The Dynamic Duo of the Nervous System

Don Quixote and Sancho Panza. Michael Jordan and Scottie Pippen. Holmes and Watson. Characters bound together by circumstance, opportunity, or fate. Sometimes they’re friends; other times, the relationship is more antagonistic. Whatever the reason and dynamics of the union, it’s always clear that one needs the other. They’re just better together. Who would Butch Cassidy be without the Sundance Kid? Could Han Solo be Han Solo without Chewbacca?

There’s something alluring about the dynamic duo narrative. It pervades literature and movies. We seek it out in sports coverage. And we try to find it in our own lives. Maybe we gravitate to the story of the dynamic duo because it’s an essential part of who we are. Maybe it is who we are.

Dynamic duos can be found all around the body. Mitochondria—the little cellular machines generating most of the energy needed by our living cells—are the descendants of engulfed prokaryotes. Bacteria line the skin, respiratory tract, and gut providing essential protective and faciliatory functions. Even viruses (despite their bad reputation) are critical to our existence. It’s estimated that at least 8 percent of the human genome contains the ghosts of ancient retroviruses that worked their way into our DNA. Viruses can provide us with helpful genes that make us more capable of functioning in our environment.

Four hundred and twenty-five million years ago, a synergy developed within the nervous system of the hinge-jawed fish, a placoderm. The pressures of adaptation and natural selection forged a "dynamic duo" relationship that set the evolutionary stage for our brains to become what it is today. Millennia ago, a cell—maybe some type of primitive phagocyte—wrapped itself around a nerve fiber. That cell wrapping was essential for the increased processing power, enhanced coordination between brain regions, and the ability to adapt that has allowed our species to thrive.

But the relationship is delicate. When the scale tips toward dysfunction, maladies of the nervous system arise. Multiple sclerosis, Alzheimer’s disease, Parkinson’s disease, depression, and anxiety are attributable, or linked to, perturbations to this dynamic duo of the nervous system.

This article, Part I of this series on the dynamic duo of the nervous system, examines the increased brain processing power allotted by this relationship between brain cells.

Increased Processing Power

Electricity is the information currency of the brain. Little bursts of it travel from neuron to neuron and neuron to cell, allowing brain regions to communicate with one another and with different parts of the body. These little bursts are called action potentials or impulses. Action potentials racing throughout the body make possible the conscious movement of our arms, legs, and other extremities. They allow us to think, reason, and remember. They allow us to feel.

Action potentials are generated by the movement of tiny particles called ions. Ions lie in wait inside and outside the neuron until it’s stimulated. They then flow across the membrane barrier of the neuron and into adjacent regions, triggering the entry of more ions. As ions in subsequent areas enter, the electrical impulse can travel.

Action potentials travel along wiry, cable-like extensions called axons. When the end of an axon is reached, a chemical messenger carries the electric signal on to the next cell. The speed the impulse can travel is, in part, dictated by how easily ions can flow without being impeded or lost to the exterior of the cell.

Speed, along with the sheer number of operations the brain can perform simultaneously, dictate the brain’s processing power. An operation can be simple—like moving your finger or computing 2 + 2—or it can be complex, say abstract reasoning or figuring out what your next move is going to be during a chess game. The speed component of processing power is limited by the speed of the action potential. The number of simultaneous operations reflects the number of elements capable of communicating using action potentials.

Hundreds of millions of years ago, the ancestors of the hinge-jawed fish were under evolutionary pressure to get bigger and faster—likely so they wouldn’t be such easy targets for predators and could become better predators themselves. But an increase in size poses a structural problem, one that can only be overcome by increasing the brain’s processing power. If the body gets bigger, neurons have to get longer. A fish cannot command the movement of its tail fin without electrical signals being passed from the brain, along the length of the body, and to the muscles responsible for the back-and-forth movement of the tail.

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If the neuron gets longer and the speed of the action potential doesn’t change, the time it takes for the signal to travel from its origin to its destination has to increase. In the case of the hinge-jawed fish, it means signals commanding the tail fin won’t get there very fast and the fish won’t be able to move as quickly as it needs to.

Biology has developed two ways to increase the speed of signal transduction. The first is to increase the diameter of the axon. An axon is a bit like a garden hose with a bunch of gunk in it (except that in the axon, the gunk is necessary cellular machinery). Water flowing through the garden hose will be impeded by all the gunk it runs into. If you increase the diameter of the garden hose, you give the water more room to get around the gunk, allowing it to move faster.

Cephalopods like the squid and octopus have adopted this approach to increase the speed of signal transduction. But it’s limited to the few neurons involved in the rapid escape response.

The second way to increase the speed of signal transduction is to insulate the axon. As ions move along the nerve fiber, some leak out, weakening the strength of the signal. If the signal isn’t strong enough to trigger the movement of ions across the membrane in neighbouring areas, the signal dies. Wrapping an axon in a material that preserves signal strength for greater lengths along the nerve fiber allows the impulse to travel faster.

In our brains, myelin is the substance wrapping axons to preserve signal strength and speed up signal transduction. Myelin is the evolutionary progeny of a cell that serendipitously wrapped an axon in the hinge-jawed fish millions of years ago.

Why did evolution choose myelin over increasing axon diameter to speed up signal transduction? It's likely because of space.

The brain and spinal cord are confined within the skull at the top of our head and the vertebrae running down the midline of our backs. This is true for us and true for much of our evolutionary line. Being tucked away behind these rigid structures introduces special constraints—which means there’s a limit to how big the brain and spinal cord can get. And this limit is reached far quicker than the increase in axon diameter needed to accommodate a modest increase in body size.

We humans need a signal transduction speed of about fifty meters per second. To attain that speed by increasing axon diameter, the spinal cord (which is made up of tracts of axons) would need to be one meter in diameter—about ten times what it is now! This isn’t exactly space-efficient.

By wrapping axons in myelin, we’re able to dramatically increase signal transduction speed while maintaining an axon diameter that is spatially viable. This allows us to maintain a high number of neurons (and other brain cells) while also being able to increase signal transduction speed. Maintaining a high number of potential contacts in the brain leaves room for increased processing power by upping the number of operations that can be performed simultaneously. In other words, it leaves room for high order brain processes like complex perception, judgment, and planning to develop. These are brain functions that require the use and coordination of many operations simultaneously.

The next part of this series is going to explore how the interaction between myelin and axons (our dynamic duo of the nervous system) paved the way for enhanced coordination between brain regions.