Size matters in biology. Whether you want to think about bone shapes, heart rates or lifespans, an elephant and an elephant shrew are enormously different despite their similarity in name and mammalian heritage. Exploring size-related effects in biology is the study of scaling, and when it comes to the study of size in relation to the nervous system, the brain seems to almost always be front and center. However, how the central nervous system communicates with the rest of the body via the peripheral nervous system has now been framed in an interesting scaling context by Heather More and Max Donelan of Simon Fraser University working with colleagues from Canada, the UK and the USA.
Think of it like this: the time it takes for an animal to sense and respond to a stimulus depends, in part, on how long it takes electrical signals traveling along axons to move from place to place. Hence, axonal conduction velocity, which depends on axonal cross-sectional area, must be an important factor determining an animal's responsiveness. Of course, the signal conveying an irritation on the foot of an elephant (and the signal conveying the appropriate response) has a lot farther to travel than similar signals in an elephant shrew. If large animals are to be as responsive as small animals, then axonal conduction velocities and thus diameters would have to increase roughly in proportion to some linear measurement of body size like limb length.
To see whether this is what happens the researchers collected three independent data sets. The first involved using electrophysiological techniques to directly measure conduction velocities along major nerves innervating the medial gastrocnemius in six least shrews and one Asian elephant (see http://www.sfu.ca/locomotionlab/2010/06/29/nervespeedlimit/ for video of the elephants). To augment this data set the researchers also performed a literature review to identify maximum axonal conduction velocities among diverse terrestrial mammals spanning a broad size range. Finally, the team used nerve fixation techniques and scanning electron microscopy to measure axon sizes in the sciatic nerves of a least shrew and an African elephant.
What the authors found is surprising. The largest elephant axons were only approximately twice the diameter of the largest shrew axons. Similarly, measurements of axon conduction velocity revealed that nerve signals traveled a little less than twice as fast in elephants (mean=70 m s–1) as in shrews (mean=42 m s–1). These data fit well within the context of the data collected in the literature review, which led to an overall scaling relationship where conduction velocity is proportional to body mass0.04, i.e. very little effect of size. Thinking through a concrete example, an elephant's limb is 100× as long as a shrew's, but its axonal conduction velocity is only twice as fast, indicating a 50-fold longer conduction delay for a signal moving through the elephant's peripheral nervous system.
In short, larger animals appear to be woefully less responsive than their smaller counterparts. Delayed response times in larger animals could mean that some of their reactions may take too long to be of much use. The authors propose the intriguing idea that large animals may need to rely more on predictive motor control strategies rather than feedback control as a result.
