengthen connections based on the close timing between presynaptic and postsynaptic excitation (‘fire together, wire together’), so it is important for signals of postsynaptic excitation to propagate back to the dendrites where the synapses are, so that the synapse may be strengthened or weakened accordingly.
In a sense, then, the augmentation of neurons in this study is similar to that seen in NR2B transgenic mice (aka ‘Doogie mice’), which overexpress the gene for a subunit of NMDA channel – a channel involved in long-term potentiation (LTP) of synaptic connections. The facilitation of more back-propagated action potentials would also result in enhanced learning and memory, if carbon nanotubes were seeded into key parts of the brain. This would, however, cause similar problems to the NR2B Tg mice. There is more to learning than simple recall, as forgetting is important too. We have a limited number of neurons to play with, and therefore a blanket increase in memorisation is an inferior solution compared to a highly-regulated memory augmentation, where things we need to remember are remembered and things we don’t wish to recall are forgotten.
But, this study was basically a random arrangement of nanotubes and neurons, which created some interactions which proved to be functional. It will be very interesting to see future neurotechnology based on carbon nanotubes, as these ‘wires’ are small enough to connect not just to a single neuron but between individual parts of that neuron. Yet still many of the issues of brain-computer interfaces will exist even with the enhanced biocompatability afforded by the carbon nanotubes – one needs to be able to interpret signals used by neurons and compute them, and this computation needs to be done with a small, preferably implantable computer. Still, I have a feeling nanotechnology will be essential for any cyborg (which is the main reason I had nanotechnology as my second field of study on my Bachelor of Science, after my first love – neuroscience).
Reference: Cellot et al (2008) Carbon nanotubes might improve neuronal performance by favouring electrical shortcuts, Nature Nanotechnology, AOP 21 December 2008, doi: 10.1038/nnano.2008.374