September 30, 2022

As you read these words, billions upon billions of electrical impulses fly through your brain. In a complex symphony of activity, all these impulses somehow encode your thoughts, feelings, and understanding. There are about 85 billion neurons in a typical adult human brain, and about ten-four billion connections – or synapses – between all those neurons (that’s ‘1’ followed by 16 zeros!). The complexity is truly staggering and difficult to wrap one’s mind around. It is an information network of a size and computational space that defies all notions of intuitive understanding.

And that’s just the neurons. There’s more 86 billion non-neuronal cells in your brain – including a class of cells that also help process information able to converse with these 85 billion neurons. This complexity is so mind-boggling that the only way to track it all and begin to try to understand it is to use mathematical modeling and data analyses. Neuroscientists may even have to wait quantum computers become available to fully simulate the brain.

And yet, the fundamental unit of all this complexity, all the information processing that takes place, are the individual electrical impulses and chemical signaling events. This is their story.

This story necessarily begins with what the anatomy and structure of a neuron brain cell looks like. There are a few key points to be aware of. A neuron has an orientation to it, a polarity – a front end and a back end. At the front are the dendrites, spidery and often dense projections that converge and meet at the cell body. The cell body contains structures and organelles (“mini-organs” inside the neuron) that keep it alive and carry out various cellular and genetic processes. Connected to the cell body is the axis. It is along the axon that action potentials—vibrations of electrical impulses or spikes—propagate downward until they reach synaptic terminals where the spikes end but begin a biochemical process that passes the signal to other neurons.

Action potentials in the axon and related but different types of electrical impulses in the dendrites are the physical carriers of the information that flows through the neuron and is then transmitted to other neurons in the network.

There is a careful directionality in the flow of information. Dendrites collect and integrate incoming signals from other neurons. If the amount of integration of these signals is sufficient – how the individual signals add up to each other – and reaches a certain critical threshold, then an action potential is triggered at the origin – the point where the axon starts in the cell body. The resulting action potential then propagates continuously and without attenuation in size (amplitude) and shape (electrical waveform) along the length of the axon until it reaches the synaptic terminals.

The signal that passes from the synapse to other neurons is not as an electrical impulse, but rather as a chemical message that then triggers new electrical impulses in the dendrites of downstream neurons, just one of tens to hundreds of thousands of such inputs occurring simultaneously in the dendrites. In this way, billions billions billions of signals are propagated independently and simultaneously throughout the brain in a vast network of 85 billion neurons.

So the result is you and who you are. What you think, how you interact with the physical world through your senses, what you imagine, how you feel, how you learn and how you remember. Your mind, as far as we know, is the result of the physical processes in your brain and all the action potentials and other cellular signals that are responsible for the collective transfer and processing of all the information the brain contains.

This is all a lot to unpack. It’s also a bit like a circle. It doesn’t matter where you start in the circle, you can start anywhere, go around and end up back in the same place. So to understand how neurons communicate with each other we have to start somewhere. Anywhere in the cycle, once a neuron receives signals from other upstream neurons it eventually transmits those signals to other downstream neurons.

But the details matter. So in a series of articles we will explore each.

Electrical stimuli in dendrites are functionally and biophysically distinct from action potentials in the axon. And the events at the synaptic terminals when the action potential reaches the end of the axon are quite different from what happens in either the dendrites or the axon. Actually, as introduced above, what happens at the synapse is not electrical at all. It is a biochemical mechanism that takes over – activated by the arrival of the action potential – which in turn passes the signal across the synapse to the next neuron. This in turn brings us back to the dendrites of the next set of neurons and back to signaling via electrical impulses. It’s all exquisite engineering.

The physical basis of electrical impulses in cells

The first fundamental thing to understand is what an electrical impulse means—what it physically is—in a neuron, or for that matter, in any cell capable of sustaining electrical activity. This is a structural principle that we will refer to again and again.

The cell membrane is like the “skin” of a cell. It separates everything inside the cell from everything outside. It consists of molecules called phospholipids, long carbon chains that interact with each other to create an internal environment within the membrane that repels water (a hydrophobic environment) and surface molecules capable of interacting with water (a hydrophilic environment). Since we are mostly made up of water, including the fluids that surround all the cells in our body, this results in phospholipids separating and keeping the inside of the cell in and the rest of the world out.

The hydrophobic environment of the cell membrane, however, is full of other molecules among the phospholipids that do various things. It provides a regulated and controlled set of mechanisms and checkpoints for the cell to interact with its local environment, not unlike the way we use our five senses to interact with the physical world around us.

A class of molecules in the cell membrane are called ion channels. These are literally channels, or pores through the cell membrane, that allow ions to flow from the inside of the cell to the outside or from the outside to the inside. An ion is an electrically charged chemical species—an element in the periodic table with an electron imbalance that gives it an electrical charge. In neurons, the most important ions to be aware of are sodium ions (Na+), potassium ions (K+), and calcium ions (Ca2+). These ions are literally the electrical charge carriers across the membrane that ultimately make up all these electrical impulses.

But the channels are not always open. They turn on and off selectively depending on what the cell needs to do when and how. Whether an ion channel is open or closed depends on the environment. What controls the opening and closing of an ion channel is something we will discuss in a later article.

In neurons, it is precisely the flow of many, many sodium and potassium ions across the cell membrane in a very specific sequence that composes the action potential and electrical activity in the dendrites. All this electrical activity—mediated by the flow of sodium and potassium ions across the neuron’s cell membrane—ultimately encodes all the information in the brain.

With this as a starting point, we will be able to understand how the flow of these ions produces electrical potential in neuronal membranes and how very fast localized transient changes in membrane potential produce the action potentials themselves.

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