Comb Jellies take on 150 years of the Neuron Doctrine

When it comes to the nervous system, you might think we’ve got the basics down. After all, it’s been over a century since the great Santiago Ramón y Cajal proposed the neuron doctrine, which basically said that the nervous system is made up of individual, discrete cells called neurons. As you may recall from our neuropharmacology experiments, Ramón y Cajal argued for discrete cells (neurons), while Gogli thought that the brain consists of groups of continuously connected cells (reticulum). Santiago’s hypothesis, known as “The Neuron Doctrine,” was later confirmed by the invention of the electron microscope, which let us see these neurons and their connections in all their glory.



But now it looks like the fight is still on! New research on ctenophores, those weird, squishy marine critters that look like a cross between a jellyfish and a feather duster, is shaking up the status quo.

The importance of these jellies cannot be overstated. As one of the first animal groups to branch off on the evolutionary tree, studying ctenophores can provide us with clues about the very origins of animal life. And while these guys have no brains, they do have a nervous system consisting of a “neural net” – a type of nervous system organization that is very understudied. Could it be that a nervous system evolved twice, independently, in our animal ancestors? That’s the question these researchers were asking.

To get their answers, a group of European researchers led by Dr. Kittelmann at Oxford Brookes University turned to high-pressure freezing-fixation techniques and a method called serial block face scanning electron microscopy (try saying that five times fast!). This gave them a stunning, 3D view of the ctenophore’s nerve net. And what they found was quite unexpected and they shared it with the world in a recent Science paper [Burkhardt et al., Science 380, 293–297 (2023)].

Unlike our own nervous system, which comprises separate neurons connected by synapses, the ctenophores’ nerve net looked more like a reticulum – a continuous network of interconnected cells. Instead of discrete neurons with synapses (small gaps between the cells), the ctenophores have a nerve net where all the nerve cells seem to be part of one, big supercell. It’s kind of like comparing a bunch of individual houses to a giant apartment complex. As seen in their figure below, the 5 separate neurons of the nerve net are actually all fused together (highlighted in white asterisks for links between neuron 1 and 2).

In the world of nerve nets, this is a pretty big deal, as it suggests that there’s more than one way to build a nervous system and that different animals might have taken different paths in their evolution to encode information and guide behaviors. The ctenophore nerve net is not just a simple precursor to our own complex brain but a complex and unique structure in its own right.

This opens up a whole new perspective on how nerve nets and nervous systems function and evolve, and reminds us that even long-held truths in science can change upon new evidence. So next time you see a ctenophore, don’t just marvel at the beauty of the squishy color-changing blob. You can also admire its nervous system that’s every bit as complex and fascinating as ours, just in its own, unique way. And who knows? Maybe we’ve got more in common with these jellies than we think. There’s so much more to be discovered!

Help us Study the Neuroscience of Sport

Help us win a Grant to Empower a New Generation of Sports Scientists!

Through our adventures at Backyard Brains over the years, we have come to love electromyography, muscle coordination, and the body in movement. We have already begun some classes where we teach muscle physiology through sport! We recently applied for a grant to devote more resources to this new area for us. We made a quick and dirty video under deadline at a local school, and to move to the next phase of the competition, we need eyeballs and likes. Help us out with your screens and your thumbs if you want to!

This is to further develop our fledgling work into making quantitative sports-science an accessible field of research for K12 students!

We’ve tried three sports so far, all favorites of Backyard Brains: baseball, basketball, and soccer. For baseball, we attached electrodes to the triceps while students threw the ball at greater and greater distances. However, the act of throwing a baseball is so violent and fast that the cables would always fly off, making an unstable interface, to put it mildly.

We next tried basketball, with muscles again on the triceps, but our students were pretty young, around 11-13, and they had a hard time launching the basketball with enough force (and good form) to actually reach the basket. Even on adults like ourselves, we did not notice an obvious difference between 2 point and 3 point throws.

Our last attempt was with soccer. We placed electrodes on the quadriceps, and we had markings on the outside gym floor with masking tape of 5 ft, 10 ft, 15 ft, 20 ft, 25 ft, etc.

With this experiment, it was very obvious that the EMG amplitude of the quadriceps contraction increased with the greater distance that the students had to kick the soccer ball, teaching about motor unit recruitment and electrophysiology in a very entertaining way. Learn more about the experiment and the results here!

We have on experiment up currently which teaches students how to study and model Rates of Muscle Fatigue – this is a great intro lab as it can be modified and applied to many sports labs!

This is just the beginning, and we will continue looking for ways to incorporate sport into our physiology experiments, as it makes teaching at the middle and high school very engaging. We would all rather be outside and move our bodies than sit at a desk, anyway.

Getting Started with the Muscle SpikerShield Pro

Hey everyone! I’m Pablo, a junior from Nido de Aguilas High School in Santiago, Chile. In my free time, I like to doodle and run.

My project is a multi-channel version of the experiment that my colleague and friend Cristian developed: it consists of using the SpikerShield Pro’s ability to get data from multiple channels to create a musical instrument. In this instrument, flexing a muscle is analogous to playing a key in a keyboard. Obviously, the amount of channels limits this keyboard to six notes, but according to my limited musical knowledge, this is enough to create a coherent melody. In fact, the Arduino program currently has four settings which can be accessed using the red button: Mary Had a Little Lamb, Frere Jacques, major pentatonic scale and the minor blues scale. All the notes are in arrays with six elements, each corresponding to a channel. To add more possibilities, holding the white button in the board makes all the notes in the current setting one octave higher. You can download my code here.

The “loop” part of the code works by reading the red button, white button, and all six channels. First, it decides which set of notes to use for that iteration of the loop, which is controlled by the red button, then it checks if the white button has been clicked, which affects the pitch of the final note it plays. The last step is to decide which tone to actually play, which the code does by selecting the largest reading of all the muscles. Now, you might be thinking that playing music with two vastly different muscles, say your forehead and your forearm, will never work because a signal from the forearm will always be bigger than the signal of even the strongest forehead flex. However, the SpikerShield Pro can control the gain from each individual channel (the little white knobs) which can make a channel more or less sensitive to a signal, so every muscle has a fair chance of being played.

One challenge I faced when I developed this project is the lack of documentation of this particular product for novice programmers. Most of the times I’ve played around with an Arduino, I’ve relied extensively on the built-in tutorials and online resources, but this time I only had the board’s schematic, which at first glance bears a closer resemblance to black spaghetti than a discernible circuit and the default program which sends the signals from the board to Spike Recorder. Running the aforementioned program was not a challenge, but reading the code, not being fully aware of what it was, proved to be confusing. I only started making progress once Tim Marzullo showed me an outdated sketch meant for this shield. However, with this project in the open, I doubt this is a problem other users will face; the heart of the code — presenting the sensor’s readings as an array and mapping those raw values to a usable scale — can be used for most projects.

The second biggest challenge was and still is, my absolute ignorance about music theory. I never learned to play an instrument, and the most complicated song I managed to play is “Hot Crossed Buns”, though that is probably a skill I’ve lost. I’ve always enjoyed music, but much like hot dogs, I preferred to enjoy the finished product rather than learning how it is made. After adding the melody of Mary Had a Little Lamb and Frere Jacques, I did not know what other songs to add. After a fair amount of research, I came upon pentatonic scales, which are comprised of five notes.

Though the musical aspect is worth examining, what attracted me more is its role in many musical traditions, ranging from the ancient Greeks to the Andes. During the 19th century, composers like Debussy used the simplicity of the scale to create a folksy in their composition, resulting in music like La fille aux cheveux de lin. Later on, rock, blues, and jazz artists adopted the scale as a tool for their respective styles of improvisation. I think this is the area where my particular instrument shows the most potential because it is only capable of playing one note at a time, and also because flexing muscles to create sound is very intuitive. However, this is a hypothesis I will let the reader confirm.