Hydrozoa are the last cnidarian class I’m going to write about. They can exist in two distinct shapes, as hydromedusa and hydropolyp (same as Scyphozoa and Cubozoa). Despite perhaps expecting hydrozoans to be the most advanced in both nervous and sensory systems, they don’t actually have any rophalium. Furthermore, some hydromedusae don’t even have nerve nets. However, they have two nerve rings (outer and inner) on the margins of their bells which are regarded as ganglia by some scientists.
These rings consist of neural pathways which process different sensory inputs (such as light and gravity). Aglantha digitale, a hydrozoan species, has been reported to have as much as 14 distinct neural pathways. A. digitale is also distinct from the other species in the class by having two swimming “modes” – slow (which is a characteristic for all hydrozoan) and escape mode. Transmission through giant ring neurons is responsible for both modes, but the escape mode requires a stronger contraction. The slow swim mode is activated by the input from the pacemaker, which triggers slow calcium spikes. Direct mechanical nerve ring stimulation by tentacles triggers fast sodium spikes. In short, giant ring neurons are capable of generating two different kinds of action potentials.
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Gap junctions are also present in (and only in) Hydrozoa, and they transfer electrical signals through the musculature. Furthermore, I would like to emphasize that despite some hydromedusae not having a nerve net, some in fact do, and so do hydropolyps. In polyps, however, some groupings of the neurons could be found around their mouth.
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.
I already wrote couple of posts on this topic, and today I’m focusing on several examples from movies and TV shows I’ve recently watched. The movies I’m going to write about are Lara Croft: Tomb Raider – The Cradle of Life, The Good Liar, Charlie’s Angels (2019 version), and Paranoid TV show. So, if you are planning to watch those, just a warning that there will probably be spoilers below!
Let’s start with the obvious and amusing misportrayal: in Lara Croft: Tomb Raider – The Cradle of Life (2003 movie), Lara is riding sharks in the opening sequence. I think no one would believe this is actually possible, but shark petting videos recently became very popular. However, many researches are strongly discouraging this type of behaviour.
The Good Liar (2019 movie) is an interesting thriller with really good cast that I actually enjoyed. Without giving out too much of the plot, I’ll just mention one scene where Helen Mirren’s character gives the haircut to the character played by Ian McKellen. She uses cut hair for the DNA analysis, factually linking McKellen’s character for the past crimes. Of course, we all know that without the root, there is no DNA to analyze, right? Not quite. Most companies that do DNA analysis require hair with follicles in order to extract DNA for analysis; however, they mostly deal with paternities, so they need the nuclear DNA. Mitochondrial DNA, on the other hand, apparently can be extracted from the hair itself. For the movie’s purposes, I think nuclear DNA was needed, so this seems like a movie mistake. But, recently scientists did manage to successfully extract nuclear DNA from rootless hair, but that DNA is often fragmented and complicated to extract. I would also just like to mention that I believe this movie explains trauma and coping surprisingly well for a Hollywoood movie.
In Charlie’s Angels (2019 movie), which I hoped would be more enjoyable than it was, a very interesting premise was introduced; in order to effortlessly communicate, members of the team get a certain tattoo (at least I think it’s a tattoo, but don’t 100% quote me on that) and they can hear thanks to it. So, my first thought was, wow, they really went sci-fi on this one, but it sounded too out there. And then I remembered bone conduction. Simply, bone conduction allows you to hear the transmission, without blocking the outside sounds. And yes, only you can hear it. There are headphones already in use for this type of thing, and they can be also used to help with some kinds of hearing impairments. These headphones are placed on the skull, and I haven’t seen it implemented in a tattoo yet, but it’s actually a neat idea.
Paranoid (2016 TV show) is, in my opinion, really bad, despite only tangential connection to Biology. This show feel anti-medication and anti-psychiatrists, portrays them in a very bad light, and chooses not to mention how much medications and psychotherapy are actually helping people. The premise is also built on one of the cliche “anti-big pharma” representations. I was honestly insulted, despite not working in any of these fields. There is a also a subplot concerning a female character who had provocative photos of her taken 15 years ago, and she is not longer in possession of said photos; she is blackmailed with the possibility of leaking those. Her love interest immediately accuses her of things I won’t write here, but you get the main idea. I mean, am I the only one who sees this as highly problematic This TV show is doing some serious damage to the health care, it is insulting to physicians, it is insulting to patients, and I honestly can’t believe that no one during the whole production didn’t say anything.
I hope you like these kind of posts, because I already have enough material for another one (there are a lot of Biology & science mistakes on the big screen). What is your favourite movie mistake related to Biology?
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!
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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!
Anthozoa (corals & anemones) are interesting animals that live exclusively in seas and oceans. Since they are regarded as sessile organisms, their nervous system is not very advanced. In a previous post, I mentioned that all cnidarians have two diffuse neural nets, and that is true for Anthozoans as well. Multipolar nerve net neurons are connected with synapses, and they also possess sensory cells that are particularly numerous around the mouth and on the tentacles.
One paper investigated traveling of electrical waves in a coral nerve network in coral colonies in genus Palythoa. It was experimentally observed that this electrical wave spreads at the constant speed from the site of simulation. Furthermore, peptidergic neurons (the ones using neuropeptides to communicate) were also noted in Anthozoa.
Did I draw this when I was 5 or 25? – A mystery.
Nematostella vectensis, also known as starlet sea anemone, is a species of Anthozoa that is known as a model organism – its genome and development have been carefully studied, including its nervous system. In this species, oral and pharyngeal nerve rings have been reported, as well as longitudinal tracts of neurites (neurites are usually axons or dendrites). These findings would suggest that some groupings of neural cells exist in at least some Anthozoan species after all. Sensory neurons, interneurons, motorneurons, and neurosecretory‐like gland cells were also reported to exist in N. vectensis.
Note: in case I didn’t mention this before, cnidocytes are often considered neural cells because they display mechanosensory properties and calcium dependent neural‐like properties as well.
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.
Cerianthus membranaecus (known as cylinder anemone or coloured tube anemone)
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.
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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!
Blood Work: A Tale of Medicine and Murder in the Scientific Revolution by Holly Tucker is a book that I first read almost 10 years ago. I got it as a gift from a dear friend, for my birthday during the time I was a student at School of Medicine (as you can guess, I decided to start anew and switch to Biology). However, this book has always stayed with me, not only because it was the first book of this genre I’ve read – I loved it because I thought it was the perfect mix of history, medicine, and macabre.
Blood Work follows a fascinating tale of the history of human transfusion, something what in today’s world we almost take for granted. Back in high school, we learned all about Rh factor and blood types (and how there is a possibility of A and B parent having an O or AB child) and voluntary blood donations are common occurrence in my country. Despite all this, I never wondered when did the actual blood transfusion procedures start and how did physicians know whose blood to use.
In the book, we follow Jean-Baptiste Denis, a 17th century French physician who administrated first documented transfusion. Since this transfusion included using sheep’s blood, it was called a xenotransfusion, which is a term that describes blood transfusion from one species to another. This experiment was successful, probably due to small amount of transfused blood and, I dare say, luck, since at the time, blood groups and agglutination were not known facts. Denis’ last transfusion experiment is, however, the one we learn about in a detail – after trying to treat an illness of psychiatric nature, and transferring a large amount of calf blood, his patient dies, and he is accused of murder. Denis was ultimately acquitted (with a true crime worthy twist in the case), but all further transfusions were banned, first by French government, then by English and even the Pope.
Blood Work doesn’t focus solely on this event – the writer masterfully describes political events of the time, both in France (rivalry with another physician, Henri-Martin de la Martinière, tensions in French Academy of Sciences founded by Louis XIV) and abroad (a competition between French and English Academies), as well as the religious ones (“playing God”, fear that this kind of transfusion could produce some sort of half-human half-animal creature). Furthermore, as Neil Blumberg wrote in his review for Journal of Clinical Investigations, these kinds of experiments were primary conducted due to the belief that transfusion could treat, or even cure, mental illness, sometimes we now know is not possible.
Lastly, it doesn’t actually matter if you have health and/or science background to find this book interesting, as long as your’re interested in history and historical non-fiction. Blood Work offers a captivating look on the beginnings of one of the most important medical procedures in the world and does it so vividly you almost feel like you’re transported to Paris, in the middle of the scientific revolution.
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I would very much liked to hear your opinion – did you read this book, and if yes, did you like it as much as I did? Do you have similar book recommendations? Please let me know in the comments 🙂
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.
Aplysina aerophoba, also of class Demospongiae, which can be found in Adriatic 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).
science-pit.blog 