The elephant and the shrew, an axonal story

(Crossposted from NeuroDojo.)

The world is different for small animals and big animals. J.B.S. Haldane said it best:

To the mouse and any smaller animal (gravity) presents practically no dangers. You can drop a mouse down a thousand-yard mine shaft; and, on arriving at the bottom, it gets a slight shock and walks away, provided that the ground is fairly soft. A rat is killed, a man is broken, a horse splashes.

What does scale mean for neurons? As an animal gets bigger, it’s going to take longer for neural signals to get from one part of the animal to another – all other things being equal. But, typically, not all things are equal. You can speed up how fast a signal travels down the length of a neuron by making that neuron larger. just like water current can flow faster through a bigger pipe, an electrical current can flow faster down a larger axon.

There’s more going on in this paper by More and colleagues (so it’s literally More going on), but that gives you a starting point for the rationale here. If you take the teensiest mammal you can find, and one of the biggest mammals you can find, how different are those axons, and how fast they can send their signals, going to be?

To test this, they did neurophysiology and neuroanatomy on the neurons of a least shrew and an Asian elephant. These two animals are about as different in size as mammals can get. The shrew’s mass measures in milligrams, and the elephant’s measures in megagrams – tonnes!

But because you don’t want to draw a line using only two data points, they also went into the existing literature for mammalian neuroanatomy, and got equivalent measures for about nine other species of mammals.

The axon sizes scaled – bigger animals had bigger axons – but much, much less than the animal’s size. I mentioned mass before, but distance is the more relevant measure, though less impressive. An elephant’s leg is about 100 times longer than a shrew’s, but the elephant’s axons are only about twice as big. That’s about as close to scale-free a relationship in biology as you’re likely to find.

Okay, the neurons may be about the same size, but maybe there’s some other mechanism that might make the actual speed of the signals a better match to the animals’ proportions. Nope. A similar story held when they stimulated the nerves electrically and measuring the delay to the muscle twitch.

All of this means that big animals are going to be slower to detect and react to the world around them. It’s a real cost to being massive, which are no doubt compensated for by other factors. Like being able to sit anywhere you want.

Unfortunately, More and company point out that all this means that dinosaurs and other large creatures probably weren’t as as agile as they are often portrayed.


Reference

More, Heather L., Hutchinson, John R., Collins, David F., Weber, Douglas J., Aung, Steven K. H., & Donelan, J. Maxwell. 2010. Scaling of sensorimotor control in terrestrial mammals Proceedings of the Royal Society B: Biological Sciences : DOI: 10.1098/rspb.2010.0898

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Comment by David D. Olmsted on July 31, 2010 at 6:57am
Interesting but the authors seem to be forgetting the physics of the problem. Structures are only as strong as the forces holding them together and that is proportional to their cross-sectional area while mass and inertia are proportional to volume. So the bigger the animal the slower it has to move if it is to stay in one piece.

So I suspect the evolution of conduction velocities is simply following the inherent physics. Still, I am surprised the scaling is so flat.

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