The more I learn about the discipline of neuroscience, the more I come to see it as the great scientific potluck of our day. While the actual meal at a potluck often seems disjointed, it allows guests to sample a wide variety of tasty foods brought by people from different culinary backgrounds. This post is the first in a series that will highlight contributions from various scientific disciplines that have furthered our understanding of how the brain and nervous system affect how we think and behave. Be on the lookout for more posts in the future!
While most scientific disciplines have contributed at least something to the neuroscience potluck party, the field of physics has brought many fundamental concepts (main dishes, if you will). The subfield of electricity and magnetism brought an especially exciting morsel: the finding that brain cells, called neurons, can send lightning-fast electrical signals, called action potentials. An action potential is an electrical wave that travels down an axon, the “sending” wire of a neuron. They occur in milliseconds, an incredibly fast event compared to other biochemical and hormonal signals in the body, which can take minutes, days, or months to communicate their messages. An action potential is enabled by a beautifully coordinated flow of electrical charge in and out of the neuron as it sails down an axon.
In neurons, electrical charges live in various atoms and small molecules that we call ions. I like to think of ions as two medieval armies (complete with swords, shields, battle axes, etc.) that are locked in a bitter feud.
Soldiers in these two groups, Team Positive and Team Negative, long for the chance to leave their army and engage in endless hand-to-hand combat with as many enemies as they can. If you place a group of Positives on one end of a field and a group of Negatives on the other, they will rush towards each other, bouncing around in a frenzy until the chaos is evenly spread.
In the context of the brain, neurons are the battlefields where tiny ionic battles are fought. Their cell membrane, which is essentially the “skin” that surrounds cells, acts as a wall to separate neuronal “guts” from the outside world. If you place Team Positive and Team Negative on either side of the cell membrane, they will sense the enemy and rush to the wall for battle. However, cell membranes are virtually impenetrable to ions, so the warmongering particles will line up close to the membrane, ready to attack at any opportunity. Fortunately for them, neurons have gates in the cell membrane called ion channels, which allow selective passage of ions through the membrane. Team Positive can pass only through Gate A, and Team Negative only through Gate B. When Gate A opens, as many positive ions that are allowed will excitedly rush into the sea of their negative foes; Gate B does the same for the negative ions. As soon as a gate closes, no more ion soldiers can cross the cell membrane.
It turns out that neurons not only tightly control how ion channels open, but they also ensure that Positives and Negatives constantly have sufficient numbers and desires to flow through an open channel and attack. This desire is known as an electrical gradient, which just means that there is a difference in overall charge between the inside and outside of the cell. Using proteins called ion pumps, neurons essentially catapult positive ions from their preferred location (inside the cell, locked in glorious battle with the Negatives) to the outside of the cell, where the rest of the Positives wait impatiently. Plucking an ion and sending it back through the membrane against its will requires a great deal of effort, as you might imagine. The brain consumes over 20% of the body’s energy, even though it makes up only 3% of the body’s overall mass. Much of this energy goes towards maintaining the electrical gradient.
Maintaining the electrical gradient and the tightly choreographed action of ion channels is how neurons are able to send signals over great distances in milliseconds. Your thoughts are made up of this lightning movement of ions across quadrillions of ion channels in billions of interconnected neurons. So, the next time you come up with a good idea, take a millisecond or two to appreciate the billions of electrical battles that were fought inside your neurons before that idea was fully formed. And, the next time you run into a physicist, thank her or him for the electrifying dish that the field of physics brought to the exciting scientific potluck that we call neuroscience.
Acknowledgements: Special thanks to AJ Rasmusson for reviewing this post for accuracy and clarity.
Edited by Clara Boothby and Ben Greulich