The Dynamic Duo of the Nervous System

Every morning, I wake up, pour myself a cup of coffee, and I read. It could be a magazine article, an interesting research paper, a book—it doesn't really matter what it is. I just find that the words and the tempo of my eyes moving back and forth across the page get me into a good head space for the day.

Minus a few years at the end of high school (a time when you’re desperately trying to fit in, and reading doesn’t seem like something the cool kids are doing), I’ve always read a lot. The Hardy Boys and The Screech Owls when I was young; books about space, turtles, or spiders in elementary school; classics like The Great Gatsby and Brothers Karamazov in my 20s; and philosophy, psychology, and statistics in my 30s. I just read and have always done it. Until I started thinking about all the little cognitive tasks that reading entails, I’d never given it much thought as a cognitive skill.

But when you break it down into its basic elements and try to understand how all the pieces fit together, you see how amazing of a mental feat reading is. Wavelengths of light bouncing off a page (or emanating from a screen) find their way to the photoreceptors at the back of the eye, get translated into electrical signals that wind their way along the optic nerve into the visual cortex, and a symphony of neural activity stitches all the pieces together. Emotions are elicited, memories are brought to the forefront of our consciousness, and new memories are formed as we interpret and understand new things.

Reading requires visual recognition, emotion, planning, and reasoning. You need to be problem-solving, using your working memory, recalling what you already know from long-term memory, and arousing your attention to stay focused. These cognitive tasks require neurons in the cortex, posterior parietal cortex, prefrontal cortex, hippocampus, hypothalamus, thalamus, and amygdala to be active and working together to create a cohesive reading experience.

Reading is just one of the marvels of the human brain we take for granted. Driving a car, playing a sport, listening to music, playing music, walking, exercising, playing a board game—all the things that make up our day-to-day lives and make life worth living require a tremendous amount of neural activity and coordination. How is this level of coordination possible?

Many millennia ago, a cell wrapped itself around an axon. What followed was a relationship fortified across millions of years of natural selection. The two cells are now inseparable. Each one is necessary for the other. They are the dynamic duo of the nervous system.

Axons are the long, wiry extensions of neurons carrying electrical signals (called action potentials or impulses). Axons make information flow between different brain regions and different parts of the body possible. Evolution has formalized the cellular wrapping around the axon into a substance called myelin.

The relationship between axons and myelin has paved the way for the increased processing power, enhanced coordination, and adaptability that characterizes the human brain and has allowed us to thrive. Part 1 of this dynamic duo series talked about increased processing power. Part 2, this article, will delve into enhanced coordination—a facet of our brain that makes complex cognitive tasks like reading, writing, and driving a car possible.

Enhanced Coordination

Everything we do—from the simple act of reaching out and grabbing a glass of water to contemplating the meaning of life and our place in the cosmos—requires neurons in different regions of the brain to coordinate. Neurons working together in this way are called networks. The neurons a network consists of and where they are located in the brain depends on the task. Listening to music, playing music, reading, and writing, for example, may all use slightly different neurons. Although there could be a considerable amount of overlap—any particular neuron or group of neurons may play a role in many different cognitive functions.

Neurons synchronize into functional networks by sending electrical signals (action potentials) back and forth at regular intervals. To understand this process, it’s best to think of regions of the brain as islands in a calm sea. For the purposes of this analogy, we’ll say each island has two inhabitants (representing neurons in the same brain region). If our islanders want to perform a task (walk the perimeter of the island, for example) in complete unison, one can call “left” every time their left foot contacts the ground. The other can then use that cue to match their gait.

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In just a couple of steps, the two island inhabitants will be walking in complete synchrony. Because of their close proximity, the utterance and interpretation of the cue can be frequent and monitored at a very high rate. This is an example of local synchronization.

In the brain, local synchronization is carried out by a specialized type of neuron called an inhibitory interneuron. Inhibitory interneurons act as mediators, fine-tuning the timing of action potentials being passed back and forth so the neural activity of two or more neurons can be synchronized.

Long-distance synchronization, however, is beyond the capabilities of the inhibitory interneuron. If our islanders want to coordinate with people living on another island, they must use a long-range form of communication (yelling or waving won’t work because the other islands are too far away). Clever as they are, the person calling “left” steps in the water with every complete revolution of the island, sending waves into the calm sea surrounding them.

The created waves cross the sea and gently lap on the shore of an island far away. The inhabitants of this island (who are also circling the perimeter in synchrony for their own reasons) notice this perturbance. In response, they begin stepping into the water at the spot the waves are breaking.

Within a few revolutions of their island, they notice something: If they step into the water too early or too late, they disrupt the wave pattern. But, if they step at just the right time, they send waves of equal or greater strength back in the other direction. They begin to time their revolutions of the island so they can step in the water at just the right moment.

Our first islanders see returning waves now breaking on their shore. They, too, begin timing their revolutions of the island so they can step into the water at just the right moment. With both sets of island inhabitants now synchronizing their activity to the waves, they have inadvertently synchronized with one another. They’re now working together.

In our brains, axons traverse the communication void between brain regions (our islands). These long fibers extending from the bodies of neurons are the wave carriers. Groups of neurons within a brain region—could be the cortex, posterior parietal cortex, or amygdala—synchronize with the help of inhibitory interneurons. They then send action potentials to other areas by way of their axons. Local synchronization of neurons in the receiving brain region is influenced by these incoming action potentials and sends signals of their own back. The reciprocal communication allows the two regions to synchronize and form a functional network.

Local paired with long-distance synchronization is how all the neurons become active while we read, drive, walk, interpret our surroundings, or chat with a friend work together to perform all the little cognitive tasks (pattern recognition, planning, etc.) that make up the whole unified experience.

For synchronization, and therefore the formation of neural networks that allow us to do all the wonderful things of life, timing is everything.

In our island analogy, there was no outside force capable of modifying the speed of waves traveling between islands. If there was, it would take some of the onus for timing off of the islanders, making their lives and synchronization easier. Luckily, our human brains have developed such a mechanism.

Myelin—that wonderous, fatty substance wrapping itself around axons—can fine-tune the speed of action potentials, so they arrive at their destination at just the right time. By subtly changing the number of wraps or the length of the wrap, the speed of the action potential can be sped up or slowed down.

Action potentials are little bursts of electricity. And electricity is generated by the movement of charged particles called ions. Myelin increases the speed of action potentials by making it easier for them to travel within the axon and by preventing ions from leaking out across the axon membrane, which weakens the signal.

By wrapping itself around the axon just a few more times, myelin can speed up an action potential by making it that much easier for ions to move. The opposite is also true: If a few myelin wraps are removed, ions can’t move as easily within the axon, and the action potential is slowed.

If myelin changes the length of its wrap, the space along the axon where an action potential can move quickly is increased or decreased. This speeds or slows the overall time it takes a signal to travel the entire length of an axon.

These are both incredibly subtle changes. But when it comes to the brain and timing, subtlety matters. Researchers estimate that even 10-percent changes in myelin (in either length or thickness) can change the arrival time of an action potential by milliseconds. And that kind of discrepancy can be the difference between synchronization and disarray among brain regions.

The island analogy had just two islands—this is relatively simple in terms of coordination. Reading, as mentioned above, involves neurons in at least seven different brain regions (the cortex, posterior parietal cortex, prefrontal cortex, hippocampus, hypothalamus, thalamus, and amygdala). The more regions, the more coordination required. And reading isn’t unique. Most cognitive skills require the coordination of at least that many areas of the brain.

Coordination becomes even more complicated when you consider that action potentials arriving in a single location from two or more places are traveling different distances and influence one another. If the timing of their arrival is right, the strength of the arriving action potential is bolstered, making it more likely to register at its destination. If it’s just a touch off, however, the signals may cancel each other out, and the action potential dies.

When this kind of precision is required, hundredths and thousands of milliseconds matter. Without myelin, coordination of this magnitude simply wouldn’t exist. And how we see the world would be drastically different. What we think of as consciousness just wouldn’t be possible.

But questions still remain: How does myelin, stuck out there in the communication void between neuron cell bodies, adapt to the changing needs of neural networks? That is the topic to be explored in Part 3 of this series: "Adaptation."