A Crucial Design Difference in Vertebrate Nerves
Editor’s note: We are pleased to present a series adapted from biologist Michael Denton’s book, Fire-Maker: How Humans Were Designed to Harness Fire and Transform Our Planet, from Discovery Institute Press. Find the whole series here. Dr. Denton’s forthcoming book, The Miracle of the Cell, will be published in September.
In addition to fast conduction speeds, there is another criterion that must be satisfied for nerves to be fit to coordinate muscle activity in organisms like ourselves. Each muscle is innervated by hundreds, if not thousands, of individual nerve fibers. As Schmidt-Nielsen points out, “vertebrate muscles… are controlled by nerves that carry hundreds or thousands of single axons” (nerve fibers).1 These are between 5 and 20 microns in diameter.2 Clearly, if these individual axons had to be much bigger for some reason to attain the necessary speeds of 120 meters per second, this would necessitate “nerve trunks of inordinate size.”3
For example, if vertebrate nerve fibers had to be the size of the fast conducting axons of invertebrates, which are up to a millimeter in diameter (fifty times the diameter of the fastest axons in mammals), then the nerve cords in mammals would be simply too large to fit into the body. As Schmidt-Nielsen points out, the optic nerve in humans has a diameter of three millimeters; if it were to contain the same number of fibers the size of large invertebrate axons conducting at the same speed as those in the optic nerve, this would require a diameter of 300 millimeters or 12 inches — larger than the head.4 And the nerves supplying the muscles of the arm would have to be larger than the arm!
The reason that vertebrate nerves can attain such high conduction speeds and remain far smaller than invertebrate nerves is because of a crucial design difference that permits the very rapid conduction of impulses. As Schmidt-Nielsen explains, rapidly conducting vertebrate axons are covered in a thin sheath of a fat-like substance, myelin, “which is interrupted at short intervals to expose the nerve membrane,” and “the exposed sites are known as nodes.”5 These nodes are separated from one another by a fraction of a millimeter up to a few millimeters.6 This design allows for what is termed “saltatory conduction,” where the nerve impulse, instead of travelling sedately and continuously down the axon, jumps from node to node, vastly increasing the speed of transmission.7 The great advantage of myelinated axons comes from their small size, which allows a highly complex nervous system with high conduction velocities without undue space occupied by the bundles of nerve fibers that make up the major nerve trunks.
Close to the Maximum
Consideration of the basic characteristics of nerve impulse propagation suggests that the speed of conduction in mammals is close to the maximum possible that is compatible with the electrical properties and general design of cells. The speed of nerve conduction is determined by a number of biophysical factors, such as the speed that sodium and potassium ions are transported across the lipid bilayer membrane, itself a physical constant. The existence and characteristics of the membrane are determined by the inherent insulating character of the lipid bilayer that surrounds all animal cells, which is the only structure known that is fit to serve as the bounding membrane of the cell.
In a counterfactual world in which the speed of nerve conduction was much less than 120 meters per second, or in which the size of axons had to be much more than 20 microns in diameter, beings of our size with our abilities of fine-motor coordination would be impossible. What this means is that the functioning of the nervous system in humans (and other mammals) is entirely dependent on the fact that speeds of 120 meters per second can be achieved with axons of less than 20 microns in diameter. This allows for nerve trunks that take up very little of the volume of the body, but can carry thousands of nerve messages to the muscles necessary for fine motor control and can carry information back to the central nervous system about heat, pain, touch, and spatial coordination from the various sense organs in the periphery.
Implications of an Upright Stance
Finally, in addition to the unique fitness our size and android design for the mastery of fire and the unique fine tuning of nature which makes our biological being possible, it is worth reflecting on the further deep implications of our upright stance. As Leon Kass comments:
Human uprightness is nothing superficial; our peculiar form is reflected in every detail of our deep structure, somatic and… psychic. Upright posture is a matter not merely of static shape, of flat-footed two-leggedness or mere verticality. It also conditions all our relations to the world and colors all our experiences of ourselves acting and suffering in the world.8
We are not just a fire-maker because of our upright stance and android design, but the same unique design played a critical role also in shaping many aspects of our humanity.
Tomorrow, “Fire and Fitness: A Summary of the Evidence.”
- Schmidt-Nielsen, (1984), Chapter Seventeen.
- Schmidt-Nielsen, (1997) Chapter Eleven, see table 11.13; Dominique Debanne, Emilie Campanac, Andrzej Bialowas, Edmond Carlier, Giséle Alcaraz, “Axon Physiology,” Physiological Reviews 91, no. 2 (April 1, 2011): 555–602. doi:10.1152/physrev.00048.2009.
- Schmidt-Nielsen, (1984), Chapter Seventeen.
- Schmidt-Nielsen, (1997) Chapter Eleven. As the author comments: “The greatest advantage of myelinated axons comes from their small size, which allows a highly complex nervous system with high conduction velocities without undue space occupied by the conduits. Let us say that we wish to increase the conduction velocity 10-fold in a given nonmyelinated fibre. This would require a 100-fold increase in its diameter, and the volume of nerve per unit length would in turn be increased 10,000-fold.”
- Leon R. Kass, The Hungry Soul: Eating and the Perfecting of Our Nature (New York, Macmillan, 1994), 64.