In the world of neurons, bigger may not be better, but it is usually faster – which is almost as good.
The wider an axon, the faster a signal travels along it. You can see this readily by playing around with a computer simulations. This is the traditional explanation for why the largest, fattest neurons are almost always found in escape circuits. Escape systems push neurons to the limit of what is physically possible to shave off every possible microsecond in the response time, because every single one could make the difference between life and death.
But when you get away from these largest neurons, what explains the differences in the axon diameter? Can everything be explained just by the need for speed?
Perge and colleagues look at the issue of axon size (which, for reasons unclear, they call “caliber”) across lots of different structures. For the most part, they use mammalian brains. They didn’t include motor neurons, because the problems motor neurons are largely forced by the distance to a muscle. They are more interested in the differences in sizes within tracts contained within the central nervous system, where all the neurons are more or less starting and stopping in the same place.
Their hypothesis is that the differences in axon size within a brain are related not to speed so much as to the ability of a neuron to convey information. The smallest axons convey information at the lowest rates, according to their discussion. Big axons can transfer information at higher rates, they argue, because there is less “jitter” in their timing. Plus, a big neuron can send a signal to many more cells downstream than a small one.
The problem, though, is that big neurons are expensive. Smaller neurons, with lower rates of spiking, convey information less rapidly, but do so more efficiently. It’s like using a Bugatti Veyron to deliver a postcard: it’s fast, but it’s an absurd extravagance. To reduce the time signals spend traveling along the axon, the authors argue that brains typically try to minimize the distance the signal has to travel instead of increasing the size of the axon.
I was also interested to see a little aside about the amount of mitochondria in neurons. Neurons are famously energy hungry cells. They noted that nonspiking neurons have fewer mitochondria than myelinated spiking neurons, and unmyelinated spiking neurons typically have the highest fractions of mitochondria.
They don’t provide any of their own original recordings of spiking rates here. The new data are all anatomical, based on electron microscope sections. Their data serves, almost incidentally, as a nice little review of neuronal diversity.
I always find papers discussing “information” to be a bit tricky, and this one is no exception. But it’s a useful paper to get you thinking about “how neurons work.”
Perge JA, Niven JE, Mugnaini E, Balasubramanian V, Sterling P. 2012. Why do axons differ in caliber? The Journal of Neuroscience 32(2): 626-638. DOI: 10.1523/JNEUROSCI.4254-11.2012
Sculture photo by nicoleversetwo on Flickr; used under a Creative Commons license.
Veyron photo by Philipp Lücke on Flickr; used under a Creative Commons license.