New article: max growth of nerve cells in big mammals and dinos

The largest dinosaurs had individual cells more than 30 meters long. How did such things develop? Read on! Illustration from Wedel (2012: fig. 2).

Here’s something that’s been in the works for a while: a popular article in Scientific American on stretch growth of axons in large, fast-growing animals:

Smith, Douglas H., Rodgers, Jeffrey M., Dollé, Jean-Pierre, and Wedel, Mathew J. 2022. Giraffes vs. blue whales vs. dinosaurs: contest reveals which one builds its nervous system fastest to evade predators. Scientific American, https://www.scientificamerican.com/article/giraffes-vs-blue-whales-vs-dinosaurs-contest-reveals-which-one-builds-its-nervous-system-fastest-to-evade-predators/

This one started a few years ago, when Doug Smith at the University of Pennsylvania saw my ‘long nerves in dinosaurs’ paper (Wedel 2012) and reached out to me to ask about the growth of nerve cells in giant dinosaurs. Among his many other interests in neurobiology, Doug has worked on the stretch growth of axons (Smith et al. 2001, Smith 2009, Purohit and Smith 2016).

As a reminder, the axon is the “sticky-out bit” of the neuron. In unipolar neurons like the one in the cartoon above, the axon transmits signals away from the nerve cell body or soma. Most primary sensory neurons — the ones that actually receive stimuli from the environment — are pseudounipolar, meaning that the axon extends in both directions, with the soma sitting off to the side like a teardrop on a tightrope.

Also worth noting is that almost all drawings of neurons are hilariously compressed and oversimplified. I drew that cartoon neuron, above, with a few dozen synapses. Here is an actual neuron from the cerebellum, drawn from a stained specimen by Spanish anatomist Santiago Ramón y Cajal in the late 1800s:

In my hand drawn neuron cartoon, the length of the axon is only three or four times the diameter of the soma. You have motor neurons that run from your lower back to your feet, in which the axon is 10,000 times as long as the soma is wide (~1 meter vs 0.1 millimeters). The difference is even more pronounced for primary sensory neurons, some of which run from your toe-tips to your brainstem, and which have somata as small as 0.02 millimeters across, or 1/100,000th of the length of their axons. In a 20-meter whale or sauropod, the axon of a primary sensory neuron could be 1 million times longer than the soma.

How do such ridiculously elongated cells develop? One method is stretch growth, and that’s what Doug has been studying for more than two decades now. Once an axon has found its innervation target, it’s stuck, like a grappling hook trailing a rope. As the body parts between the soma and the axon terminals grow, the axon is forced to grow in length to keep up (in the grapping hook analogy, playing out more rope). This can be done in the lab, by getting neurons to connect to two plates, and then cranking the plates apart.

How fast can axons possibly grow by stretching? For that we have to look at the maximum linear growth rates of the largest and fastest-growing mammals and dinosaurs. Doug and I and our coauthors wrote a whole article about that, and it’s short. Check it out — here’s that link again.

References

• Purohit, P.K. and Smith, D.H. 2016. A model for stretch growth of neurons. Journal of Biomechanics 49(16): 3934-3942.
• Smith, D.H. 2009. Stretch growth of integrated axon tracts: extremes and exploitations. Progress in Neurobiology 89(3): 231-239.
• Smith, D.H., Wolf, J.A. and Meaney, D.F. 2001. A new strategy to produce sustained growth of central nervous system axons: continuous mechanical tension. Tissue Engineering 7(2): 131-139.
• Wedel, M.J. 2012. A monument of inefficiency: the presumed course of the recurrent laryngeal nerve in sauropod dinosaurs. Acta Palaeontologica Polonica 57(2):251-256.