From Toes to Brain: The Secret Life of a Nerve Signal

Have you ever had your leg fall asleep? That strange buzzing or fizzing as it comes back to life is your nervous system flickering back on. It’s a funny feeling, but it shows how much is happening beneath the surface all the time.

Now, I picture something more serious. A patient is lying on an operating table, completely unconscious, while the surgeon is working close to the spinal cord. They can’t speak or react. So, how does the medical team know if a nerve is being stretched, compressed, or damaged?

That’s where something called SSEPs comes in. It stands for Somatosensory Evoked Potentials. This method lets doctors send a tiny electrical pulse into the body and track whether it reaches the brain. If the signal goes through clearly, things are likely fine. If it weakens or disappears, that could mean something is wrong.

SSEPs are part of a broader toolset called Intraoperative Neurophysiological Monitoring, or IONM. Think of it like live tracking for the nervous system. It gives the surgical team information in real time so they can respond immediately if anything starts to go off track.

In this project, we’re going to follow the journey of that pulse. It begins near the ankle and climbs all the way to the brain. At each stop, we’ll look at what doctors are measuring and why it matters to keep patients safe.

Step 1: The First Spark

Our story starts just behind the ankle bone, where the posterior tibial nerve rests. This nerve helps send sensory signals from the leg up to the brain. It’s part of a long highway of communication running through the body.

In surgery, the patient is asleep and can’t give any feedback. So, the team sends a test signal into this nerve using a small electrode. The pulse is gentle and painless, and the patient doesn't feel anything. But that pulse begins a journey that can reveal a lot.

It’s kind of like tapping the first domino in a long row. If the rest falls in order, everything is working. But if they stop or fall out of rhythm, something might be wrong along the way.

Doctors choose this nerve for a reason. It’s easy to reach, it travels a predictable path up to the brain, and the signals it produces are usually clear and easy to read.

That small pulse begins traveling almost instantly. It moves up the leg, through the spinal cord, past the brainstem, and eventually reaches the part of the brain that handles touch and body awareness. The full trip takes just milliseconds, but it tells a complete story.

Step 2: How Do We Know It’s Working?

After that tiny zap is sent into the nerve, it sets off like a traveler on a journey. But how do we know it’s moving? Or making it to the brain?

That’s where these clever little checkpoints come in, electrodes placed along the body to “listen in” as the signal goes by. It’s kind of like having microphones placed along a parade route, each one reporting back: “Yep, just passed by here!”

You can think of it like tracking a package. One update says it left the warehouse, another says it made it to a sorting facility, and if the updates stop halfway through, you know something’s gone wrong.

In SSEP monitoring, we are basically doing the same thing with signals traveling from the ankle to the brain. A few key checkpoints we listen to:

• Behind the knee: This is where we see something called the N9 wave. It means the nerve is doing its job and carrying the message upward.

• At the base of the neck: This is where we expect to see P31 and N34. If that one shows up, it means the spinal cord is involved and the signals are still alive.

• On the scalp: Here, we catch the P37 wave. This one tells us the brain has received the message.

Each of these is like a little flag waving to say, “I’m still here.” And if any of them start to fade, arrive late, or disappear, it gives us valuable clues:

Maybe there’s some kind of pressure on the nerve. Maybe blood flow is down. Maybe something got pinched or pulled.

Even though these signals just look like simple lines on a screen, they’re telling a detailed story in real time, and during surgery, that story can make all the difference.

Step 3: Where the Signal Comes From

Okay, so we’ve got this signal traveling through the body, hitting different checkpoints. But who’s actually “speaking”? Where do these waves even come from?

Each of those signals that the neuromonitoring team watches on the screen, like N9, P31, N34, and P37, is produced by a different part of the nervous system. These aren’t just random blips; they’re tied to real structures inside the body, kind of like voices from different rooms in a house.

Here’s what each one tells us:

• N9 is the first voice we hear. It’s created right where the signal starts, in the posterior tibial nerve. If this one doesn’t show up, the signal might not have even left the station, like sending a text but forgetting to hit send.

• P31 & N34 come from the brainstem, more specifically, the dorsal columns in the lower back. If N9 is present but P31 is missing, the signal started out okay, but something happened before it reached the cord, possibly due to a blockage or damage in that stretch of the pathway.

• P37 is the brain’s response, picked up over the somatosensory cortex. This wave tells us the brain received the message. If it’s missing or fading, something is stopping the signal from completing the journey, either up in the brain or somewhere more global, like poor circulation or anesthesia effects.

Think of each part of the pathway like a musician in a band. Each one plays their part at the right time to keep the song going. If one player drops out, the whole sound changes, and it tells the conductor something is off.

That’s what makes these signals so useful. Doctors don’t just know that something is wrong, they can figure out where and why. It’s one thing to say the lights went out, but another to know which wire snapped.

This kind of detail is what gives SSEP monitoring its strength. It’s not just about catching a problem, it’s about catching it early, knowing where it is, and giving the team time to fix it before it causes real damage.

Step 4: When the Signal Doesn’t Look Right

So, why even bother watching these signals during surgery? Simply, you want to catch anything weird before it turns into a real problem.

Normally, everything’s smooth. The signal travels as expected, shows up strongly, and hits every checkpoint on time. But every so often, something shifts. Maybe the waves are getting smaller. Maybe it’s late. Maybe it’s just... gone.

There are several reasons why this might happen:

• The nerve could be getting pushed or stretched.

• Blood flow might not be great.

• Something might be irritating the nerve or spine.

• Even stuff like cold body temperature or low oxygen can mess with things.

Neurophysiologists can tell what’s going on based on what the signal looks like:

• A smaller wave? The signal’s weaker.

• A delay? It’s moving slower than it should.

• No wave at all? The signal didn’t make it.

And where the signal drops out gives even more info:

• If N9 disappears, the issue is probably near the ankle. It could be a loose electrode or a bad contact.

• If P31 or N34 is gone but N9 is fine, that points to a spinal cord issue.

• If P37 fades out, it might be something up with the brain, or something affecting the whole system, like meds or blood flow.

The best part? It’s all happening live. If something changes, the neurophysiologist sees it right away and tells the team. They can pause, move stuff, and fix it before it causes damage.

And it works. People who might’ve woken up with numb legs or worse have walked out totally fine, just because someone spotted a wave getting quiet and jumped on it fast.

Step 5: When a Signal Saves a Life

Picture the operating room.

It’s quiet, focused. A patient is out cold on the table. The surgeon is deep into a spinal fusion, trying to fix years of pain. They're working closely just millimeters from the spinal cord.

Off to the side, someone’s watching a screen. They’re not looking at video or X-rays, just a bunch of lines that rise and fall: N9, P31, N34, P37. Each one is a sign that the nerve signal is still moving from the leg up to the brain.

Then something changes.

It’s subtle. One of the waves starts dipping. First P37, then N34 and P31. It’s not dramatic, but it’s different, and that’s enough. The neurophysiologist speaks up, and the room responds. The surgeon pulls back. The anesthesiologist checks the vitals. Everyone’s on alert.

A few seconds pass, and the signals start coming back.

Nothing broke. Nothing got cut. The team just adjusted fast. The surgery continues, a little more carefully now.

The next morning, the patient wakes up. They stretch their legs. Feel their toes. Walk. They’ll never know how close they were to losing that or how a dip on a screen and a quick response made all the difference.

This type of save doesn’t just happen occasionally; it happens every day, quietly, without alarms or drama. It’s simply about someone paying attention and knowing what to do when the signal shifts.

Step 6: The Journey from Ankle to Brain - All in One View

Now that we’ve walked through every step, let’s pull back and look at the big picture. That little zap near the ankle? It travels far, far farther than you’d expect.

It starts with the posterior tibial nerve, right behind the ankle bone. From there, the signal races up through the leg, past the knee, into the spinal cord, and through the brainstem. Then it hits the thalamus, the brain’s sorting center, and finally lands in the somatosensory cortex, where the brain “hears” the message.

In the visuals, we see that journey laid out in a candy-themed world, it’s light-hearted, but it tracks. There's the ankle spark, the knee checkpoint (N9), the sorting station (thalamus), and the candy castle at the top (cortex). The neck site, Cv5, where we check P31, isn’t shown in the picture, but it’s still part of the path.

Here’s a quick recap of what each stop tells us:

• Ankle (stimulation site): This is where the signal starts.

• Knee (popliteal fossa): N9 shows the nerve that carries the message.

• Neck (Cv5): P31 tells us the spinal cord’s doing its job.

• Thalamus: Sorting center. If N34 shows up, the signal made it.

• Scalp (CPz): P37 proves the brain received it.

If one of these signals is missing, it doesn’t just mean something’s wrong, it tells you where to look. That’s what makes SSEP so powerful. You’re not just watching electricity. You’re tracking communication. And you’re doing it in real time, while the patient sleeps.

References

• Cruccu, G., Aminoff, M. J., Curio, G., Guerit, J. M., Kakigi, R., Mauguière, F., Rossini, P. M., Treede, R.-D., & Garcia‑Larrea, L. (2008). 
  Recommendations for the clinical use of somatosensory‑evoked potentials. Clinical Neurophysiology, 119(8), 1705–1719. https://doi.org/10.1016/j.clinph.2008.03.016 MDPI+7PubMed+7Aalborg Universitets forskningsportal+7 

• Jahangiri, F. (2024). Intraoperative neurophysiological monitoring lecture series [PowerPoint slides]. University of Texas at Dallas. 

• Nuwer, M. R., Emerson, R. G., Galloway, G., Legatt, A. D., Lopez, J., Minahan, R., Yamada, T., Goodin, D. S., Armon, C., Chaudhry, V., Gronseth, G. S., & Harden, C. L. (2012). 
  Evidence-based guideline update: Intraoperative spinal monitoring with somatosensory and transcranial electrical motor evoked potentials. Neurology, 78(8), 585–589. https://doi.org/10.1212/WNL.0b013e318247fa0e.

• Toleikis JR, Pace C, Jahangiri FR, Hemmer LB, Toleikis SC. Intraoperative somatosensory evoked potential (SEP) monitoring: an updated position statement by the American Society of Neurophysiological Monitoring. J Clin Monit Comput. 2024 Oct;38(5):1003-1042. doi: 10.1007/s10877-024-01201-x.