Nerves (Part Two): How the Nervous System Actually Works – By John Macy
Last time I wrote about what nerves are structurally and how they are laid out in the body. This time I want to talk about how the nervous system actually works, how it creates, edits and transmits information so we can move, think, and live in the physical world. This is an area that whole textbooks are written on but I want to boil it down to the basics of how the system works. Next time, I will go into how we use the nervous system, some of the things that can go wrong, and how these problems are treated.
The Three Types of Nerves
I mentioned last time that sensory nerves bring information to the spinal cord and motor nerves carry information out from the spinal cord to the parts of the body. There is a third type of nerve made of interneurons. They link one neuron to another neuron and are what the spinal cord and brain are made up of. They work the same way that sensory and motor nerves work, they just don’t connect to anything but neurons.
The Simple Version (before the details)
The mechanics of how nervous system works, in principle, is pretty straight forward if you know just a few things. Your fingertip gets pinched and that information goes to spinal cord, up to your brainstem and into your brain. Then you decide if you need to move your hand and, if so, you send signals back down to your muscles to move your hand.
“It is pretty simple. But, as they say, the devil is in the details.”
Helpful Comparison: the Phone System
You might compare the arrangement in the nervous system with that of a phone system. Making a call is easy. You pick up the receiver, press seven or ten digits and the system connects you to an individual phone…
And then there are the details…
There are multiple routes for your individual signal to get to the other phone, with multiple places to be switched from one route to another… Each one of these conversion points is where something in the initial signal pattern can be altered.
Why the Nervous System is Similar
The nervous system has the same kinds of details to deal with – multiple transfer points, filtering and directing traffic, making decisions about what signals get handled where and when and figuring out what is noise and what is real signal.
“The nervous system is just a huge collection of these neurons all working the same basic way.”
How a Neuron Works
Neurons carry a signal from one end, where there is a sensor or a dendrite, to the other end called the axon. Signals always go one way in an individual neuron, dendrite to axon.
To see how that works you need to know two things:
First, neurons have many pores along their length that are made for letting sodium atoms, which have a positive charge, in and out of the neuron. There are gates on the pore to prevent sodium coming in the pores and the neuron has active pumps for kicking sodium out of the neuron. (The pumps really are little molecules that grab sodium atoms and toss them out through the pores.)
Second, the neuron is constantly pumping out positively charged sodium atoms that diffuse in from the body fluid outside the neuron. The pumps pull in two positively charged potassium ions for every three sodium tossed out. The net result is that the fluid inside of the neuron has a more negative charge than the fluid outside the neuron. (It is a little more involved than that but close enough for our purposes.)
From Pressure to Signal
With that knowledge let’s look at how a nerve pulse or signal is transmitted from your finger to your brain when your fingertip gets squeezed. When the finger is squeezed a sensor (actually many of them but let’s keep it simple) in your finger gets deformed by the pressure. This sensor, like most sensors, structurally surrounds the tip of a neuron. By design, when the sensor deforms it stretches the membrane on the neuron in a way that lets a lot of positively charged sodium ions leak into the tip of the neuron, more than the pumps can kick out immediately, Now the little area inside the tip has more positive charges than the rest of the inside of the neuron. The regional change to being more positive makes the sodium pores nearby open so more sodium comes in which opens the next pores down the neuron, sodium comes in there and opens the next pores and so on. This starts a wave of sodium flooding into the neuron that races down the length of the neuron all the way to the axon.
Within several ten thousandths to thousandths of a second the first influx of sodium ions from the deformed pores is pumped out, so fast that fluids inside the neuron near where the influx started returns to a negative charge long before the sodium influx wave has moved the length of the neuron. (Remember, some neurons can be several feet long.) The sensor can start another wave of positive sodium influx as soon as the pumps have cleared the immediate area. This means the neuron can potentially send hundreds or thousands of electrical waves down the
neuron’s length in a second.
“This means the neuron can potentially send hundreds or thousands of electrical waves down the neuron’s length in a second.”
What Firing A Nerve Means
The wave of ions is electrical; from the outside you would see a positive charge racing down the length of the neuron from the tip to the axon as sodium floods in and is kicked back out. This process of the electrical wave going along a nerve is commonly called firing the nerve and may get repeated thousands of times in a second depending on the type of sensor, the type of neuron, and amount of pressure on the finger.
The speed that the wave is carried along nerves varies, and many nerves have an insulation coating, called myelin, that lets a signal travel much faster. Speeds vary from 80 meters per second for proprioception (where your body parts are in space) to less than 2 meters per second for what we perceive as pain. I will touch on the importance of this difference in the next post.
The Synapse: Where Chemistry Takes Over
Once the signal gets to tip of the axon it needs to be converted so it can be handed off to the next neuron on the way up to the brain just like the electrical impulse in your phone needs to be made into a different type of signal for the cell tower to handle. This conversion happens in an area called a synapse, where the axon tips of one neuron meet with dendrites of the next neuron. Unlike electronics, though, this transition uses chemistry instead of electricity. (Why? Evolution. Many invertebrates actually use a method that carries a charged particle across the gap.)
At the synapse the axon branches out into multiple little fibers that sit very close to the dendrite of the next nerve in the chain. The dendrite is branched out in little fibers so that axons from multiple neurons may be connecting to the dendrite. (We will come back to the importance of that in a minute.) The gap between neuron and dendrite is tiny, 20 – 40 nanometers, and filled with interstitial fluid. That gap is so small you need an electron microscope to visualize it. You are in the realm of the sizes of molecules. For perspective, one nanometer is to your length as
you are to three times the distance to the moon.
When the positive charge arrives at the tip of the axon it causes little sacks, called vesicles, of neurotransmitter molecules to be dumped into the gap between the axons and dendrites. The neurotransmitter molecules then float across the gap and dock onto sites on the dendrite. There are many varieties of neurotransmitters but each individual neuron only uses one kind.
“Unlike electronics… this transition uses chemistry instead of electricity.”
Neurotransmitters: The Whisper System
When enough neurotransmitter molecules are docked at the same time on the dendrite, how many varies from neuron to neuron, the dendrite opens up pores for sodium and the whole process of sodium flooding in and creating an electrical wave in the neuron starts and the second neuron is fired. Think of it this way – each molecule that docks whispers to the dendrite “fire the nerve.” When there are enough whispers at once, the call is loud enough for the dendrite to open up the sodium pores in the area and fire the nerve. If it is not loud enough
nothing happens.
The neurotransmitter molecules don’t stay docked on the dendrite for long, they are broken apart by enzymes or pushed off the dock very fast so the docking site is reopened for another molecule to use. This process is fast enough to keep up with the impulses coming up to the axon tip so there may be thousands of releases and re-uptake of neurotransmitter molecules in a second. The vesicles on the axon will reclaim the molecules for reuse or pick up broken parts to make more neurotransmitter and restock vesicles for the next use. However, there are limits to how long this can keep pace with the signals to be sent across the synapse.
“Each molecule… whispers to the dendrite ‘fire the nerve.’ When there are enough whispers at once… the nerve fires.”
Signal Control: Excite vs. Inhibit
Now, recall there may be multiple axons near the dendrite. Some of them will be using neurotransmitters that are telling the dendrite to not open the sodium pores, a process called inhibition. The result is a competition between neurotransmitters for controlling if the dendrite will start a neuron firing. Changing the balance of neurotransmitters at a synapse, thereby controlling if signals cross or are blocked, is how we exert control on which signals get to go up and down the nerves. Interfering with neurotransmitters or docking sites at a synapse is also how many poisons and drugs actually affect us; they may prevent signals from crossing a synapse successfully or make it impossible to stop any signals at the synapse. (More on that next post.)
Getting a neuron to fire is a binary process – either it fires fully or it doesn’t fire at all. There is not an in-between setting. Once the process starts at the dendrite the positive charge goes all the way along the neuron to the end of the axon. The axon tip meets another dendrite at a synapse and the whole process of sending neurotransmitter across the synapse to get the next dendrite to let sodium in is repeated. This continues from one neuron to another all the way to the brain and within the brain. If you decide to move your pinched finger the same process is
used to send signals back down to the finger muscles.
Of note, there are not a lot of synapses between your finger and the top of the spinal cord or brainstem, usually just three or four. In your brain and spinal cord, the neurons have much more branching and interconnections among them (one neuron’s axons may be connecting to hundreds of other neurons at a time) but each neuron and synapse works the same way as I just described.
There is no in between setting – it’s all or nothing signalling.
Simple System – Complex Outcomes
The process of a wave of sodium ions coming in and out of a neuron to carry an electrical pulse to the end of an axon, the release of neurotransmitter to cross the synapse and the continuation of a pulse in the next neuron is how nerves work, all of them. It isn’t too complex at the level of one neuron. Like the switching networks handling your phone calls, the central nervous system coordinates multiple inputs and makes complex decisions on where signals should go. Still, it comes down to using arrays of simple on/off switches that make feedback loops to control each
other, just like all the logic circuits in a computer chip. Except the nervous system uses both chemical and electrical processes.
If a neuron is used constantly there comes a time when the neuron will not carry a signal because it needs to reset the levels of chemicals and ions. The pumps need to catch up, neurotransmitter vesicles refilled, and the things of just being a living cell need to be tended to. This down time is known as neural fatigue and varies by neuron, the availability of replacement minerals and ions, and other factors. In some ways this is a fail-safe mechanism. In the case of a seizure and convulsions where self-perpetuating feedback loops in the brain are triggering dysfunctional activity, it is believed that the neural fatigue in the pathways involved play a major role in limiting how long seizures continue.
Why It All Matters?
There are volumes written about how the nervous system works but all of it revolves around the simple structures of neurons and synapses and the interplay of electrical and chemical processes. It is an elegant system that allows us to build a very complex and robust system to process and control information, direct and balance the body’s systems, and adapt to our world.
A look at how that happens and some of the problems that can arise is what we will cover next time.