Ctenophora, commonly known as comb jellies, are a rather perplexing phylum of beautiful pelagic creatures. Their evolutionary position has been debated for many years as is the origin of their nervous system (some scientists believe they are older than sponges and that sponges lost their nervous system, while others advocate the theory about the nervous system forming independently twice, once in cnidarians and once in ctenophores). Ctenophora have two nerve nets: subepidermal and less organized subgastrodermal, which recent research identifies as a mesogleal nerve net. Nerve cells from this layer communicate with muscles by synapses and affect the locomotion of the body. The subepidermal net is denser around the mouth, the pharynx, and under the comb rows (comb rows are strips that run the length of the ctenophore body and contain cilia called “ctenes”). Ctenophore neurons can be iso- and multipolar.
They have sensory cells on the whole surface of the body and those correspond to vibrations and thermal and chemical stimuli: more receptors are located around the mouth and pharynx. Ctenophora also have an apical and aboral sensory organ. Such sensory organ consists of a statocyst, a sensor that contains a statolith that balances on four groups of long cilia connected to the comb rows. These organs help the orientation of the ctenophore body. What’s extremely interesting is that ctenophores use different chemical signalling system than the ones described in the previous posts, mainly because these animals simply lack the neurotransmitters (and genes), such as serotonin, dopamine, noradrenaline, and acetylcholine; glutamate is the only neurotransmitter currently known to be present.
I gathered all this information from different resources, and some are sometimes contradictory or are generalizing conclusions about the whole phylum from the data of only one ctenophora species. This is the best overview I could manage, to show both the similarities and the differences of the ctenophora nervous system, when compared to the Cnidarian system. These lovely animals are not very well researched and I’m sure many wonderful breakthroughs about their anatomy, physiology, and their place in the evolutionary tree are to come.
Cubozoa, or box jellyfish, are another cnidarian class. Their name stems from their distinct cube-like shape. Cubozoa are also distinct from other cnidarian because their venom can be fatal to humans. As with all cnidarians, box jellyfish have two nerve nets and, like Scyphozoa, rophalia. However, box jellyfish also have a distinct nerve ring, as well as more developed eyes that consist of a lens, cornea, pupil, and a layer of retinal cells. Altogether, Cubozoa have 24 eyes, which makes them the most advanced cnidarian class in the sensory aspect.
Rophalia are mutually connected via the mentioned nerve ring. This ring is believed to be an integration center for the swimming, visual, and tentacle systems; it is comprised of oversized neurons, as well as some smaller neurites. The communication between the nerve net and jellyfish muscles is regulated by chemical synapses.
Most of the information relating to Cubozoa, I already mentioned in the previous post about Scyphozoa, so I only wanted to relay the main differences between the two. These two classes are so similar that, until recently, they were actually considered one class.
Scyphozoa (true jellyfish) are much more interesting (in a neurobiological way) than previously described corals. One major difference is that Scyphozoa are pelagic animals, which means they are not fixed to the ground. They also have two diffuse nerve nets (subepidermal and subgastrodermal) that consist of bipolar and multipolar neurons – the impulse conduction has been measured at 0,15 m/s. Both nets coordinate the movements of an animal towards the food. Some scientists, however, differentiate one diffuse and one motor nerve net. The motor net is in charge of the activation of muscle contractions after receiving signals from the so-called pacemaker organs (which are in charge of the swimming rhythm). The diffuse net, in this case, is in charge of marginal tentacle contraction and it is also believed it communicates sensory information to jellyfish musculature. Neurons of the motor nerve net are connected by chemical synapses, while neurons of diffuse nerve net are connected by peptidergic synapses that were noted in Anthozoa as well.
Sycphozoa also have much more developed sensory organs than any of the animals previously mentioned. These sensory structures are called rhopalia and they are located on the edges of the jellyfish bell – there are usually four of them (or a number that’s a multiple of four). Rhopalia contain multiple sensory receptors – statocyst (balance receptor), ocelli (light sensitivity), a mechanoreceptor, a chemoreceptor, and aforementioned pacemaker neurons.
I would also like to note here that some authors (I’m referring here to the article “Do jellyfish have central nervous systems?” by R. A. Satterlie) believe this kind of nerve net explanation is rather simplified and that there exist some evidence suggesting that jellyfish have a centralized nervous system, mainly that rophalia are in fact rudimentary ganglia and could be regarded as integrative centers. However, any communications between rhopalia themselves exist only through the nerve nets.
I don’t know about you, but I just love doing online courses, especially when they deal in subjects I don’t get to explore in my college courses. Over the years, I tried many different platforms, such as YouTube, Google Digital Garage, Khan Academy, Udemy, and, my favourite, Coursera. As a matter of fact, I discovered Coursera back when they started in 2012; most of the courses I took were on topics of Neuroscience and Molecular Biology. At first, courses and certificates were completely free, but with time, they started offering paid vs. free, as well as many specializations and even some college degree courses. However, many of the courses are still available to watch and do quizzes, just without the certification.
Disclaimer: this post is not sponsored by Coursera.
This course, offered by Hebrew University of Jerusalem, is actually the very first course I took, and at the time was one of the rare Neuroscience courses. I have only good memories about this one – it is a good introductory course into the field and the professor explained the curriculum very well. Also, the course mentions real projects that deal with neural networks and brain reconstructions, such as Blue Brain Project. I didn’t mention this previously, but every course also comes with subtitles (in English at least) and transcripts, so you can follow along easier.
The Addicted Brain (by Emory University) is another course appropriate for beginners in this topic – I was initially interested not only because of the topic of addiction, but also because various mechanisms of how drugs interact with the brain were presented. The course also covers the topic of drugs in society, although this part mainly concerns United States of America. Also, I don’t know if this is something that’s important to you, but I followed professor’s narration easily – his voice is calming and he speaks very understandably.
Medical Neuroscience (by Duke University) is not only the most advanced course of the ones mentioned here, but the most advanced course I ever took. Actually, I started it once or twice before, but dropped out because it required a lot of time and dedication that, at times, I just didn’t have due to my University obligations. This course is really extensive and requires some before-knowledge, but is also very satisfactory when you finish it. The only problem I had with this one is that sometimes I felt that questions in quizzes were asking for details that to me seemed almost overlooked in the videos. However, I felt like this course was quite important for my studies, since I have a strong interest in Neuroscience, but lacked the medicinal perspective.
All quizzes are multiple choice answers, with usually one correct answer (sometimes more correct answers). I vaguely remember some questions where you had to connect some phrases (like 1-d, 2-c, etc) as well, but haven’t came across those recently. Also, the quizzes I did were never timed and you can take one quiz 3 times every eight hours (they keep your highest score).
Coursera also offers financial aid – you can fill out an application where you explain why is the course you’re applying for important to you and why you can’t afford it. So far, I’ve heard of many positive experiences where they gave grants.
There are also two Neuroscience related courses I am planning to take – Human Neuroanatomy (to revise a bit) and Computational Neuroscience, which deals with using Python in Neuroscience research. I would very happily review those for you, in a greater detail, if this is something you’d like to read about!
What is your opinion on online courses – do you think they’re useful or a waste of time? Did you perhaps take some of the ones I mentioned? If yes, I would love to hear from your!
A nerve net is a type of nervous system that consists of many neurons but there is no brain or cephalization. Nerve nets are found in animals with radial symmetry (Cnidaria) and biradial symmetry (Ctenophora). Despite being called a net, there sometimes exist some groupings of neural cells in some Cnidaria classes, which I will write more about during the next couple of weeks. Cnidaria are specific due to their specialized organelles, cnidocytes, which they utilize to hunt for food or use for securing itself to a surface. Some cnidocytes contain toxins that can paralyze their prey (the burning sensation you may have felt when touching a sea anemone 😉). As a rule, Cnidaria have two diffuse nerve nets, one in the epidermal layer and a second one in the gastrodermal layer. In between these two layers is the mesoglea, a layer that functions as sort of a skeleton. The epidermal net consists of bipolar and multipolar nerve cells, while the gastrodermal net is made up of only multipolar cells.
Cnidarian nerve systems are fascinating but also quite unexplored. What is known is that nerve cells consist of two types of neurons, sensory neurons that respond to stimuli and motor neurons which ultimately trigger a response. Chemical synapses exist and provide the communication between the neurons. Hormones have also been reported in some cnidarians (steroids, neuropeptides) but it is still not known how exactly these signalling molecules work.
In the next couple of weeks, I will write a post about every cnidarian class and also ctenophores, focusing on their nervous and sensory systems. If you have any questions or would like me to focus on something, please let me know!
Sponges (phylum Porifera) are sessile multicellular organisms that live predominantly in seas and oceans. They don’t have tissues or organs, and therefore, they don’t actually have a nervous system. However, they do have bipolar and multipolar cells that resemble nerve cells, which are found in the middle, “jelly-like”, layer. Sequencing of some sponge species showed the presence of many genes associated with neural cells, such as genes that code enzymes for neurotransmitter synthesis and synaptic transmission. It is important to note that these genes have other functions in the organism. It has also been observed that some sponge larvae can respond to outer stimuli and show various “taxis” behaviour – phototaxis (response to light), geotaxis (response to gravity), rheotaxis (response to water current). Phototaxis has been closely studied in species Amphimedon queenslandica (class Demospongiae), a sponge native to Coral Sea.
Potassium channels have been observed in that same species, as well as glutamate, GABA, and NO systems, which have been investigated in Ephydatia muelleri, another species of class Demospongiae. Electrical signalling has been noted in glass sponges (class Hexactinellida). These sponges have bodies comprised of a syncitial tissue and their skeleton is made of silicon dioxide. The scientists were able to measure the action potential (5s long, with 29s refractory period) and deduce this signal relies on potassium and calcium ions. Some scientists even suggest that sponges used to have a nervous system, but lost it during evolution – they introduced several hypothetical scenarios for this event, proposing that sponges lost their nervous system in order to focus on filtering.
For the next couple of weeks, I would like to write a bit about the evolution of the nervous system, from early nerve cells to the human nervous system and brain evolution. Alongside nervous I will also focus, to a lesser extent, on sensory systems. These posts will be published on my Instagram account, but I decided to publish them on the blog as well.
Mostly, these posts will be about various animals and the nerve systems they have – nerve nets, nerve cords, complete systems. The main process behind this is called cephalization, and it starts with the groupings of nerve cells and ganglia at one end of the body. After some (long) time, this process led to us having a head with sensory organs and a brain inside it.
But when did all of it start? It is kind of hard to say, for even single-celled organisms, such as bacteria, have voltage-gated channels and genes that support the theory of possible synaptic transmission. These channels are potassium (the oldest), calcium, and, rarely, sodium channels as well. Action potentials have been detected in some algae and diatoms, although their function is mostly unclear. In Chlamydomonas (unicellular green algae) on the other hand, potentials were detected in flagellums, which clearly suggest they play the part in the movement of the algae. Action potentials were also recorded in the cilia of some protists, such as Parmecium.
Of course, the exact evolutionary processes are unknown, and there is a possibility that these organisms acquired the mentioned features later than scientists now assume. It is also possible that some more evolved organisms, such as sponges, subsequently lost some of the features discussed here (more about this in the next week’s post).
As promised (to my two and a half readers and myself), I’m diligently writing another post, and this one combines my two greatest scientific loves – neuroscience and biospeleology!
Last weekend, I attended two very important conferences – 3rd Rijeka forum on neurodgenerative diseases – diagnosis and treatment in early stage of disease and 2nd Dinaric symposium on subterreanean Biology. There was one day overlap between the two, so I chose to spend my Friday in Rijeka, but more about that later on. Since this trip included a lot of walking and transport, for the simplicity I packed only the bare necessities, which included only one pair of booties (this comes in play later on).
The topic of neurodegenerative diseases is something I would really like to do all my future research in, and it’s also a subject of my master thesis (precisely, neurofibrillary tangles in rat brains). Also, there was no attending fee, which is pretty much amazing for the students. We (Bruno and me) arrived in Rijeka (from Zagreb) very early in the Thursday morning by bus and set to find the University campus where forum was taking place. This wasn’t my first time in Rijeka, so we didn’t have any problems with finding the venue. The forum consisted of plenary lectures held by renowned professors from all over the Europe (Sweden, Italy, Slovenia, England) and Croatia of course, and it was roughly divided into two parts, basic neuroscience and clinical research. As a biologist, I didn’t pay too much attention to clinical part, and instead was dutifully solving my programming homework. However, it was amazing to be there for both days, and just listening what was new in the world of neurodegenerative diseases. I didn’t find courage to ask questions (during the Q&A time, or even after), but I hope that one day I will. We didn’t “touristly” roam around Rijeka, but we stayed overnight in the most gorgeous apartment in the centre, I was seriously impressed!
On Friday, after lecture, I stayed in the venue for a little while (internet access!) and then we went to the train station. I love travelling by trains, and I try to do it as much as possible. Luckily, this train to Ljubljana (yes, Slovenia again!) also went through our next stop, Postojna. If that name rings a bell, that’s surely because of the Postojna Cave, one of the most famous caves open to tourists in the area. It is also special because there are many blind cave salamanders, which are part of the vivarium as well. Our plan was to visit this cave at Sunday, while Saturday was reserved for the Dinaric symposium. The symposium was held in Karst museum in Slovenia, and we caught the last plenary session and some posters. There was a short break (we went to the town centre to stock-pile some food) and then, the excursion!
We were driven by car by our hosts, Teo and Ester (who we met during this summer – I will write about that in the next post!) to Rakov Škocjan. I would like to use this opportunity to say that Slovenia is really beautiful, full of intact nature and small roads. In Rakov Škocjan, we set for a quite a long walk to visit two natural bridges and caves near them. I had my shiny new camera with them, so I was taking as much photos and videos as possible, and honestly, I also used this as an excuse to stay behind, because I had a hard time keeping up with the rest of the group (I’m chronically out of shape). The trails itself is beautiful, a part of it also goes near the river Rak. Soon we came to the first natural bridge, which is completely made of stone. The only thing human hand built were protective barriers, so people don’t fall down. The story behind both bridges is that they used to be huge caves, but they collapsed, leaving only the bridges. Parts of the caves still remain, so most of the group went down to enjoy the cave and river that goes through it. I stayed behind, because I assumed it would take me too long to climb down and back up, and I didn’t want to hold back the whole group.
After the first bridge, we went back to the meeting point, which was the hotel where we had lunch. Next, the second bridge, which is 5-minute-long car ride away, and about 20 minutes by foot from there. Usually, the passage under this large natural bridge is almost completely under water, but we were lucky, as it was completely dry. We carefully walked in the river canyon, up the entrance of the second cave. Since Bruno and I didn’t bring any headlamps, one of the organizers kindly borrowed us spare helmets with the lamp on. Which lamp, you may ask? Scurion, the best of the best! I was so excited to wear it that I made Bruno take countless pictures of me!
Me, wearing a really awesome helmet!
Bruno in the cave
Then, we naturally went to explore the cave, which wasn’t as dry as the canyon. Sometimes, it was really hard to see how deep the water level is, and that’s how I ended up with a very wet right foot. Like, full on water in my boot, because the water level was just a bit too high then I anticipated. This didn’t stop me to try out my camera, and I was very satisfied with the photos, since they are a big step forward from the ones I took this summer (again, this will my next post!). The light from the Scurion was on occasion so strong, that some of the photos look almost burned, I couldn’t believe it.
A cave detail
A cave detail
Some folks went through the whole cave, but I wanted to go back the same way, to take more pictures of the bridge and cave entrance, since I didn’t have enough time to do it the first time we went through. Honestly, at moments, I felt like everything is a bit too fast, especially for someone who wanted to take a quick break and just enjoy the nature or take a few snaps for the photo album. The whole time however, I was thinking only how wet my sock is, and couldn’t wait to go back to our apartment and change. However, the group had other plans and first we went to the small café close to the big lake, which I honestly didn’t see. We did say to Teo that we would like to go back and would skip dinner (programming!), so he organized a transport back for us, which we were quite grateful for.
Big Natural Bridge of Rakov Škocjan
A view from under the Big Bridge
The next day, the big day! Postojna cave, or at least we thought so. After waking up, we realized we should walk two kilometers up to the cave, with both of our backpacks and I also had a bag full of photography equipment. There were some taxi companies, but no one answered, and we also didn’t have almost any cash, since I’m used to paying with a bank card. This was actually a problem in Postojna – apart from big chain stores, everything was to be paid in cash, and all ATM-s were located in the city centre. All of this, combined with our train schedule and my sill wet boot, contributed to us giving up on Postojna cave, and heading for a train station… Where we realized that the ticket office is closed, and we can’t pay with bank card in the train. So, Bruno quickly headed back to the centre, and came back with some pastries as well. Our way back went smoothly – first train to Ljubljana, trying out new burgers at McDonalds, and then train to Zagreb. During that time, we decided to come back to Postojna during the winter, and explore that cave, as well as going back to Rakov Škocjan on our own, setting our own pace and excursion plan. However, apart from finishing my homework assignment, the most important thing, excluding pretty photos, is the fact that I finally finished reading Dracula, the book I struggled with for almost two years, for the reasons I still don’t understand.