A Look Inside Your Head

Neurons undergo a surprisingly treacherous journey to allow you to think, move, and feel.

Photo by Lulu Loverling/Flickr, Creative Commons.

“Will you get him tonight?” she asks her husband pleadingly, glancing at the clock. “My interview is so early tomorrow.” They lay there for a silent moment, listening to the baby’s cries. He sighs, mumbles something incomprehensible, and gets up to soothe their newborn child. She closes her eyes and tries to get her brain, fully awake from being startled by the crying, to quiet down so she can get some rest. But her brain cells refuse to be quiet and her mind continues to wander.

These brain cells that keep her awake are called neurons. They are cells that connect to each other in complex and purposeful networks and constantly communicate with each other. This communication between neurons is what makes her baby know it wants to cry, allows her to sense the crying and wake up, and gives her the idea to ask her husband to take care of it because she has an important interview in the morning. But how do these neurons know where to send their messages? How does each one know, for example, which cell to send the ‘wake up your husband to take care of it’ message to next so this idea comes to fruition?

The answer comes from thinking about the way that neurons become connected in a network. As the nervous system develops, each new neuron starts off as a simple round cell. But over time, that cell is bombarded with chemical signals that cause it to start to grow extensions out of its round origin. These extensions are what allow neurons to communicate with each other because they end up travelling to connect to extensions from other neurons. These connections form synapses, which are the places where messages are sent from neuron to neuron.

The extensions that a neuron uses to send messages to other cells are called axons. Axons begin as small protrusions from the original round neurons and grow rapidly once the proper signals are sent to them. These signals tell them that they must embark on journey into the unknown to find their way to their destined partner neurons.

Photo by clappstar/Flickr, Creative Commons.

Many of these axon journeys are still well underway in the baby’s developing brain as her cries startle her parents awake. Inside her tiny head, millions of axons are asking for directions to their destination, like lost tourists in a new city. The twist is: it’s a city where thousands of different languages are spoken, so finding someone to ask is not simple. Like people from different countries only understand one or two languages, axons from different classes of neurons will only be able to understand certain types of signals. So despite the presence of thousands of chemical signals floating around in the brain, an axon only responds to signals it has the machinery to grab onto and interpret.

Once received, these signals essentially play a game of ‘hot and cold’ with the axons, telling them either “warmer, warmer, you’re headed in the right direction” or “too cold! Turn around!” This game continues, causing the axon to get longer and longer as it reaches toward the signals that say “come over here! Your partner neuron is this way!” Then, suddenly, this journey ends. The axon receives a signal that is stronger than any of the others which tells it that its destination has finally been reached. “Stop here,” it says, “you made it.” The axon reaches its final length and begins to form a synapse with its partner neuron so that they can talk to each other and play their part in the complicated network of the brain.

Every axon undergoes this journey, and although it may seem like it is somewhat random, it’s actually very controlled and precise. Partner neurons don’t find each other by chance; they find each other and connect because this signaling system is tightly controlled by the body. For the brain to function normally, the right neurons need to find each other and connect.

Image of hippocampal neuron in culture by Shelley Halpain, UC San Diego.

So when the baby cries, she might not really understand what she’s crying about because all of these connections have not yet been made. But in her mom’s brain, specific networks of neurons are talking, trying to come up with a solution to the problem. When she wakes up her husband, his neurons send messages down their axons to be received by other neurons, and he is able to tell himself to get up and calm their daughter because his wife needs her sleep tonight. And this is all able to happen because of that initial journey that axons went on. Our thoughts and movements are only able to occur because our axons were able to navigate their way through complex and unknown territory to find their partners as our brains developed.

Marilyn Day is a senior biology major at Ursinus College and a fellow of the Center for Science and the Common Good.

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