As you read these words, billions upon billions of electrical impulses are flying through your brain. In a complex symphony of activity all those impulses are somehow encoding your thoughts, feelings, and understanding. There are about 85 billion neurons in a typical adult human brain, and about ten quadrillion connections – or synapses – between all those neurons (that’s a ‘1’ followed by 16 zeros!) . The complexity is truly staggering and hard to wrap one’s mind around it. It is an information network of a size and computational space that defies any notion of intuitive grasp.
And that is just the neurons. There are another 86 billion non-neuronal cells in your brain – including a class of cells that also contributes to information processing capable of cross-talking with those 85 billion neurons. This complexity is so staggering that the only way to keep track of it all and begin to attempt to understand it is to use mathematical modeling and data analyses. Neuroscientists may even need to wait for quantum computers to become available in order to fully simulate the brain.
And yet, the fundamental unit of all this complexity, of all the information processing taking place, are 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 parts that we need to be aware of. A neuron has an orientation to it, a polarity – a front end and back end. At the front are the dendrites, spidery and often dense projections that all 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 axon. It is along the axon that action potentials – jolts of electrical impulses or spikes – propagate down until they reach the synaptic terminals where the spikes end but initiate a biochemical process that passes the signal along to other neurons.
Action potentials in the axon and related but different types of electrical impulses in the dendrites are the physical carries of the information coursing through the neuron that then gets passed onto other neurons in the network.
There is a careful directionality to the flow of information. The dendrites collect and integrate incoming signals from other neurons. If the amount of integration of those signals is sufficient – how the individual signals add up with each other – and they reach a certain critical threshold, then an action potential is triggered at the initial segment – the spot where the axon starts at the cell body. The subsequent action potential then propagates uninterrupted and unattenuated in size (amplitude) and shape (electrical waveform) down the entire length of the axon until it reaches the synaptic terminals.
The signal that crosses over at the synapse to other neurons is not as an electrical impulse, but rather a chemical message that then triggers new electrical impulses in the dendrites of the downstream neurons, just one of as many as tens to hundreds of thousands such inputs simultaneously occurring on the dendrites . In this way, billions upon billions upon billions of signals independently and simultaneously propagate through the entire brain across the massive network of 85 billion neurons.
The result is, well, 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 responsible for collectively carrying and processing all the information the brain contains.
This is all a lot to unpack. It is also a bit like a circle. It does not matter where on the circle you start, you can start anywhere, go around, and you end up back in the same place. So to understand how neurons communicate with each other we have to start somewhere. Anywhere on the circle, since a neuron receiving signals from other upstream neurons is eventually passing those signals off to other neurons downstream.
But the details matter. So in a series of articles we will explore each one.
The electrical impulses in the dendrites are functionally and biophysically different than the action potentials in the axon. And the events at the synaptic terminals when the action potential reaches the end of the axon are completely different than what is taking place in either the dendrites or the axon. In fact, as introduced above, what happens at the synapse is not electrical at all. It is a biochemical mechanism that takes over – triggered by the arrival of the action potential – which in turn passes the signal across the synapse to the next neuron. Which in turn puts us back at the dendrites of the next set of neurons, and back to signaling via electrical impulses. It is 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 building block principle we will refer back to over and over again.
The cell membrane is like the ‘skin’ of a cell. It separates everything inside the cell from everything outside. It is made up of molecules called phospholipids, long chains of carbon that interact with each other creating an internal environment inside the membrane that repels water (a hydrophobic environment), and surface molecules capable of interacting with water (a hydrophilic environment). Since we are made up of mostly water, including the fluids that surround all the cells in our body, this results in the phospholipids separating and keeping the cell’s innards inside and the rest of the world outside.
The hydrophobic environment of the cell membrane though is full of other molecules in between the phospholipids that do various things. It provides a regulated and controlled set of mechanisms and control points 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.
One 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 in. An ion is an electrically charged chemical species – an element from the periodic table with an imbalance of electrons that gives it an electrical charge. In neurons, the most important ions we will need to know are sodium ions (Na+), potassium ions (K+), and calcium ions (Ca2+). These ions are literally the carriers of electric charge across the membrane which ultimately make up all those electrical impulses.
The channels are not always open though. They selectively open and close depending on what the cell needs to do when and how. Whether an ion channel is open or closed is context dependent. What controls the opening and closing of an ion channel is something we will discuss in s subsequent 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 make up 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 encode all the information in the brain.
With this as a starting point, we will be able to understand how the flow of these ions produce an electric potential in the membranes of neurons, and how very rapid localized transient changes in the membrane potential produce the action potentials themselves.
Life Sciences, Forbes – Healthcare